Introduction to Programming BIT701/Introduction to Programming CS201 book HANDOUTS in pdf

<b><span style="font-size: large;">Introduction to Programming BIT701/Introduction to Programming CS201 book HANDOUTS in pdf</span></b><br /><br />Handouts | Lectures | Contents | Books<div> <a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi4yurSJ7EV3zQG5ebWgMX4gecQG1fnf50hRDZaxhAI3ISSAAlad5EQyTA3ZNajPtNAykUSZS8IgEEYMZsAd5zXAa80R5PYJK894jwDBWMFWUlLWOTmw1eS6FZSGmaXhhqP6BRV9bpYMwE/s1600/unnamed.png" imageanchor="1"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi4yurSJ7EV3zQG5ebWgMX4gecQG1fnf50hRDZaxhAI3ISSAAlad5EQyTA3ZNajPtNAykUSZS8IgEEYMZsAd5zXAa80R5PYJK894jwDBWMFWUlLWOTmw1eS6FZSGmaXhhqP6BRV9bpYMwE/s640/unnamed.png" width="640" /></a></div> <div> <br /></div> <div> <div> Lecture No. 1</div> <div> Summary</div> <div> o What is programming</div> <div> o Why programming is important</div> <div> o What skills are needed</div> <div> o Develop a basic recipe for writing programs</div> <div> o Points to remember</div> <div> What is programming</div> <div> As this course is titled “Introduction to programming”, therefore it is most essential</div> <div> and appropriate to understand what programming really means. Let us first see a</div> <div> widely known definition of programming.</div> <div> Definition: "A program is a precise sequence of steps to solve a particular problem.”</div> <div> It means that when we say that we have a program, it actually means that we know</div> <div> about a complete set activities to be performed in a particular order. The purpose of</div> <div> these activities is to solve a given problem.</div> <div> Alan Perlis, a professor at Yale University, says:</div> <div> "It goes against the grain of modern education to teach children to program. What fun</div> <div> is there in making plans, acquiring discipline in organizing thoughts, devoting</div> <div> attention to detail and learning to be self-critical? "</div> <div> It is a sarcastic statement about modern education, and it means that the modern</div> <div> education is not developing critical skills like planning, organizing and paying</div> <div> attention to detail. Practically, in our day to day lives we are constantly planning,</div> <div> organizing and paying attention to fine details (if we want our plans to succeed). And</div> <div> it is also fun to do these activities. For example, for a picnic trip we plan where to go,</div> <div> what to wear, what to take for lunch, organize travel details and have a good time</div> <div> while doing so.</div> <div> When we talk about computer programming then as Mr. Steve Summit puts it</div> <div> “At its most basic level, programming a computer simply means telling it what to do,</div> <div> and this vapid-sounding definition is not even a joke. There are no other truly</div> <div> fundamental aspects of computer programming; everything else we talk about will</div> <div> simply be the details of a particular, usually artificial, mechanism for telling a</div> <div> computer what to do. Sometimes these mechanisms are chosen because they have</div> <div> been found to be convenient for programmers (people) to use; other times they have</div> <div> been chosen because they're easy for the computer to understand. The first hard thing</div> <div> about programming is to learn, become comfortable with, and accept these artificial</div> <div> mechanisms, whether they make ``sense'' to you or not. “</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 4</div> <div> Why Programming is important</div> <div> The question most of the people ask is why should we learn to program when there</div> <div> are so many application software and code generators available to do the task for us.</div> <div> Well the answer is as give by the Matthias Felleisen in the book ‘How to design</div> <div> programs’</div> <div> “The answer consists of two parts. First, it is indeed true that traditional forms of</div> <div> programming are useful for just a few people. But, programming as we the authors</div> <div> understand it is useful for everyone: the administrative secretary who uses</div> <div> spreadsheets as well as the high-tech programmer. In other words, we have a broader</div> <div> notion of programming in mind than the traditional one. We explain our notion in a</div> <div> moment. Second, we teach our idea of programming with a technology that is based</div> <div> on the principle of minimal intrusion. Hence, our notion of programming teaches</div> <div> problem-analysis and problem-solving skills without imposing the overhead of</div> <div> traditional programming notations and tools.”</div> <div> Hence learning to program is important because it develops analytical and problem</div> <div> solving abilities. It is a creative activity and provides us a mean to express abstract</div> <div> ideas. Thus programming is fun and is much more than a vocational skill. By</div> <div> designing programs, we learn many skills that are important for all professions. These</div> <div> skills can be summarized as:</div> <div> o Critical reading</div> <div> o Analytical thinking</div> <div> o Creative synthesis</div> <div> What skills are needed</div> <div> Programming is an important activity as people life and living depends on the</div> <div> programs one make. Hence while programming one should</div> <div> o Paying attention to detail</div> <div> o Think about the reusability.</div> <div> o Think about user interface</div> <div> o Understand the fact the computers are stupid</div> <div> o Comment the code liberally</div> <div> Paying attention to detail</div> <div> In programming, the details matter. This is a very important skill. A good programmer</div> <div> always analyzes the problem statement very carefully and in detail. You should pay</div> <div> attention to all the aspects of the problem. You can't be vague. You can't describe</div> <div> your program 3/4th of the way, then say, "You know what I mean?'', and have the</div> <div> compiler figure out the rest.</div> <div> Furthermore you should pay attention to the calculations involved in the program, its</div> <div> flow, and most importantly, the logic of the program. Sometimes, a grammatically</div> <div> correct sentence does not make any sense. For example, here is a verse from poem</div> <div> "Through the Looking Glass" written by Lewis Carol:</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 5</div> <div> “Twas brillig, and the slithy toves</div> <div> Did gyre and gimble in the wabe “</div> <div> The grammar is correct but there is no meaning. Similarly, the sentence, "Mr. ABC</div> <div> sleeps thirty hours every day", is grammatically correct but it is illogical.</div> <div> So it may happen that a program is grammatically correct. It compiles and runs but</div> <div> produces incorrect or absurd results and does not solve the problem. It is very</div> <div> important to pay attention to the logic of the program.</div> <div> Think about the reusability</div> <div> When ever you are writing a program, always keep in mind that it could be reused at</div> <div> some other time. Also, try to write in a way that it can be used to solve some other</div> <div> related problem. A classic example of this is:</div> <div> Suppose we have to calculate the area of a given circle. We know the area of a circle</div> <div> is (Pi * r2). Now we have written a program which calculates the area of a circle with</div> <div> given radius. At some later time we are given a problem to find out the area of a ring.</div> <div> The area of the ring can be calculated by subtracting the area of outer circle from the</div> <div> area of the inner circle. Hence we can use the program that calculates the area of a</div> <div> circle to calculate the area of the ring.</div> <div> Think about Good user interface</div> <div> As programmers, we assume that computer users know a lot of things, this is a big</div> <div> mistake. So never assume that the user of your program is computer literate. Always</div> <div> provide an easy to understand and easy to use interface that is self explanatory.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 6</div> <div> Understand the fact that computers are stupid</div> <div> Computers are incredibly stupid. They do exactly what you tell them to do: no more,</div> <div> no less-- unlike human beings. Computers can't think by themselves. In this sense,</div> <div> they differ from human beings. For example, if someone asks you, “What is the</div> <div> time?”, “Time please?” or just, “Time?” you understand anyway that he is asking the</div> <div> time but computer is different. Instructions to the computer should be explicitly</div> <div> stated. Computer will tell you the time only if you ask it in the way you have</div> <div> programmed it.</div> <div> When you're programming, it helps to be able to "think'' as stupidly as the computer</div> <div> does, so that you are in the right frame of mind for specifying everything in minute</div> <div> detail, and not assuming that the right thing will happen by itself.</div> <div> Comment the code liberally</div> <div> Always comment the code liberally. The comment statements do not affect the</div> <div> performance of the program as these are ignored by the compiler and do not take any</div> <div> memory in the computer. Comments are used to explain the functioning of the</div> <div> programs. It helps the other programmers as well as the creator of the program to</div> <div> understand the code.</div> <div> Program design recipe</div> <div> In order to design a program effectively and properly we must have a recipe to follow.</div> <div> In the book name ‘How to design programs’ by Matthias Felleisen.and the co-worker,</div> <div> the idea of design recipe has been stated very elegenlty as</div> <div> “Learning to design programs is like learning to play soccer. A player must learn to</div> <div> trap a ball, to dribble with a ball, to pass, and to shoot a ball. Once the player knows</div> <div> those basic skills, the next goals are to learn to play a position, to play certain</div> <div> strategies, to choose among feasible strategies, and, on occasion, to create variations</div> <div> of a strategy because none fits. “</div> <div> The author then continue to say that:</div> <div> “A programmer is also very much like an architect, a composers, or a writer. They are</div> <div> creative people who start with ideas in their heads and blank pieces of paper. They</div> <div> conceive of an idea, form a mental outline, and refine it on paper until their writings</div> <div> reflect their mental image as much as possible. As they bring their ideas to paper, they</div> <div> employ basic drawing, writing, and playing music to express certain style elements of</div> <div> a building, to describe a person's character, or to formulate portions of a melody. They</div> <div> can practice their trade because they have honed their basic skills for a long time and</div> <div> can use them on an instinctive level.</div> <div> Programmers also form outlines, translate them into first designs, and iteratively</div> <div> refine them until they truly match the initial idea. Indeed, the best programmers edit</div> <div> and rewrite their programs many times until they meet certain aesthetic standards.</div> <div> And just like soccer players, architects, composers, or writers, programmers must</div> <div> practice the basic skills of their trade for a long time before they can be truly creative.</div> <div> Design recipes are the equivalent of soccer ball handling techniques, writing</div> <div> techniques, arrangements, and drawing skills. “</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 7</div> <div> Hence to design a program properly, we must:</div> <div> o Analyze a problem statement, typically expressed as a word problem.</div> <div> o Express its essence, abstractly and with examples.</div> <div> o Formulate statements and comments in a precise language.</div> <div> o Evaluate and revise the activities in light of checks and tests and</div> <div> o Pay attention to detail.</div> <div> All of these are activities that are useful, not only for a programmer but also for a</div> <div> businessman, a lawyer, a journalist, a scientist, an engineer, and many others.</div> <div> Let us take an example to demonstrate the use of design recipe:</div> <div> Suppose we have to develop a payroll system of a company. The company has</div> <div> permanent staff, contractual staff, hourly based employees and per unit making</div> <div> employees. Moreover, there are different deductions and benefits for permanent</div> <div> employees and there is a bonus for per unit making employees and overtime for</div> <div> contractual employees.</div> <div> We need to analyze the above problem statement. The company has four categories of</div> <div> employees; i.e.; Permanent staff, Contractual staff, hourly based employees and per</div> <div> unit making employees. Further, permanent staff has benefits and deductions</div> <div> depending upon their designation. Bonus will be given to per unit making employees</div> <div> if they make more than 10 pieces a day. Contractual employee will get overtime if</div> <div> they stay after office hours.</div> <div> Now divide the problem into small segments and calculations. Also include examples</div> <div> in all segments. In this problem, we should take an employee with his details from</div> <div> each category. Let’s say, Mr. Ahmad is a permanent employee working as Finance</div> <div> Manager. His salary is Rs.20000 and benefits of medical, car allowance and house</div> <div> rent are Rs.4000 and there is a deduction of Rs.1200. Similarly, we should consider</div> <div> employees from other categories. This will help us in checking and testing the</div> <div> program later on.</div> <div> The next step is to formulate these statements in a precise language, i.e. we can use</div> <div> the pseudo code and flowcharting. which will be then used to develop the program</div> <div> using computer language.</div> <div> Then the program should be evaluated by testing and checking. If there are some</div> <div> changes identified, we revise the activities and repeat the process. Thus repeating the</div> <div> cycle, we achieve a refined solution.</div> <div> Points to remember</div> <div> Hence the major points to keep in mind are:</div> <div> o Don’t assume on the part of the users</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 8</div> <div> o User Interface should be friendly</div> <div> o Don’t forget to comment the code</div> <div> o PAY ATTENTION TO DETAIL</div> <div> o Program, program and program, not just writing code, but the whole</div> <div> process of design and development</div> </div> <div> <div> Lecture No. 2</div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program chapter 1</div> <div> 1.2, 1.3, 1.4, 1.6,</div> <div> 1.7</div> <div> 1.11, 1.12, 1.13</div> <div> Summary</div> <div> o Software Categories</div> <div> o System Software</div> <div> o Application Software</div> <div> o History of C language</div> <div> o Development Environment of ‘C’</div> <div> Software Categories</div> <div> Software is categorized into two main categories</div> <div> o System Software</div> <div> o Application Software</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 10</div> <div> System Software</div> <div> The system software controls the computer. It communicates with computer’s</div> <div> hardware (key board, mouse, modem, sound card etc) and controls different aspects of</div> <div> operations. Sub categories of system software are:</div> <div> o Operating system</div> <div> o Device drivers</div> <div> o Utilities</div> <div> Operating system</div> <div> An operating system (sometimes abbreviated as "OS") is the program that manages all</div> <div> the other programs in a computer. It is a integrated collection of routines that service</div> <div> the sequencing and processing of programs by a computer. Note: An operating system</div> <div> may provide many services, such as resource allocation, scheduling, input/output</div> <div> control, and data management.</div> <div> Definition</div> <div> “Operating system is the software responsible for controlling the allocation and usage of hardware</div> <div> resources such as memory, central processing unit (CPU) time, disk space, and peripheral devices. The</div> <div> operating system is the foundation on which applications, such as word processing and spreadsheet</div> <div> programs, are built. (Microsoft)”</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 11</div> <div> Device drivers</div> <div> The device driver software is used to communicate between the devices and the</div> <div> computer. We have monitor, keyboard and mouse attached to almost all PC’s; if we</div> <div> look at the properties of these devices we will see that the operating system has</div> <div> installed special software to control these devices. This piece of software is called</div> <div> device driver software. When we attach a new device with the computer, we need</div> <div> software to communicate with this device. These kinds of software are known as</div> <div> device drivers e.g. CD Rom driver, Sound Card driver and Modem driver. Normally</div> <div> manufacturer of the device provide the device driver software with the device. For</div> <div> scanners to work properly with the computers we install the device driver of the</div> <div> scanner. Nowadays if you have seen a scanner, it comes with TWAIN Drivers.</div> <div> TWAIN stands for Technology Without An Interesting Name.</div> <div> Utility Software</div> <div> Utility software is a program that performs a very specific task, usually related to</div> <div> managing system resources. You would have noticed a utility of Disk Compression.</div> <div> Whenever you write a file and save it to the disk, Compression Utility compresses the</div> <div> file (reduce the file size) and write it to the disk and when you request this file from</div> <div> the disk, the compression utility uncompressed the file and shows its contents.</div> <div> Similarly there is another utility, Disk Defragmentation which is used to defragment</div> <div> the disk. The data is stored on the disks in chunks, so if we are using several files and</div> <div> are making changes to these files then the different portions of file are saved on</div> <div> different locations on the disk. These chunks are linked and the operating system</div> <div> knows how to read the contents of file from the disk combining all the chunks.</div> <div> Similarly when we delete a file then the place where that file was stored on the disk is</div> <div> emptied and is available now to store other files. As the time goes on, we have a lot of</div> <div> empty and used pieces on the disk. In such situation we say that the disk is</div> <div> fragmented now. If we remove this fragmentation the chunks of data on the disk will</div> <div> be stored close to each other and thus reading of data will be faster. For the purpose of</div> <div> removing fragmentation on the disk the Defragmentation utility is used.</div> <div> The compilers and interpreters also belong to the System Software category.</div> <div> Application software</div> <div> A program or group of programs designed for end users. For example a program for</div> <div> Accounting, Payroll, Inventory Control System, and guided system for planes. GPS</div> <div> (global positioning system), another application software, is being used in vehicles,</div> <div> which through satellite determines the geographical position of the vehicle</div> <div> History of C language</div> <div> The C language was developed in late 60’s and early 70’s, in Bell Laboratories. In</div> <div> those days BCPL and B languages were developed there. The BCPL language was</div> <div> developed in 1967 by Martin Richards as a language for writing operating systems</div> <div> software and compilers. In 1970 Ken Thompson used B language to create early</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 12</div> <div> versions of the UNIX operating system at Bell Laboratories. Thus both the languages</div> <div> were being used to develop various system software even compilers. Both BCPL and</div> <div> B were ‘type less’ languages, every data item occupied one ‘word’ in memory and the</div> <div> burden of treating a data item as a whole number or real number, for example was the</div> <div> responsibility of the programmer.</div> <div> Dennis Ritchie developed a general purpose language, called C language, by using</div> <div> different features of BCPL and B languages. C uses many important concepts of</div> <div> BCPL and B while adding data typing and other features. In the start C became</div> <div> widely known as the development language of the UNIX operating system, and the</div> <div> UNIX operating system was written by using this C language. The C language is so</div> <div> powerful that the compiler of C and other various operating systems are written in C.</div> <div> C language has almost unlimited powers to do with computers. You can program to</div> <div> turn on or off any device of computer. You can do a lot to hard disk and other</div> <div> peripherals. It is very easy to write a program in C that stops the running of computer.</div> <div> So be careful while programming in C.</div> <div> The C language and UNIX operating system widely spread in educational and</div> <div> research institutions. There was C and UNIX everywhere. Due to the wide spread of</div> <div> C, different researchers started to add their features in the language. And thus</div> <div> different variations in C came into existence. Many universities developed their own</div> <div> C by adding different features to the C language developed by Ritchie. These</div> <div> variations led to the need of a standard version of C. In 1983 a technical committee</div> <div> was created under the American National Standards Committee on Computer and</div> <div> Information Processing to provide an unambiguous and machine-independent</div> <div> definition of the language. In 1989 the standard was approved. ANSI cooperated with</div> <div> the International Standard Organization (ISO) to standardize C worldwide.</div> <div> Tools of the trade</div> <div> As programmer we need different tools to develop a program. These tools are needed</div> <div> for the life cycle of programs</div> <div> Editors</div> <div> First of all we need a tool for writing the code of a program. For this purpose we used</div> <div> Editors in which we write our code. We can use word processor too for this, but word</div> <div> processors have many other features like bold the text, italic, coloring the text etc, so</div> <div> when we save a file written in a word processor, lot of other information including the</div> <div> text is saved on the disk. For programming purposes we don’t need these things we</div> <div> only need simple text. Text editors are such editors which save only the text which we</div> <div> type. So for programming we will be using a text editor</div> <div> Compiler and Interpreter</div> <div> As we write the code in English and we know that computers can understand only 0s</div> <div> and 1s. So we need a translator which translates the code of our program into machine</div> <div> language. There are two kinds of translators which are known as Interpreter and</div> <div> Compilers. These translators translate our program which is written in C-Language</div> <div> into Machine language. Interpreters translates the program line by line meaning it</div> <div> reads one line of program and translates it, then it reads second line, translate it and so</div> <div> on. The benefit of it is that we get the errors as we go along and it is very easy to</div> <div> correct the errors. The drawback of the interpreter is that the program executes slowly</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 13</div> <div> as the interpreter translates the program line by line. Another drawback is that as</div> <div> interpreters are reading the program line by line so they cannot get the overall picture</div> <div> of the program hence cannot optimize the program making it efficient.</div> <div> Compilers also translate the English like language (Code written in C) into a language</div> <div> (Machine language) which computers can understand. The Compiler read the whole</div> <div> program and translates it into machine language completely. The difference between</div> <div> interpreter and compiler is that compiler will stop translating if it finds an error and</div> <div> there will be no executable code generated whereas Interpreter will execute all the</div> <div> lines before error and will stop at the line which contains the error. So Compiler needs</div> <div> syntactically correct program to produce an executable code. We will be using</div> <div> compiler in our course</div> <div> Debugger</div> <div> Another important tool is Debugger. Every programmer should be familiar with it.</div> <div> Debugger is used to debug the program i.e. to correct the logical errors. Using</div> <div> debugger we can control our program while it is running. We can stop the execution</div> <div> of our program at some point and can check the values in different variables, can</div> <div> change these values etc. In this way we can trace the logical errors in our program and</div> <div> can see whether our program is producing the correct results. This tool is very</div> <div> powerful, so it is complex too</div> <div> Linker</div> <div> Most of the time our program is using different routines and functions that are located</div> <div> in different files, hence it needs the executable code of those routines/functions.</div> <div> Linker is a tool which performs this job, it checks our program and includes all those</div> <div> routines or functions which we are using in our program to make a standalone</div> <div> executable code and this process is called Linking</div> <div> Loader</div> <div> After a executable program is linked and saved on the disk and it is ready for</div> <div> execution. We need another process which loads the program into memory and then</div> <div> instruct the processor to start the execution of the program from the first instruction</div> <div> (the starting point of every C program is from the main function). This processor is</div> <div> known as loader. Linker and loaders are the part of development environment. These</div> <div> are part of system software.</div> <div> The following figure represents a graphical explanation of all the steps involved in</div> <div> writing and executing a program.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 14</div> <div> Preprocessor program</div> <div> processes the code.</div> <div> Loader puts program</div> <div> in memory.</div> <div> CPU takes each</div> <div> instruction and</div> <div> executes it, possibly</div> <div> storing new data</div> <div> values as the</div> <div> program executes.</div> <div> Compiler creates object</div> <div> code and stores</div> <div> it on disk.</div> <div> Linker links the object</div> <div> code with the libraries</div> <div> Loader</div> <div> Primary Memory</div> <div> Compiler</div> <div> Editor</div> <div> Preprocessor</div> <div> Linker</div> <div> Primary Memory</div> <div> ...</div> <div> ...</div> <div> ...</div> <div> ...</div> <div> Disk</div> <div> Disk</div> <div> Disk</div> <div> CPU</div> <div> Disk</div> <div> Disk</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 15</div> <div> Lecture No. 3</div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program chapter 1</div> <div> 1.19, 1.20, 1.21,</div> <div> 1.22</div> <div> Summary</div> <div> First C program</div> <div> Variables</div> <div> Data Types</div> <div> Arithmetic Operators</div> <div> Precedence of Operators</div> <div> Tips</div> <div> First C program</div> <div> The best way to learn C is to start coding right away. So here is our very first program</div> <div> in C.</div> <div> # include <iostream.h></div> <div> main()</div> <div> {</div> <div> cout << "Welcome to Virtual University of Pakistan";</div> <div> }</div> <div> We will look at this code line by line and try to understand them.</div> <div> # include <iostream.h></div> <div> #include: This is a pre-processor directive. It is not part of our program; it is an</div> <div> instruction to the compiler. It tells the C compiler to include the contents of a file, in</div> <div> this case the system file iostream.h. The compiler knows that it is a system file, and</div> <div> therefore looks for it in a special place. The features of preprocessor will be discussed</div> <div> later. For the time being take this line on faith. You have to write this line. The sign #</div> <div> is known as HASH and also called SHARP.</div> <div> <iostream.h></div> <div> This is the name of the library definition file for all Input Output Streams. Your</div> <div> program will almost certainly want to send stuff to the screen and read things from the</div> <div> keyboard. iostream.h is the name of the file in which has code to do that work for you</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 16</div> <div> main()</div> <div> The name main is special, in that the main is actually the one which is run when your</div> <div> program is used. A C program is made up of a large number of functions. Each of</div> <div> these is given a name by the programmer and they refer to each other as the program</div> <div> runs. C regards the name "main" as a special case and will run this function first. If</div> <div> you forget to have a main function, or mistype the name, the compiler will give you</div> <div> an error.</div> <div> Notice that there are parentheses (“( )”, normal brackets) with main. Here the</div> <div> parentheses contain nothing. There may be something written inside the parentheses.</div> <div> It will be discussed in next lectures.</div> <div> { }</div> <div> Next, there is a curly bracket also called braces("{ }"). For every open brace there</div> <div> must be a matching close. Braces allows to group together pieces of a program. The</div> <div> body of main is enclosed in braces. Braces are very important in C; they enclose the</div> <div> blocks of the program.</div> <div> cout << “ Welcome to Virtual University of Pakistan”</div> <div> cout:</div> <div> This is known as out put stream in C and C++. Stream is a complicated thing, you will</div> <div> learn about it later. Think a stream as a door. The data is transferred through stream,</div> <div> cout takes data from computer and sends it to the output. For the moment it is a</div> <div> screen of the monitor. hence we use cout for output.</div> <div> <<</div> <div> The sign << indicates the direction of data. Here it is towards cout and the function of</div> <div> cout is to show data on the screen.</div> <div> “ Welcome to Virtual University of Pakistan”</div> <div> The thing between the double quotes (“ ”) is known as character string. In C</div> <div> programming character strings are written in double quotes. Whatever is written after</div> <div> << and within quotation marks will be direct it to cout, cout will display it on the</div> <div> screen.</div> <div> ;</div> <div> There is a semicolon (;) at the end of the above statement. This is very important. All</div> <div> C statements end with semicolon (;). Missing of a semicolon (;) at the end of</div> <div> statement is a syntax error and compiler will report an error during compilation. If</div> <div> there is only a semicolon (;) on a line than it will be called a null statement. i.e. it does</div> <div> nothing. The extra semicolons may be put at the end but are useless and aimless. Do</div> <div> not put semicolon (;) at a wrong place, it may cause a problem during the execution of</div> <div> the program or may cause a logical error.</div> <div> In this program we give a fixed character string to cout and the program prints it to</div> <div> the screen as:</div> <div> Variables</div> <div> During programming we need to store data. This data is stored in variables. Variables</div> <div> are locations in memory for storing data. The memory is divided into blocks. It can be</div> <div> viewed as pigeon-holes. You can also think of it as PO Boxes. In post offices there are</div> <div> different boxes and each has an address. Similarly in memory, there is a numerical</div> <div> address for each location of memory (block). It is difficult for us to handle these</div> <div> numerical addresses in our programs. So we give a name to these locations. These</div> <div> names are variables. We call them variables because they can contain different values</div> <div> at different times.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 17</div> <div> The variable names in C may be started with a character or an underscore ( _ ). But</div> <div> avoid starting a name with underscore ( _ ). C has many libraries which contain</div> <div> variables and function names normally starting with underscore ( _ ). So your variable</div> <div> name starting with underscore ( _ ) may conflict with these variables or function</div> <div> names.</div> <div> In a program every variable has</div> <div> Name</div> <div> Type</div> <div> Size</div> <div> Value</div> <div> The variables having a name, type and size (type and size will be discussed later) are</div> <div> just empty boxes. They are useless until we put some value in them. To put some</div> <div> value in these boxes is known as assigning values to variables. In C language, we use</div> <div> assignment operator for this purpose.</div> <div> Assignment Operator</div> <div> In C language equal-to-sign (=) is used as assignment operator. Do not confuse the</div> <div> algebraic equal-to with the assignment operator. In Algebra X = 2 means the value of</div> <div> X is 2, whereas in C language X = 2 (where X is a variable name) means take the</div> <div> value 2 and put it in the memory location labeled as X, afterwards you can assign</div> <div> some other value to X, for example you can write X = 10, that means now the</div> <div> memory location X contains the value 10 and the previous value 2 is no more there.</div> <div> Assignment operator is a binary operator (a binary operator has two operands). It must</div> <div> have variable on left hand side and expression (that evaluates to a single value) on</div> <div> right hand side. This operator takes the value on right hand side and stores it to the</div> <div> location labeled as the variable on left hand side, e.g. X = 5, X = 10 + 5, and X = X</div> <div> +1.</div> <div> In C language the statement X = X + 1 means that add 1 to the value of X and then</div> <div> store the result in X variable. If the value of X is 10 then after the execution of this</div> <div> statement the value of X becomes 11. This is a common practice for incrementing the</div> <div> value of the variable by ‘one in C language. Similarly you can use the statement X =</div> <div> X - 1 for decrementing the value of the variable by one. The statement X = X + 1 in</div> <div> algebra is not valid except when X is infinity. So do not confuse assignment operator</div> <div> (=) with equal sign (=) in algebra. Remember that assignment operator must have a</div> <div> variable name on left hand side unlike algebra in which you can use expression on</div> <div> both sides of equal sign (=). For example, in algebra, X +5 = Y + 7 is correct but</div> <div> incorrect in C language. The compiler will not understand it and will give error.</div> <div> Data Types</div> <div> A variable must have a data type associated with it, for example it can have data types</div> <div> like integer, decimal numbers, characters etc. The variable of type Integer stores</div> <div> integer values and a character type variable stores character value. The primary</div> <div> difference between various data types is their size in memory. Different data types</div> <div> have different size in memory depending on the machine and compilers. These also</div> <div> affect the way they are displayed. The ‘cout’ knows how to display a digit and a</div> <div> character. There are few data types in C language. These data types are reserved</div> <div> words of C language. The reserve words can not be used as a variable name.</div> <div> Let’s take a look into different data types that the C language provides us to deal with</div> <div> whole numbers, real numbers and character data.</div> <div> Whole Numbers</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 18</div> <div> The C language provides three data types to handle whole numbers.</div> <div> int</div> <div> short</div> <div> long</div> <div> int Data Type</div> <div> The data type int is used to store whole numbers (integers). The integer type has a</div> <div> space of 4 bytes (32 bits for windows operating system) in memory. And it is</div> <div> mentioned as ‘int’ which is a reserved word of C, so we can not use it as a variable</div> <div> name.</div> <div> In programming before using any variable name we have to declare that variable with</div> <div> its data type. If we are using an integer variable named as ‘i’, we have to declare it as</div> <div> int i ;</div> <div> The above line is known as declaration statement. When we declare a variable in this</div> <div> way, it reserves some space in memory depending on the size of data type and labels</div> <div> it with the variable name. The declaration statement int i ; reserves 4 bytes of memory</div> <div> and labels it as ‘i’. This happens at the execution time.</div> <div> Sample Program 1</div> <div> Let’s consider a simple example to explain int data type. In this example we take two</div> <div> integers, add them and display the answer on the screen.</div> <div> The code of the program is written below.</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> int x;</div> <div> int y;</div> <div> int z;</div> <div> x = 5;</div> <div> y = 10;</div> <div> z = x + y;</div> <div> cout << “x = “;</div> <div> cout << x;</div> <div> cout << “ y=“;</div> <div> cout << y;</div> <div> cout << “ z = x + y = “;</div> <div> cout << z;</div> <div> }</div> <div> The first three lines declare three variables x, y and z as following.</div> <div> int x;</div> <div> int y;</div> <div> int z;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 19</div> <div> These three declarations can also be written on one line. C provides us the comma</div> <div> separator (,). The above three lines can be written in a single line as below</div> <div> int x, y, z;</div> <div> As we know that semicolon (;) indicates the end of the statement. So we can write</div> <div> many statements on a single line. In this way we can also write the above declarations</div> <div> in the following form</div> <div> int x; int y; int z;</div> <div> For good programming practice, write a single statement on a single line.</div> <div> Now we assign values to variables x and y by using assignment operator. The lines x</div> <div> = 5; and y = 10 assign the values 5 and 10 to the variables x and y, respectively. These</div> <div> statements put the values 5 and 10 to the memory locations labeled as x and y.</div> <div> The next statement z = x + y; evaluates the expression on right hand side. It takes</div> <div> values stored in variables x and y (which are 5 and 10 respectively), adds them and by</div> <div> using the assignment operator (=), puts the value of the result, which is 15 in this case,</div> <div> to the memory location labeled as z.</div> <div> Here a thing to be noted is that the values of x and y remains the same after this</div> <div> operation. In arithmetic operations the values of variables used in expression on the</div> <div> right hand side are not affected. They remain the same. But a statement like x = x + 1;</div> <div> is an exceptional case. In this case the value of x is changed.</div> <div> The next line cout << “ x = “ ; is simple it just displays ‘ x = ‘ on the screen.</div> <div> Now we want to display the value of x after ‘x =’. For this we write the statement</div> <div> cout << x ;</div> <div> Here comes the affect of data type on cout. The previous statement cout << “x = “ ;</div> <div> has a character string after << sign and cout simply displays the string. In the</div> <div> statement cout << x; there is a variable name x. Now cout will not display ‘x’ but the</div> <div> value of x. The cout interprets that x is a variable of integer type, it goes to the</div> <div> location x in the memory and takes its value and displays it in integer form, on the</div> <div> screen. The next line cout << ”y =”; displays ‘ y = ‘ on the screen. And line cout << y;</div> <div> displays the value of y on the screen. Thus we see that when we write something in</div> <div> quotation marks it is displayed as it is but when we use a variable name it displays the</div> <div> value of the variable not name of the variable. The next two lines cout << “z = x + y =</div> <div> ”; and cout << z; are written to display ‘z = x + y = ’ and the value of z that is 15.</div> <div> Now when we execute the program after compiling, we get the following output.</div> <div> x = 5 y = 10 z = x + y = 15</div> <div> short Data type</div> <div> We noted that the integer occupies four bytes in memory. So if we have to store a</div> <div> small integer like 5, 10 or 20 four bytes would be used. The C provides another data</div> <div> type for storing small whole numbers which is called short. The size of short is two</div> <div> bytes and it can store numbers in range of -32768 to 32767. So if we are going to use</div> <div> a variable for which we know that it will not increase from 32767, for example the</div> <div> age of different people, then we use the data type short for age. We can write the</div> <div> above sample program by using short instead of int.</div> <div> /*This program uses short data type to store values */</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 20</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> short x;</div> <div> short y;</div> <div> short z;</div> <div> x = 5;</div> <div> y = 10;</div> <div> z = x + y;</div> <div> cout << “x = “;</div> <div> cout << x;</div> <div> cout << “ y=“;</div> <div> cout << y;</div> <div> cout << “ z = x + y = “;</div> <div> cout << z;</div> <div> }</div> <div> long Data Type</div> <div> On the other side if we have a very large whole number that can not be stored in an int</div> <div> then we use the data type long provided by C. So when we are going to deal with very</div> <div> big whole numbers in our program, we use long data type. We use it in program as:</div> <div> long x = 300500200;</div> <div> Real Numbers</div> <div> The C language provides two data types to deal with real numbers (numbers with</div> <div> decimal points e.g. 1.35, 735.251). The real numbers are also known as floating point</div> <div> numbers.</div> <div> float</div> <div> double</div> <div> float Data Type</div> <div> To store real numbers, float data type is used. The float data type uses four bytes to</div> <div> store a real number. Here is program that uses float data types.</div> <div> /*This program uses short data type to store values */</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> float x;</div> <div> float y;</div> <div> float z;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 21</div> <div> x = 12.35;</div> <div> y = 25.57;</div> <div> z = x + y;</div> <div> cout << “ x = “;</div> <div> cout << x;</div> <div> cout << “ y = “;</div> <div> cout << y;</div> <div> cout << “ z = x + y = “;</div> <div> cout << z;</div> <div> }</div> <div> double Data Type</div> <div> If we need to store a large real number which cannot be store in four bytes, then we</div> <div> use double data type. Normally the size of double is twice the size of float. In</div> <div> program we use it as:</div> <div> double x = 345624.769123;</div> <div> char Data Type</div> <div> So far we have been looking on data types to store numbers, In programming we do</div> <div> need to store characters like a,b,c etc. For storing the character data C language</div> <div> provides char data type. By using char data type we can store characters in variables.</div> <div> While assigning a character value to a char type variable single quotes are used</div> <div> around the character as ‘a’.</div> <div> /* This program uses short data type to store values */</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> char x;</div> <div> x = ’a’;</div> <div> cout << “The character value in x = “;</div> <div> cout << x;</div> <div> }</div> <div> Arithmetic Operators</div> <div> In C language we have the usual arithmetic operators for addition, subtraction,</div> <div> multiplication and division. C also provides a special arithmetic operator which is</div> <div> called modulus. All these operators are binary operators which means they operate on</div> <div> two operands. So we need two values for addition, subtraction, multiplication,</div> <div> division and modulus.</div> <div> ARITHMETIC</div> <div> OPERATION</div> <div> ARITHMETIC</div> <div> OPERATOR</div> <div> ALGEBRAIC</div> <div> EXPRESSION</div> <div> C</div> <div> EXPRESSION</div> <div> Addition + x + y x + y</div> <div> Subtraction - x – y x - y</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 22</div> <div> Multiplication * Xy x * y</div> <div> Division / x ÷ y, x / y x / y</div> <div> Modulus % x mod y x % y</div> <div> Addition, subtraction and multiplication are same as we use in algebra.</div> <div> There is one thing to note in division that when we use integer division (i.e. both</div> <div> operands are integers) yields an integer result. This means that if, for example, you</div> <div> are dividing 5 by 2 (5 / 2) it will give integer result as 2 instead of actual result 2.5.</div> <div> Thus in integer division the result is truncated to the whole number, the fractional part</div> <div> (after decimal) is ignored. If we want to get the correct result, then we should use float</div> <div> data type.</div> <div> The modulus operator returns the remainder after division. This operator can only be</div> <div> used with integer operands. The expression x % y returns the remainder after x is</div> <div> divided by y. For example, the result of 5 % 2 will be 1, 23 % 5 will be 3 and 107%10</div> <div> will be 7.</div> <div> Precedence of Operators</div> <div> The arithmetic operators in an expression are evaluated according to their precedence.</div> <div> The precedence means which operator will be evaluated first and which will be</div> <div> evaluated after that and so on. In an expression, the parentheses ( ) are used to force</div> <div> the evaluation order. The operators in the parentheses ( ) are evaluated first. If there</div> <div> are nested parentheses then the inner most is evaluated first.</div> <div> The expressions are always evaluated from left to right. The operators *, / and % have</div> <div> the highest precedence after parentheses. These operators are evaluated before + and –</div> <div> operators. Thus + and – operators has the lowest precedence. It means that if there are</div> <div> * and + operators in an expression then first the * will be evaluated and then its result</div> <div> will be added to other operand. If there are * and / operators in an expression (both</div> <div> have the same precedence) then the operator which occurs first from left will be</div> <div> evaluated first and then the next, except you force any operator to evaluate by putting</div> <div> parentheses around it.</div> <div> The following table explains the precedence of the arithmetic operators:</div> <div> OPERATORS OPERATIONS PRECEDENCE (ORDER OF</div> <div> EVALUATION)</div> <div> ( ) Parentheses Evaluated first</div> <div> *, /, or % Multiplication,</div> <div> Division, Modulus</div> <div> Evaluated second. If there are</div> <div> several, they are evaluated from</div> <div> left to right</div> <div> + or - Addition, Subtraction Evaluated last. If there are several,</div> <div> they are evaluated from left to</div> <div> right</div> <div> Lets look some examples.</div> <div> What is the result of 10 + 10 * 5 ?</div> <div> The answer is 60 not 100. As * has higher precedence than + so 10 * 5 is evaluated</div> <div> first and then the answer 50 is added to 10 and we get the result 60. The answer will</div> <div> be 100 if we force the addition operation to be done first by putting 10 + 10 in</div> <div> parentheses. Thus the same expression rewritten as (10 + 10) * 5 will give the result</div> <div> 100. Note that how the parentheses affect the evaluation of an expression.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 23</div> <div> Similarly the expression 5 * 3 + 6 / 3 gives the answer 17, and not 7. The evaluation</div> <div> of this expression can be clarified by writing it with the use of parentheses as (5 * 3) +</div> <div> (6 / 3) which gives 15 + 2 = 17. Thus you should be careful while writing arithmetic</div> <div> expressions.</div> <div> Tips</div> <div> Use spaces in the coding to make it easy to read and understand</div> <div> Reserved words can not be used as variable names</div> <div> There is always a main( ) in a C program that is the starting point of execution</div> <div> Write one statement per line</div> <div> Type parentheses ’( )’ and braces ‘{ }’ in pairs</div> <div> Use parentheses for clarification in arithmetic expressions</div> <div> Don’t forget semicolon at the end of each statement</div> <div> C Language is case sensitive so variable names x and X are two different variables</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 24</div> <div> Lecture No. 4</div> </div> <div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program chapter 1</div> <div> 1.22</div> <div> Summary</div> <div> o Sample Program</div> <div> o Examples of Expressions</div> <div> o Use of Operators</div> <div> o Tips</div> <div> Sample Program</div> <div> Problem statement:</div> <div> Calculate the average age of a class of ten students. Prompt the user to enter the age of</div> <div> each student.</div> <div> Solution:</div> <div> Lets first sort out the problem. In the problem we will take the ages of ten students</div> <div> from the user. To store these ages we will use ten variables, one variable for each</div> <div> student’s age. We will take the ages of students in whole numbers (in years only, like</div> <div> 10, 12, 15 etc), so we will use the variables of data type int. The variables declaration</div> <div> statement in our program will be as follow:</div> <div> int age1, age2, age3, age4, age5, age6, age7, age8, age9, age10;</div> <div> We have declared all the ten variables in a single line by using comma separator ( , ).</div> <div> This is a short method to declare a number of variables of the same data type.</div> <div> After this we will add all the ages to get the total age and store this total age in a</div> <div> variable. Then we will get the average age of the ten students by dividing this total</div> <div> age by 10. For the storage of total and average ages we need variables. For this</div> <div> purpose we use variable TotalAge for the total of ages and AverageAge for average of</div> <div> ages respectively.</div> <div> int TotalAge, AverageAge;</div> <div> We have declared AverageAge as int data type so it can store only whole numbers.</div> <div> The average age of the class can be in real numbers with decimal point (for example if</div> <div> total age is 173 then average age will be 17.3). But the division of integers will</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 25</div> <div> produce integer result only and the decimal portion is truncated. If we need the actual</div> <div> result then we should use real numbers (float or double) in our program.</div> <div> Now we have declared variables for storing different values. In the next step we</div> <div> prompt the user to enter the age of first student. We simply show a text line on the</div> <div> screen by using the statement:</div> <div> cout << “Please enter the age of first student : ” ;</div> <div> So on the screen the sentence “Please enter the age of first student:” will appear.</div> <div> Whenever we are requesting user to enter some information we need to be very clear</div> <div> i.e. write such sentences that are self explanatory and user understands them</div> <div> thoroughly and correctly. Now with the above sentence everyone can understand that</div> <div> age would be entered for the first student. As we are expecting only whole numbers</div> <div> i.e. age in years only i.e. 10, 12 etc, our program is not to expect ages as 13.5 or 12.3</div> <div> or 12 years and 3 months etc. We can refine our sentence such, that the user</div> <div> understands precisely that the age would be entered in whole number only.</div> <div> After this we allow the user to enter the age. To, get the age entered by the user into a</div> <div> variable, we use the statement:</div> <div> cin >> age1;</div> <div> Lets have a look on the statement cin >> age1; cin is the counter part of the cout.</div> <div> Here cin is the input stream that gets data from the user and assigns it to the variable</div> <div> on its right side. We know that the sign >> indicates the direction of the flow of data.</div> <div> In our statement it means that data comes from user and is assigned to the variable</div> <div> age1, where age1 is a variable used for storing the age entered for student1. Similarly</div> <div> we get the ages of all the ten students and store them into respective variables. That</div> <div> means the age of first student in age1, the age of second student in age2 and so on up</div> <div> to 10 students. When cin statement is reached in a program, the program stops</div> <div> execution and expects some input from the user. So when cin >> age1; is executed,</div> <div> the program expects from the user to type the age of the student1. After entering the</div> <div> age, the user has to press the 'enter key'. Pressing 'enter key' conveys to the program</div> <div> that user has finished entering the input and cin assigns the input value to the variable</div> <div> on the right hand side which is age1 in this case. As we have seen earlier that in an</div> <div> assignment statement, we can have only one variable on left hand side of the</div> <div> assignment operator and on right hand side we can have an expression that evaluates</div> <div> to a single value. If we have an expression on the left hand side of assignment</div> <div> operator we get an error i.e. x = 2 + 4; is a correct statement but x + y = 3+ 5; is an</div> <div> incorrect statement as we can not have an expression on the left hand side. Similarly</div> <div> we can not have an expression after the >> sign with cin. So we can have one and</div> <div> only one variable after >> sign i.e. cin >> x; is a correct statement and cin >> x + y;</div> <div> is an incorrect statement.</div> <div> Next, we add all these values and store the result to the variable TotalAge. We use</div> <div> assignment operator for this purpose. On the right hand side of the assignment</div> <div> operator, we write the expression to add the ages and store the result in the variable,</div> <div> TotalAge on left hand side. For this purpose we write the statement as follow:</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 26</div> <div> TotalAge = age1 + age2 + age3 + age4 + age5 + age6 + age7 + age8 +</div> <div> age9 + age10 ;</div> <div> The expression on the right hand side uses many addition operators ( + ). As these</div> <div> operators have the same precedence, the expression is evaluated from left to right.</div> <div> Thus first age1 is added to age2 and then the result of this is added to age3 and then</div> <div> this result is added to age4 and so on.</div> <div> Now we divide this TotalAge by 10 and get the average age. We store this average</div> <div> age in the variable i.e. AverageAge by writing the statement:</div> <div> AverageAge = TotalAge / 10;</div> <div> And at the end we display this average age on the screen by using the following</div> <div> statement:</div> <div> cout << “ The average age of the students is : “ << AverageAge;</div> <div> Here the string enclosed in the quotation marks, will be printed on the screen as it is</div> <div> and the value of AverageAge will be printed on the screen.</div> <div> The complete coding of the program is given below:</div> <div> /* This program calculates the average age of a class of ten students after prompting</div> <div> the user to enter the age of each student. */</div> <div> #include <iostream.h></div> <div> main ()</div> <div> {</div> <div> // declaration of variables, the age will be in whole numbers</div> <div> int age1, age2, age3, age4, age5, age6, age7, age8, age9, age10;</div> <div> int TotalAge, AverageAge;</div> <div> // take ages of the students from the user</div> <div> cout << “Please enter the age of student 1: ”;</div> <div> cin >> age1;</div> <div> cout << “Please enter the age of student 2: ”;</div> <div> cin >> age2;</div> <div> cout << “Please enter the age of student 3: ”;</div> <div> cin >> age3;</div> <div> cout << “Please enter the age of student 4: ”;</div> <div> cin >> age4;</div> <div> cout << “Please enter the age of student 5: ”;</div> <div> cin >> age5;</div> <div> cout << “Please enter the age of student 6: ”;</div> <div> cin >> age6;</div> <div> cout << “Please enter the age of student 7: ”;</div> <div> cin >> age7;</div> <div> cout << “Please enter the age of student 8: ”;</div> <div> cin >> age8;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 27</div> <div> cout << “Please enter the age of student 9: ”;</div> <div> cin >> age9;</div> <div> cout << “Please enter the age of student 10: ”;</div> <div> cin >> age10;</div> <div> // calculate the total age and average age</div> <div> TotalAge = age1 + age2 + age3 + age4 + age5 + age6 + age7 + age8 + age9 +</div> <div> age10;</div> <div> AverageAge = TotalAge / 10;</div> <div> // Display the result ( average age )</div> <div> cout << “Average age of class is: “ << AverageAge;</div> <div> }</div> <div> A sample output of the above program is given below.</div> <div> In the above output the total age of the students is 123 and the actual average should</div> <div> be 12.3 but as we are using integer data types so the decimal part is truncated and the</div> <div> whole number 12 is assigned to the variable AverageAge.</div> <div> Examples of Expressions</div> <div> We have already seen the precedence of arithmetic operators. We have expressions</div> <div> for different calculations in algebraic form, and in our programs we write them in the</div> <div> form of C statements. Let’s discuss some more examples to get a better</div> <div> understanding.</div> <div> We know about the quadratic equation in algebra, that is y = ax2 + bx + c. The</div> <div> quadratic equation in C will be written as y = a * x * x + b * x + c. In C, it is not an</div> <div> equation but an assignment statement. We can use parentheses in this statement, this</div> <div> Please enter the age of student 1: 12</div> <div> Please enter the age of student 2: 13</div> <div> Please enter the age of student 3: 11</div> <div> Please enter the age of student 4: 14</div> <div> Please enter the age of student 5: 13</div> <div> Please enter the age of student 6: 15</div> <div> Please enter the age of student 7: 12</div> <div> Please enter the age of student 8: 13</div> <div> Please enter the age of student 9: 14</div> <div> Please enter the age of student 10: 11</div> <div> Average age of class is: 12</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 28</div> <div> will make the expression statement easy to read and understand. Thus we can rewrite</div> <div> it as y = a * (x * x) + (b * y) + c.</div> <div> Note that we have no power operator in C, just use * to multiply the same value.</div> <div> Here is another expression in algebra: x = ax + by + cz2. In C the above expression</div> <div> will be as:</div> <div> x = a * x + b * y + c * z * z</div> <div> The * operator will be evaluated before the + operator. We can rewrite the above</div> <div> statement with the use of parentheses. The same expressions can be written as:</div> <div> x = (a * x) + (b * y) + c * ( z * z)</div> <div> Lets have an other expression in algebra as x = a(x + b(y + cz2)). The parentheses in</div> <div> this equation force the order of evaluation. This expression will be written in C as:</div> <div> x = a * (x + b * (y + c * z * z))</div> <div> While writing expressions in C we should keep in mind the precedence of the</div> <div> operators and the order of evaluation of the expressions (expressions are evaluated</div> <div> from left to right). Parentheses are used in complicated expressions. In algebra, there</div> <div> may be curly brackets { } and square brackets [ ] in an expression but in C we have</div> <div> only parentheses</div> <div> ( ). Using parentheses, we can make a complex expression easy to read and</div> <div> understand and can force the order of evaluation. We have to be very careful while</div> <div> using parentheses, as parentheses at wrong place can cause an incorrect result. For</div> <div> example, a statement x = 2 + 4 * 3 results x = 14. As * operator is of higher</div> <div> precedence, 4 * 3 is evaluated first and then result 12 is added to 4 which gives the</div> <div> result 14. We can rewrite this statement, with the use of parentheses to show it clearly,</div> <div> that multiplication is performed first. Thus we can write it as x = 2 + (4 * 3). But the</div> <div> same statement with different parentheses like x = (2 + 4) * 3 will give the result 18,</div> <div> so we have to be careful while using parenthesis and the evaluation order of the</div> <div> expression.</div> <div> Similarly the equation (b2 – 4ac)/2a can be written as ( b * b – 4 * a * c) / ( 2 * a ).</div> <div> The same statement without using parentheses will be as b * b – 4 * a * c / 2 * a. This</div> <div> is wrong as it evaluates to b2 – 4ac/2a (i.e. 4ac is divided by 2a instead of (b2-4ac)).</div> <div> Use of Operators</div> <div> Here are sample programs which will further explain the use of operators in</div> <div> programming.</div> <div> Problem Statement:</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 29</div> <div> Write a program that takes a four digits integer from user and shows the digits on the</div> <div> screen separately i.e. if user enters 7531, it displays 1,3,5,7 separately.</div> <div> Solution:</div> <div> Let’s first analyze the problem and find out the way how to program it.</div> <div> Analysis:</div> <div> First of all, we will sort the problem and find out how we can find digits of an integer.</div> <div> We know that when we divide a number by 10, we get the last digit of the number as</div> <div> remainder. For example when we divide 2415 by 10 we get 5 as remainder. Similarly</div> <div> 3476 divided by 10 gives the remainder 6. We will use this logic in our problem to get</div> <div> the digits of the number. First of all, we declare two variables for storing number and</div> <div> the digit. Let’s say that we have a number 1234 to show its digits separately. In our</div> <div> program we will use modulus operator ( % ) to get the remainder. So we get the first</div> <div> digit of the number 1234 by taking its modulus with 10 (i.e. 1234 % 10). This will</div> <div> give us the digit 4. We will show this digit on the screen by using cout statement.</div> <div> After this we have to find the next digit. For this we will divide the number by 10 to</div> <div> remove its last digit. Here for example the answer of 1234 divided by 10 is 123.4, we</div> <div> need only three digits and not the decimal part. In C we know that the integer division</div> <div> truncates the decimal part to give the result in whole number only. We will use integer</div> <div> division in our program and declare our variable for storing the number as int data</div> <div> type. We will divide the number 1234 by 10 (i.e. 1234 / 10). Thus we will get the</div> <div> number with remaining three digits i.e. 123. Here is a point to be noted that how can</div> <div> we deal with this new number (123)?</div> <div> There are two ways, one is that we declare a new variable of type int and assign the</div> <div> value of this new number to it. In this way we have to declare more variables that</div> <div> mean more memory will be used. The second way is to reuse the same variable</div> <div> (where number was already stored). As we have seen earlier that we can reassign</div> <div> values to variables like in the statement x = x + 1, which means, add 1 to the value of</div> <div> x and assign this resultant value again to x. In this way we are reusing the variable x.</div> <div> We will do the same but use the division operator instead of addition operator</div> <div> according to our need. For this purpose we will write number = number / 10. After</div> <div> this statement we have value 123 in the variable number.</div> <div> Again we will get the remainder of this number with the use of modulus operator,</div> <div> dividing the number by 10 (i.e. 123 % 10). Now we will get 3 and display it on the</div> <div> screen. To get the new number with two digits, divide the number by 10. Once again,</div> <div> we get the next digit of the number (i.e. 12) by using the modulus operator with 10,</div> <div> get the digit 2 and display it on the screen. Again get the new number by dividing it</div> <div> by 10</div> <div> (i.e. 1). We can show it directly, as it is the last digit, or take remainder by using</div> <div> modulus operator with 10. In this way, we get all the digits of the number.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 30</div> <div> Now let’s write the program in C by following the analysis we have made. The</div> <div> complete C program for the above problem is given below. It is easy to understand as</div> <div> we are already familiar with the statements used in it.</div> <div> /* A program that takes a four digits integer from user and shows the digits on the</div> <div> screen separately i.e. if user enters 7531, it displays 7,5,3,1 separately. */</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> // declare variables</div> <div> int number, digit;</div> <div> // prompt the user for input</div> <div> cout << "Please enter 4-digit number:";</div> <div> cin >> number;</div> <div> // get the first digit and display it on screen</div> <div> digit = number % 10;</div> <div> cout << "The digits are: ";</div> <div> cout << digit << ", ";</div> <div> // get the remaining three digits number</div> <div> number = number / 10;</div> <div> // get the next digit and display it</div> <div> digit = number % 10;</div> <div> cout << digit << ", ";</div> <div> // get the remaining two digits number</div> <div> number = number / 10;</div> <div> // get the next digit and display it</div> <div> digit = number % 10;</div> <div> cout << digit << ", ";</div> <div> // get the remaining one digit number</div> <div> number = number / 10;</div> <div> // get the next digit and display it</div> <div> digit = number % 10;</div> <div> cout << digit;</div> <div> }</div> <div> A sample output of the above program is given below.</div> <div> Please enter 4-digit number: 5678</div> <div> The digits are: 8, 7, 6, 5</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 31</div> <div> Problem Statement:</div> <div> Write a program that takes radius of a circle from the user and calculates the diameter,</div> <div> circumference and area of the circle and display the result.</div> <div> Solution:</div> <div> In this problem we take the input (radius of a circle) from the user. For that we can</div> <div> use cin statement to prompt the user to enter the radius of a circle. We store this radius</div> <div> in a variable. We also need other variables to store diameter, circumference and area</div> <div> of the circle. To obtain the correct result, we declare these variables of type float,</div> <div> instead of int data type, as we know that the int data type stores the whole numbers</div> <div> only. Here in our problem the area or circumference of the circle can be in decimal</div> <div> values. After getting the radius we use the formulae to find the diameter,</div> <div> circumference and area of the circle and then display these results on the screen. The</div> <div> solution of this program in coding form is given below.</div> <div> /* Following program takes the radius of a circle from the user and calculates the</div> <div> diameter, circumference and area of the circle and displays the result. */</div> <div> #include <iostream.h></div> <div> main ()</div> <div> {</div> <div> // declare variables</div> <div> float radius, diameter, circumference, area;</div> <div> // prompt the user for radius of a circle</div> <div> cout << "Please enter the radius of the circle " ;</div> <div> cin >> radius ;</div> <div> // calculate the diameter, circumference and area of the circle</div> <div> // implementing formula i.e. diameter = 2 r circumference = 2 ח r and area = ח r2</div> <div> diameter = radius * 2 ;</div> <div> circumference = 2 * 3.14 * radius ; // 3.14 is the value of ח (Pi)</div> <div> area = 3.14 * radius * radius ;</div> <div> // display the results</div> <div> cout << "The diameter of the circle is : " << diameter ;</div> <div> cout << "The circumference of the circle is : " << circumference ;</div> <div> cout << "The area of the circle is : " << area ;</div> <div> }</div> <div> A sample output of the above program is given below.</div> <div> Please enter the radius of the circle 5</div> <div> The diameter of the circle is : 10</div> <div> The circumference of the circle is : 31.4</div> <div> The area of the circle is : 78.5</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 32</div> <div> Tips</div> <div> o Use descriptive names for variables</div> <div> o Indent the code for better readability and understanding</div> <div> o Use parenthesis for clarity and to force the order of evaluation in an</div> <div> expression</div> <div> o Reuse the variables for better usage of memory</div> <div> o Take care of division by zero</div> <div> o Analyze the problem properly, and then start coding (i.e. first think and then</div> <div> write)</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 33</div> <div> Lecture No. 5</div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program chapter 2</div> <div> 2.4, 2.5, 2.6, 2.19,</div> <div> 2.20</div> <div> Summary</div> <div> o Conditional Statements</div> <div> o Flow Charting</div> <div> • Sample Program 1</div> <div> o if/else structure</div> <div> o Logical Operators</div> <div> • Sample Program 2</div> <div> o Tips</div> <div> Conditional Statements (Decision Making)</div> <div> In every day life, we are often making decisions. We perform different tasks while</div> <div> taking decisions. For example, the statement ‘if the milk shop is open, bring one liter</div> <div> of milk while returning home from college’, involves this phenomenon.</div> <div> In this statement, there is an element of decision making. We bring one litre of milk if</div> <div> the shop is open. And if the shop is closed, we come back to home without milk.</div> <div> Thus we are making a decision on the condition that the shop is open. The decisionmaking</div> <div> process is everywhere in our daily life. We see that the college gives</div> <div> admission to a student if he has the required percentage in his previous examination</div> <div> and/or in the entry test. Similarly administration of a basketball team of the college</div> <div> decides that the students having height more than six feet can be members of the</div> <div> team.</div> <div> In the previous lectures, we have written simple elementary programs. For writing</div> <div> interesting and useful programs, we have to introduce the decision making power in</div> <div> them. Now we will see what kind of decisions are there in programming and how</div> <div> these can be used.</div> <div> Every programming language provides a structure for decision making.</div> <div> 'C' also provides this structure. The statement used for decisions in 'C' language is</div> <div> known as the 'if statement'. The if statement has a simple structure. That is</div> <div> if ( condition )</div> <div> Statement (or group of statements)</div> <div> The above statements mean, If condition is true, then execute the statement or a group</div> <div> of statements. Here the condition is a statement which explains the condition on</div> <div> which a decision will be made. We can understand it from the example that Ali can</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 34</div> <div> become the member of the basket ball team if he has a height more than six feet .In</div> <div> this case, the condition will be</div> <div> if (Ali’s height is greater than six feet)</div> <div> Ali can be a member of team</div> <div> We have written the condition in English language. Now let's see how we can</div> <div> implement this in terms of variables, operators and C statements. In the program, we</div> <div> will write the condition in parentheses, followed by a statement or group of statements</div> <div> to be executed.</div> <div> Now here is the concept of block of statements. We use braces { } to make a group</div> <div> (block) of a number of statements. We put ‘{’ before first statement and ‘}’ after the</div> <div> last statement. Thus if we have to do many things after the if statement. The structure</div> <div> of if statement becomes as under</div> <div> if (condition)</div> <div> {</div> <div> statement;</div> <div> statement;</div> <div> .</div> <div> .</div> <div> statement;</div> <div> }</div> <div> Note the indentation of the lines and semi-colon after each statement. Semi-colons are</div> <div> necessary after every C statement. The indentation is only a matter of style. It makes</div> <div> the code easy to read and understand from where a block starts, ends and what kind of</div> <div> block it is. It does not affect the logic of the program. But the braces can affect the</div> <div> logic. We can also write a comment line to state the purpose of code block.</div> <div> Let's consider a simple example to explain the if statement. Suppose, we have ages of</div> <div> two students (say for the time being we have got these ages in variables). These</div> <div> variables are- age1 and age2. Now we say that if the age1 is greater than age2, then</div> <div> display the statement ‘Student 1 is older than student 2’.</div> <div> The coding for this program will be as below</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> int age1, age2;</div> <div> age1 = 12;</div> <div> age2 = 10;</div> <div> if(age1 > age2)</div> <div> cout << “Student 1 is older than student 2”;</div> <div> }</div> <div> Here, in our code we see a new operator i.e. ‘ > ‘ (greater than) in the if statement. We</div> <div> need such operators (like greater than, less than, equal to etc) while making decisions.</div> <div> These operators are called 'relational operators'. These are almost the same relational</div> <div> operators we use in algebra. Following table summarizes the relational operators.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 35</div> <div> Algebraic In C</div> <div> language</div> <div> Example Meaning</div> <div> Greater than > > x > y x is greater than y</div> <div> Equal to = == x == y x is equal to y</div> <div> Less than < < x < y x is less than y</div> <div> Greater than or</div> <div> equal to</div> <div> > >= x >= y x is greater than or</div> <div> equal to y</div> <div> Less than or</div> <div> equal to</div> <div> < <= x <= y x is less than or equal</div> <div> to y</div> <div> Not equal to ≠ != x != y x is not equal to y</div> <div> Note that there is no space between ==, >=, <= and !=.</div> <div> These are considered as single operators.</div> <div> The operator == (equal to) is different from the operator =. We know that operator =</div> <div> is the assignment operator which is used in assignment statement to assign a value to</div> <div> a variable.</div> <div> Don't confuse the assignment operator (=) with equal to operator (==). If we write</div> <div> single = in condition of if statement. For example, if we write if ( x = 2 ), the compiler</div> <div> will not give error. This means that it is not a syntax error. The conditional expression</div> <div> in if statement returns a value. In this case, x = 2 will also have some value but it will</div> <div> not in the form of true or false. So it will create a logical error. So be careful while</div> <div> using equal to condition in if statement.</div> <div> Flow Charting</div> <div> There are different techniques that are used to analyze and design a program. We will</div> <div> use the flow chart technique. A flow chart is a pictorial representation of a program.</div> <div> There are labeled geometrical symbols, together with the arrows connecting one</div> <div> symbol with other.</div> <div> A flow chart helps in correctly designing the program by visually showing the</div> <div> sequence of instructions to be executed. A programmer can trace and rectify the</div> <div> logical errors by first drawing a flow chart and then simulating it.</div> <div> Flow Chart Symbols</div> <div> Below are some of the main symbols used in the flow chart.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 36</div> <div> The flow chart for the if structure is shown in the figure below.</div> <div> Sample Program 1</div> <div> Now let’s see the usage of relational operators by an example. There are two students</div> <div> Amer and Amara. We take their ages from the user, compare them and tell who is</div> <div> older?</div> <div> As there are two students to be compared in terms of age, we need to declare two</div> <div> variables to store their ages. We declare two variables AmerAge and AmaraAge of</div> <div> type int. The variable names are one continuous word as we can’t use spaces in a</div> <div> variable name.</div> <div> Here is an important point about variables declaration. We should assign an initial</div> <div> value (preferably 0 for integers) to variables when we declare them. This is called</div> <div> initialization of variables.</div> <div> We can do this in one line while declaring a variable like int x = 0; This statement</div> <div> will declare a variable of name x with data type int and will assign a value 0 to this</div> <div> variable. Initializing a variable in this way is just a matter of style. You can initialize a</div> <div> variable on a separate line after declaring it. It is a good programming practice to</div> <div> initialize a variable.</div> <div> Now we prompt the user to enter Amer’s age and store it into variable AmerAge.</div> <div> Then similarly we get Amara’s age from the user in the variable AmaraAge.</div> <div> While comparing the ages, we will use the if statement to see whether Amer’s age is</div> <div> greater than Amara’s. We will use > (greater than) operator to compare the ages. This</div> <div> can be written as if ( AmerAge > AmaraAge) .</div> <div> With this if statement, we write the statement cout << "Amer is greater than Amara" ;</div> <div> It’s a simple one line test i.e. ‘if Amer’s age is greater than Amara's’, then display the</div> <div> message ‘Amer is older than Amara’.</div> <div> The flow chart for the above problem is as under.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 37</div> <div> The complete code of the program is given below.</div> <div> /* This program test that if the age of Amer is greater than Amara’s age and displays the result.</div> <div> */</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> int AmerAge, AmaraAge;</div> <div> //prompt the user to enter Amer’s age</div> <div> cout << “Please enter Amer’s age “ ;</div> <div> cin >> AmerAge;</div> <div> //prompt the user to enter Amara’s age</div> <div> cout << “Please enter Amara’s age “ ;</div> <div> cin >> AmaraAge;</div> <div> //perform the test</div> <div> if(AmerAge > AmaraAge )</div> <div> cout << “ Amer is older than Amara”;</div> <div> }</div> <div> In our program, we write a single statement with the if condition. This statement</div> <div> executes if the condition is true. If we want to execute more than one statements, then</div> <div> we have to enclose all these statements in curly brackets { }. This comprises a block</div> <div> of statements which will execute depending upon the condition. This block may</div> <div> contain a single statement just like in our problem. So we can write the if statement as</div> <div> follow.</div> <div> if (AmerAge > AmaraAge )</div> <div> {</div> <div> cout << " Amer is older than Amara";</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 38</div> <div> A sample execution of the program provides the following output.</div> <div> Please enter Amer’s age 16</div> <div> Please enter Amara’s age 14</div> <div> Amer is older than Amara</div> <div> Now think what happens if the condition in the if statement is not true i.e. Amer’s age</div> <div> is not greater than Amara’s. In this case, if the user enters Amer’s age less than</div> <div> Amara’s, then our program does nothing. So to check this condition, another if</div> <div> statement after the first if statement is required. Then our program will be as:</div> <div> /* This program checks the age of Amer and Amara’s and</div> <div> displays the appropriate the message. The program is using</div> <div> two if statements.*/</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> int AmerAge, AmaraAge;</div> <div> //prompt the user to enter Amer’s age</div> <div> cout << “Please enter Amer’s age “ ;</div> <div> cin >> AmerAge;</div> <div> //prompt the user to enter Amara’s age</div> <div> cout << “Please enter Amara’s age “ ;</div> <div> cin >> AmaraAge;</div> <div> //perform the test</div> <div> if (AmerAge > AmaraAge )</div> <div> {</div> <div> cout << “ Amer is older than Amara”;</div> <div> }</div> <div> if (AmerAge < AmaraAge )</div> <div> {</div> <div> cout << “ Amer is younger than Amara”;</div> <div> }</div> <div> }</div> <div> Now our program decides properly about the ages entered by the user.</div> <div> After getting ages from the user, the if statements are tested and if statement will be</div> <div> executed if the condition evaluates to true.</div> <div> If/else Structure</div> <div> We have seen that the if structure executes its block of statement(s) only when the</div> <div> condition is true, otherwise the statements are skipped. The if/else structure allows the</div> <div> programmer to specify that a different block of statement(s) is to be executed when</div> <div> the condition is false. The structure of if/else selection is as follows.</div> <div> if ( condition)</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 39</div> <div> {</div> <div> statement(s);</div> <div> }</div> <div> else</div> <div> {</div> <div> statement(s);</div> <div> }</div> <div> Thus using this structure we can write the construct of our program as</div> <div> if (AmerAge > AmaraAge )</div> <div> {</div> <div> cout << " Amer is older than Amara";</div> <div> }</div> <div> else</div> <div> {</div> <div> cout << " Amer is younger than Amara";</div> <div> }</div> <div> In this construct, the program checks the condition in if statement .If the condition is</div> <div> true, then the line "Amer is greater than Amara" is printed. Otherwise (if condition is</div> <div> not true), the statement related to else is executed and the message "Amer is younger</div> <div> than Amara" is printed. Here in if/else structure an important thing is that the else part</div> <div> is executed for all the cases (conditions) other than the case which is stated in the if</div> <div> condition.</div> <div> And in the comparison, we know that there are three conditions i.e. first value is</div> <div> greater than the second value, first value is less than the second value and first value</div> <div> is equal to the second value. Here in the above program construct the else part</div> <div> competes the greater than conditions and covers both less than and equal to</div> <div> conditions.</div> <div> Thus in the above program construct, the message "Amer is younger than Amara" is</div> <div> displayed even if Amer’s age is the same as Amara’s age. This is logically incorrect</div> <div> and so to make this correct, we should display the message "Amer is younger than or</div> <div> is of the same age as Amara". Now this statement describes both the cases other than</div> <div> the one ‘Amer is greater than Amara'.</div> <div> The use of else saves us from writing different if statements to compare different</div> <div> conditions, in this way it cover the range of checks to complete the comparison.</div> <div> If we want to state the condition "Amer is greater than or is of the same age as</div> <div> Amara’s" then we use the greater than or equal to operator (i.e. >=) in the if statement</div> <div> and less than operator ( < ) in the else statement to complete the comparison.</div> <div> It is very important to check all the conditions while making decisions for good,</div> <div> complete and logical results. Make sure that all cases are covered and there is no such</div> <div> case in which the program does not respond.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 40</div> <div> Logical Operators</div> <div> There are many occasions when we face complex conditions to make a decision. This</div> <div> means that a decision depends upon more than one condition in different ways. Here</div> <div> we combine the conditions with AND or OR. For example, a boy can be selected in</div> <div> basket ball team only if he is more than 18 years old and has a height of 6 feet. In this</div> <div> statement a boy who wants to be selected in the basket ball team must have both the</div> <div> conditions fulfilled. This means that AND forces both the conditions to be true.</div> <div> Similarly we say that a person can be admitted to the university if he has a BCS</div> <div> degree OR BSC degree. In this statement, it is clear that a person will be admitted to</div> <div> the university if he has any one of the two degrees.</div> <div> In programming we use logical operators ( && and || ) for AND and OR respectively</div> <div> with relational operators. These are binary operators and take two operands. These</div> <div> operators use logical expressions as operands, which return TRUE or FALSE.</div> <div> The following table (called truth table) can be used to get the result of the &&</div> <div> operator and || operator with possible values of their operands. It is used to explain the</div> <div> result obtained by the && and || operators.</div> <div> Expression 1 Expression 2 Expression 1 &&</div> <div> Expression 2</div> <div> Expression 1 ||</div> <div> Expression 2</div> <div> True False false True</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 41</div> <div> True True true True</div> <div> False False false False</div> <div> False True false True</div> <div> The && operator has a higher precedence than the || operator. Both operators</div> <div> associate from left to right. An expressions containing && or || is evaluated only until</div> <div> truth or falsehood is known. Thus evaluation of the expression (age > 18) && (height</div> <div> > 6) will stop immediately if age > 18 is false (i.e. the entire expression is false) and</div> <div> continue if age > 18 is true (i.e. the entire expression could still be true if the</div> <div> condition height > 6 is true ).</div> <div> There is another logical operator that is called logical negation. The sign ! is used for</div> <div> this operator. This operand enables a programmer to ‘reverse’ the meaning of a</div> <div> condition. This is a unary operator that has only a single condition as an operand. The</div> <div> operator ! is placed before a condition. If the original condition (without the !</div> <div> operator) is false then the ! operator before it converts it to true and the statements</div> <div> attached to this are executed.</div> <div> Look at the following expression</div> <div> if ( ! (age > 18 ))</div> <div> cout << “ The age is less than 18”;</div> <div> Here the cout statement will be executed if the original condition (age > 18) is false</div> <div> because the ! operator before it reverses this false to true.</div> <div> The truth table for the logical negation operator ( ! ) is given below.</div> <div> Expression ! Expression</div> <div> True False</div> <div> False True</div> <div> Sample Program 2</div> <div> Problem statement</div> <div> A shopkeeper announces a package for customers that he will give 10 % discount on</div> <div> all bills and if a bill amount is greater than 5000 then a discount of 15 %. Write a C</div> <div> program which takes amount of the bill from user and calculates the payable amount</div> <div> by applying the above discount criteria and display it on the screen.</div> <div> Solution</div> <div> In this problem we are going to make decision on the basis of the bill amount, so we</div> <div> will be using if statement. We declare three variables amount, discount and</div> <div> netPayable and initialize them. Next we prompt the user to enter the amount of the</div> <div> bill. After this we implement the if statement to test the amount entered by the user.</div> <div> As we see in the problem statement that if the amount is greater than 5000 then the</div> <div> discount rate is 15 % otherwise (i.e. the amount is less than or equal to 5000) the</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 42</div> <div> discount rate is 10 %. So we check the amount in if statement. If it is greater than</div> <div> 5000 then the condition is true then the if block is executed otherwise if amount is not</div> <div> greater than 5000 then the else block is executed.</div> <div> The analysis and the flow of the program is shown by the following flow chart.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 43</div> <div> The complete program code is given below:</div> <div> /* This program calculates the discount amount for a customer. As different discount</div> <div> percentage applies on different amount so program is using if statement for deciding</div> <div> which discount is applicable and display the result. */</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> double amount, discount, netPayable ;</div> <div> amount = 0 ;</div> <div> netPayable = 0 ;</div> <div> discount = 0 ;</div> <div> // prompt the user to enter the bill amount</div> <div> cout << "Please enter the amount of the bill " ;</div> <div> cin >> amount ;</div> <div> //test the conditions and calculate net payable</div> <div> if ( amount > 5000 )</div> <div> {</div> <div> //calculate amount at 15 % discount</div> <div> discount = amount * (15.0 / 100);</div> <div> netPayable = amount - discount;</div> <div> cout << "The discount at the rate 15 % is Rupees " << discount << endl;</div> <div> cout << "The payable amount is Rupees " << netPayable ;</div> <div> }</div> <div> else</div> <div> {</div> <div> // calculate amount at 10 % discount</div> <div> discount = amount * (10.0 / 100);</div> <div> netPayable = amount - discount;</div> <div> cout << "The discount at the rate 10 % is Rupees " << discount << endl ;</div> <div> cout << "The payable amount is Rupees " << netPayable ;</div> <div> }</div> <div> }</div> <div> In the program we declared the variables as double. We do this to get the correct</div> <div> results (results may be in decimal points) of the calculations. Look at the statement</div> <div> which calculates the discount. The statement is</div> <div> discount = amount * (15.0 / 100) ;</div> <div> Here in the above statement we write 15.0 instead of 15. If we write here 15 then the</div> <div> division 15 / 100 will be evaluated as integer division and the result of division (0.15)</div> <div> will be truncated and we get 0 and this will result the whole calculation to zero. So it</div> <div> is necessary to write at least one operand in decimal form to get the correct result by</div> <div> division and we should also declare the variables as float or double. We do the same</div> <div> in the line discount = amount * (10.0 / 100);</div> <div> A sample execution of the program is given below</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 44</div> <div> Please enter the amount of the bill 6500</div> <div> The discount at the rate 15 % is Rupees 975</div> <div> The payable amount is Rupees 5525</div> <div> Tips</div> <div> • Always put the braces in an if/else structure</div> <div> • Type the beginning and ending braces before typing inside them</div> <div> • Indent both body statements of an if and else structure</div> <div> • Be careful while combining the conditions with logical operators</div> <div> • Use if/else structure instead of a number of single selection if</div> <div> statements</div> </div> <div> <div> Lecture No. 6</div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program chapter 2</div> <div> 2.7, 2.8, 2.9, 2.20</div> <div> Summary</div> <div> Repetition Structure (Loop)</div> <div> Overflow Condition</div> <div> Sample Program 1</div> <div> Sample Program 2</div> <div> Infinite Loop</div> <div> Properties of While loop</div> <div> Flow Chart</div> <div> Sample Program 3</div> <div> Tips</div> <div> Repetition Structure (Loop)</div> <div> In our day to day life, most of the things are repeated. Days and nights repeat</div> <div> themselves 30 times a month. Four seasons replace each other every year. We can see</div> <div> similar phenomenon in the practical life. For example, in the payroll system, some</div> <div> procedures are same for all the employees. These are repeatedly applied while dealing</div> <div> with the employees. So repetition is very useful structure in the programming.</div> <div> Let’s discuss a problem to understand it thoroughly. We have to calculate the sum of</div> <div> first 10 whole numbers i.e. add the numbers from 1 to 10. Following statement may</div> <div> be one way to do it.</div> <div> cout << “Sum of first 10 numbers is = “ << 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 +</div> <div> 10;</div> <div> This method is perfectly fine as the syntax is right. The answer is also correct. This</div> <div> procedure can also be adopted while calculating the sum of numbers from 1 to 100.</div> <div> We can write the above statement adding all the digits from 1 to 100. But this method</div> <div> will not be suitable for computing the sum of numbers from 1 to 1000.The addition of</div> <div> a very big number of digits will result in a very ugly and boring statement. Let’s</div> <div> analyze it carefully. Our first integer is 1, is there any other way to find out what is the</div> <div> next integer? Yes, we can add 1 to the integer and get the next integer which is 2. To</div> <div> find the next integer (i.e. 3) we add 1 to the previous integer (i.e. 2) and get the next</div> <div> integer which is 3. So whenever we have to find out the next integer, we have to add 1</div> <div> to the previous integer.</div> <div> We have to calculate the sum of first 1000 integers by taking a variable sum of type</div> <div> int. It is a good programming practice to initialize the variable before using it. Here,</div> <div> we initialize the variable sum with zero.</div> <div> int sum = 0;</div> <div> Now we get the first integer i.e. 1. We add this to the sum (sum becomes 0 + 1 = 1).</div> <div> Now get the next integer which can be obtained by adding 1 to the previous integer</div> <div> i.e. 2 and add it to the sum (sum becomes 1 + 2 = 3). Get the next integer by adding 1</div> <div> to the previous integer and add it to the sum (sum becomes 3 + 3 = 6) and so on.</div> <div> This way, we get the next integer by adding 1 to the previous integer and the</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 46</div> <div> new integer to the sum. It is obvious that we are repeating this procedure again and</div> <div> again i.e. adding 1 to the previous integer and add this new integer to the sum. So we</div> <div> need some repetition structure in the programming language. There are many looping</div> <div> constructs in C Language. The repetition structure we are discussing in this lecture is</div> <div> 'while loop structure'. ‘while’ is also a key word of 'C' so it cannot be used as a</div> <div> variable name.</div> <div> While means, 'do it until the condition is true'. The use of while construct can be</div> <div> helpful in repeating a set of instructions under some condition. We can also use curly</div> <div> braces with while just like we used with if. If we omit to use the braces with while</div> <div> construct, then only one statement after while will be repeatedly executed. For good</div> <div> programming practices, always use braces with while irrespective of the number of</div> <div> statements in while block. The code will also be indented inside the while block as</div> <div> Indentation makes the code easy to understand.</div> <div> The syntax of while construct is as under:</div> <div> while ( Logical Expression ) {</div> <div> statement1;</div> <div> statement2;</div> <div> ………….</div> <div> }</div> <div> The logical expression contains a logical or relational operator. While this logical</div> <div> expression is true, the statements will be executed repeatedly. When this logical</div> <div> expression becomes false, the statements within the while block, will not be executed.</div> <div> Rather the next statement in the program after while block, will be executed.</div> <div> Let’s discuss again the same problem i.e. calculation of the sum of first 1000 integers</div> <div> starting from 1. For this purpose, we need a variable to store the sum of integers and</div> <div> declare a variable named sum. Always use the self explanatory variable names. The</div> <div> declaration of the variable sum in this case is:</div> <div> int sum = 0;</div> <div> The above statement has performed two tasks i.e. it declared the variable sum of type</div> <div> int and also initialized it with zero. As it is good programming practice to initialize all</div> <div> the variables when declared, the above statement can be written as:</div> <div> int sum;</div> <div> sum = 0;</div> <div> Here we need a variable to store numbers. So we declare a variable number of type</div> <div> int. This variable will be used to store integers.</div> <div> int number;</div> <div> As we have declared another variable of int data type, so the variables of same data</div> <div> type can be declared in one line.</div> <div> int sum, number;</div> <div> Going back to our problem, we need to sum up all the integers from 1 to 1000. Our</div> <div> first integer is 1. The variable number is to be used to store integers, so we will</div> <div> initialize it by 1 as our first integer is 1:</div> <div> number = 1;</div> <div> Now we have two variables- sum and number. That means we have two memory</div> <div> locations labeled as sum and number which will be used to store sum of integers and</div> <div> integers respectively. In the variable sum, we have to add all the integers from 1 to</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 47</div> <div> 1000. So we will add the value of variable number into variable sum, till the time the</div> <div> value of number becomes 1000. So when the value of number becomes 1000, we will</div> <div> stop adding integers into sum. It will become the condition of our while loop. We can</div> <div> say sum the integers until integer becomes 1000. In C language, this condition can be</div> <div> written as:</div> <div> while ( number <= 1000 ) {</div> <div> ………Action ………</div> <div> }</div> <div> The above condition means, 'perform the action until the number is 1000 or less than</div> <div> 1000'. What will be the Action? Add the number, the value of number is 1 initially,</div> <div> into sum. This is a very simple statement:</div> <div> sum = sum + number;</div> <div> Let’s analyze the above statement carefully. We did not write sum = number; as this</div> <div> statement will replace the contents of sum and the previous value of sum will be</div> <div> wasted as this is an assignment statement. What we did? We added the contents of</div> <div> sum and contents of number first (i.e. 0 + 1) and then stored the result of this (i.e. 1) to</div> <div> the sum.</div> <div> Now we need to generate next integer and add it to the sum. How can we get the next</div> <div> integer? Just by adding 1 to the integer, we will get the next integer. In ‘C’, we will</div> <div> write it as:</div> <div> number = number + 1;</div> <div> Similarly in the above statement, we get the original contents of number (i.e. 1). Add</div> <div> 1 to them and then store the result (i.e. 2) into the number. Now we need to add this</div> <div> new number into sum:</div> <div> sum = sum + number;</div> <div> We add the contents of sum (i.e. 1) to the contents of number (i.e. 1) and then store</div> <div> the result (i.e. 2) to the sum. Again we need to get the next integer which can be</div> <div> obtained by adding 1 to the number. In other words, our action consists of only two</div> <div> statements i.e. add the number to the sum and get the next integer. So our action</div> <div> statements will be:</div> <div> sum = sum + number;</div> <div> number = number + 1;</div> <div> Putting the action statements in while construct:</div> <div> while ( number <= 1000 ) {</div> <div> sum = sum + number;</div> <div> number = number + 1;</div> <div> }</div> <div> Let's analyze the above while loop. Initially the contents of number is 1. The</div> <div> condition in while loop (i.e. number <= 1000) will be evaluated as true, contents of</div> <div> sum and contents of number will be added and the result will be stored into sum. Now</div> <div> 1 will be added to the contents of number and number becomes 2. Again the condition</div> <div> in while loop will be evaluated as true and the contents of sum will be added to the</div> <div> contents of number .The result will be stored into sum. Next 1 will be added to the</div> <div> contents of number and number becomes 3 and so on. When number becomes 1000,</div> <div> the condition in while loop evaluates to be true, as we have used <= (less than or equal</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 48</div> <div> to) in the condition. The contents of sum will be added to the contents of number (i.e.</div> <div> 1000) and the result will be stored into the sum. Next 1 will be added to the contents</div> <div> of number and number becomes 1001. Now the condition in while loop is evaluated to</div> <div> false, as number is no more less than or equal to 1000 (i.e. number has become 1001).</div> <div> When the condition of while loop becomes false, loop is terminated. The control of</div> <div> the program will go to the next statement following the ending brace of the while</div> <div> construct. After the while construct, we can display the result using the cout</div> <div> statement.</div> <div> cout << “ The sum of first 1000 integers starting from 1 is “ << sum;</div> <div> The complete code of the program is as follows:</div> <div> /* This program calculate the sum of first 1000 integers */</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> //declaration of variables</div> <div> int sum, number;</div> <div> //Initialization of the variables</div> <div> sum = 0;</div> <div> number = 1;</div> <div> // using the while loop to find out the sum of first 1000 integers starting from 1</div> <div> while(number <= 1000)</div> <div> {</div> <div> // Adding the integer to the contents of sum</div> <div> sum = sum + number;</div> <div> // Generate the next integer by adding 1 to the integer</div> <div> number = number + 1;</div> <div> }</div> <div> cout << "The sum of first 1000 integers starting from 1 is " << sum;</div> <div> }</div> <div> The output of the program is:</div> <div> The sum of first 1000 integers starting from 1 is 500500</div> <div> While construct is a very elegant and powerful construct. We have seen that it is very</div> <div> easy to sum first 1000 integers just with three statements. Suppose we have to</div> <div> calculate the sum of first 20000 integers. How can we do that? We just have to change</div> <div> the condition in the while loop (i.e. number <= 20000).</div> <div> Overflow Condition:</div> <div> We can change this condition to 10000 or even more. Just try some more numbers.</div> <div> How far can you go with the limit? We know that integers are allocated a fixed space</div> <div> in memory (i.e. 32 bits in most PCs) and we can not store a number which requires</div> <div> more bits than integer, into a variable of data type, int. If the sum of integers becomes</div> <div> larger than this limit (i.e. sum of integers becomes larger than 32 bits can store), two</div> <div> things can happen here. The program will give an error during execution, compiler</div> <div> can not detect such errors. These errors are known as run time errors. The second</div> <div> thing is that 32 bits of the result will be stored and extra bits will be wasted, so our</div> <div> result will not be correct as we have wasted the information. This is called overflow.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 49</div> <div> When we try to store larger information in, than a data type can store, overflow</div> <div> condition occurs. When overflow condition occurs either a run-time error is generated</div> <div> or wrong value is stored.</div> <div> Sample Program 1:</div> <div> To calculate the sum of 2000 integers, we will change the program (i.e. the while</div> <div> condition) in the editor and compile it and run it again. If we need to calculate the sum</div> <div> of first 5000 integers, we will change the program again in the editor and compile and</div> <div> run it again. We are doing this work again in a loop. Change the program in the editor,</div> <div> compile, execute it, again change the program, compile and execute it and so on. Are</div> <div> we doing this in a loop? We can make our program more intelligent so that we don’t</div> <div> need to change the condition every time. We can modify the condition as:</div> <div> int upperLimit;</div> <div> while (number <= upperLimit)</div> <div> where upperLimit is a variable of data type int. When the value of upperLimit is 1000,</div> <div> the program will calculate the sum of first 1000 integers. When the value of</div> <div> upperLimit is 5000, the program will calculate the sum of first 5000 integers. Now we</div> <div> can make it re-usable and more effective by requesting the user to enter the value for</div> <div> upper limit:</div> <div> cout << “Please enter the upper limit for which you want the sum ”;</div> <div> cin >> upperLimit;</div> <div> We don’t have to change our program every time when the limit changes. For the sum</div> <div> of integers, this program has become generic. We can calculate the sum of any</div> <div> number of integers without changing the program. To make the display statement</div> <div> more understandable, we can change our cout statement as:</div> <div> cout << “ The sum of first “ << upperLimit << “ integers is “ << sum;</div> <div> Sample Program 2:</div> <div> Problem statement:</div> <div> Calculate the sum of even numbers for a given upper limit of integers.</div> <div> Solution:</div> <div> We analyze the problem and know that while statement will be used. We need to sum</div> <div> even numbers only. How can we decide that a number is even or not? We know that</div> <div> the number that is divisible by 2 is an even number. How can we do this in C</div> <div> language? We can say that if a number is divisible by 2, it means its remainder is</div> <div> zero, when divided by 2. To get a remainder we can use C’s modulus operator i.e. %.</div> <div> We can say that for a number if the expression (number % 2) results in zero, the</div> <div> number is even. Putting this in a conditional statement:</div> <div> If ( ( number % 2) == 0 )</div> <div> The above conditional statement becomes true, when the number is even and false</div> <div> when the number is odd (A number is either even or odd).</div> <div> The complete code of the program is as follows:</div> <div> /* This program calculates sum of even numbers for a given upper limit of</div> <div> integers */</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> //declaration of variables</div> <div> int sum, number, upperLimit;</div> <div> //Initialization of the variables</div> <div> sum = 0;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 50</div> <div> number = 1;</div> <div> // Prompt the user to enter upper limit of integers</div> <div> cout << “Please enter the upper limit for which you want the sum ” ;</div> <div> cin >> upperLimit;</div> <div> // using the while loop to find out the sum of first 1000 integers starting from 1</div> <div> while(number <= upperLimit)</div> <div> {</div> <div> // Adding the even integer to the contents of sum</div> <div> if ( ( number % 2 ) == 0 )</div> <div> {</div> <div> sum = sum + number;</div> <div> }</div> <div> // Generate the next integer by adding 1 to the integer</div> <div> number = number + 1;</div> <div> }</div> <div> cout << "The sum of even numbers of first “ << upperLimit << “ integers starting</div> <div> from 1 is " << sum;</div> <div> }</div> <div> The output of the program is:</div> <div> Please enter the upper limit for which you want the sum 10</div> <div> The sum of even numbers of first 10 integers starting from 1 is 30</div> <div> Suppose if we don’t have modulus operator in the C language. Is there any other way</div> <div> to find out the even numbers? We know that in C integer division gives the integer</div> <div> result and the decimal portion is truncated. So the expression (2 * (number / 2)) gives</div> <div> the number as a result, if the number is even only. So we can change our condition in</div> <div> if statement as:</div> <div> if ( ( 2 * ( number /2 ) ) == number )</div> <div> Infinite Loop:</div> <div> Consider the condition in the while structure that is (number <= upperLimit) and in</div> <div> the while block the value of number is changing (number = number + 1) to ensure that</div> <div> the condition is tested again next time. If it is true, the while block is executed and so</div> <div> on. So in the while block statements, the variable used in condition must change its</div> <div> value so that we have some definite number of repetitions. What will happen if we do</div> <div> not write the statement number = number + 1; in our program? The value of number</div> <div> will not change, so the condition in the while loop will be true always and the loop</div> <div> will be executed forever. Such loops in which the condition is always true are known</div> <div> as infinite loops as there are infinite repetitions in it.</div> <div> Property of while loop:</div> <div> In the above example, if the user enters 0, as the value for upper limit. In the while</div> <div> condition we test (number <= upperLimit) i.e. number is less than or equal to</div> <div> upperLimit ( 0 ), this test return false. The control of the program will go to the next</div> <div> statement after the while block. The statements in while structure will not be executed</div> <div> even for a single time. So the property of while loop is that it may execute zero or</div> <div> more time.</div> <div> The while loop is terminated, when the condition is tested as false. Make sure that the</div> <div> loop test has an adequate exit. Always use braces for the loop structure. If you forget</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 51</div> <div> to put the braces, only one statement after the while statement is considered in the</div> <div> while block.</div> <div> Flow Chart:</div> <div> The basic structure of while loop in structured flow chart is:</div> <div> At first, we will draw a rectangle and write while in it. Then draw a line to its right</div> <div> and use the decision symbol i.e. diamond diagram. Write the loop condition in the</div> <div> diamond and draw a line down to diamond which represents the flow when the</div> <div> decision is true. All the repeated processes are drawn here using rectangles. Then a</div> <div> line is drawn from the last process going back to the while and decision connection</div> <div> line. We have a line on the right side of diamond which is the exit of while loop. The</div> <div> while loop terminates, when the loop condition evaluates to false and the control gets</div> <div> out of while structure.</div> <div> Here is the flow chart for sample program 2:</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 52</div> <div> So far, we have been drawing flow charts after coding the program but actually we</div> <div> have to draw the flow chart first and then start coding.</div> <div> Sample Program 3:</div> <div> Problem statement:</div> <div> Calculate the factorial of a given number.</div> <div> Solution:</div> <div> The factorial of a number N is defined as:</div> <div> N(N-1)(N-2)………….3.2.1</div> <div> By looking at the problem, we can see that there is a repetition of multiplication of</div> <div> numbers. A loop is needed to write a program to solve a factorial of a number. Let's</div> <div> think in terms of writing a generic program to calculate the factorial so that we can get</div> <div> the factorial of any number. We have to multiply the number with the next</div> <div> decremented number until the number becomes 1. So the value of number will</div> <div> decrease by 1 in each repetition.</div> <div> Here is the flow chart for the factorial.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 53</div> <div> Here is the code of the program.</div> <div> /*This program calculates the factorial of a given number.*/</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> //declaration of variables</div> <div> int factorial, number;</div> <div> //Initialization of the variables</div> <div> factorial = 1;</div> <div> number = 1;</div> <div> // Prompt the user to enter upper limit of integers</div> <div> cout << “Please enter the number for factorial ” ;</div> <div> cin >> number;</div> <div> // using the while loop to find out the factorial</div> <div> while(number > 1)</div> <div> {</div> <div> factorial = factorial * number;</div> <div> number = number - 1;</div> <div> }</div> <div> cout << "The factorial is “ << factorial;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 54</div> <div> }</div> <div> Exercise:</div> <div> Calculate the sum of odd integers for a given upper limit. Also draw flow chart of the</div> <div> program.</div> <div> Calculate the sum of even and odd integers separately for a given upper limit using</div> <div> only one loop structure. Also draw flow chart of the program.</div> <div> Tips</div> <div> Always use the self explanatory variable names</div> <div> Practice a lot. Practice makes a man perfect</div> <div> While loop may execute zero or more time</div> <div> Make sure that loop test (condition) has an acceptable exit.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 55</div> <div> Lecture No. 7</div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program Chapter 2</div> <div> 2.11, 2.12, 2.14, 2.15, 2.17</div> <div> Summary</div> <div> Do-While Statement</div> <div> Example</div> <div> for Statement</div> <div> Sample Program 1</div> <div> Increment/decrement Operators</div> <div> Sample Program 2</div> <div> Tips</div> <div> Do-While Statement</div> <div> We have seen that there may be certain situations when the body of while loop does</div> <div> not execute even a single time. This occurs when the condition in while is false. In</div> <div> while loop, the condition is tested first and the statements in the body are executed</div> <div> only when this condition is true. If the condition is false, then the control goes directly</div> <div> to the statement after the closed brace of the while loop. So we can say that in while</div> <div> structure, the loop can execute zero or more times. There may be situations where we</div> <div> may need that some task must be performed at least once.</div> <div> For example, a computer program has a character stored from a-z. It gives to</div> <div> user five chances or tries to guess the character. In this case, the task of guessing the</div> <div> character must be performed at least once. To ensure that a block of statements is</div> <div> executed at least once, C provides a do-while structure. The syntax of do-while</div> <div> structure is as under:</div> <div> do</div> <div> {</div> <div> statement(s);</div> <div> }</div> <div> while ( condition ) ;</div> <div> Here we see that the condition is tested after executing the statements of the loop</div> <div> body. Thus, the loop body is executed at least once and then the condition in do while</div> <div> statement is tested. If it is true, the execution of the loop body is repeated. In case, it</div> <div> proves otherwise (i.e. false), then the control goes to the statement next to the do</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 56</div> <div> while statement. This structure describes ‘execute the statements enclosed in braces in</div> <div> do clause' when the condition in while clause is true.</div> <div> Broadly speaking, in while loop, the condition is tested at the beginning of the loop</div> <div> before the body of the loop is performed. Whereas in do-while loop, the condition is</div> <div> tested after the loop body is performed.</div> <div> Therefore, in do-while loop, the body of the loop is executed at least once.</div> <div> The flow chart of do-while structure is as follow:</div> <div> Example</div> <div> Let’s consider the example of guessing a character. We have a character in the</div> <div> program to be guessed by the user. Let’s call it ‘z’. The program allows five tries</div> <div> (chances) to the user to guess the character. We declare a variable tryNum to store</div> <div> the number of tries. The program prompts the user to enter a character for guessing.</div> <div> We store this character in a variable c.</div> <div> We declare the variable c of type char. The data type char is used to store a single</div> <div> character. We assign a character to a variable of char type by putting the character in</div> <div> single quotes. Thus the assignment statement to assign a value to a char variable will</div> <div> be as c = ‘a’. Note that there should be a single character in single quotes. The</div> <div> statement like c = ‘gh’ will be a syntax error.</div> <div> Here we use the do-while construct. In the do clause we prompt the user to enter a</div> <div> character.</div> <div> After getting character in variable c from user, we compare it with our character i.e</div> <div> ‘z’. We use if\else structure for this comparison. If the character is the same as ours</div> <div> then we display a message to congratulate the user else we add 1 to tryNum variable.</div> <div> And then in while clause, we test the condition whether tryNum is less than or equal</div> <div> to 5 (tryNum <= 5). If this condition is true, then the body of the do clause is repeated</div> <div> again. We do this only when the condition (tryNum <= 5) remains true. If it is</div> <div> otherwise, the control goes to the first statement after the do-while loop.</div> <div> If guess is matched in first or second try, then we should exit the loop. We know that</div> <div> the loop is terminated when the condition tryNum <= 5 becomes false, so we assign a</div> <div> value which is greater than 5 to tryNum after displaying the message. Now the</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 57</div> <div> condition in the while statement is checked. It proves false (as tryNum is greater than</div> <div> 5). So the control goes out of the loop. First look here the flow chart for the program.</div> <div> The code of the program is given below.</div> <div> //This program allows the user to guess a character from a to z</div> <div> //do-while construct is used to allow five tries for guessing</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> //declare & initialize variables</div> <div> int tryNum = 0 ;</div> <div> char c ;</div> <div> // do-while construct</div> <div> do</div> <div> {</div> <div> cout << “Please enter a character between a-z for guessing : “ ;</div> <div> cin >> c ;</div> <div> //check the entered character for equality</div> <div> if ( c == ‘z’)</div> <div> {</div> <div> cout << “Congratulations, Your guess is correct” ;</div> <div> tryNum = 6;</div> <div> }</div> <div> else</div> <div> {</div> <div> tryNum = tryNum + 1;</div> <div> }</div> <div> }</div> <div> while ( tryNum <= 5);</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 58</div> <div> }</div> <div> There is an elegant way to exit the loop when the correct number is guessed. We</div> <div> change the condition in while statement to a compound condition. This condition will</div> <div> check whether the number of tries is less than or equal to 5 and the variable c is not</div> <div> equal to ‘z’. So we will write the while clause as while (tryNum <= 5 && c != ‘z’ );</div> <div> Thus when a single condition in this compound condition becomes false, then the</div> <div> control will exit the loop. Thus we need not to assign a value greater than 5 to variable</div> <div> tryNum. Thus the code of the program will be as:</div> <div> //This program allows the user to guess a character from a to z</div> <div> //do-while construct is used to allow five tries for guessing</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> //declare & initialize variables</div> <div> int tryNum = 0 ;</div> <div> char c ;</div> <div> // do-while construct, prompt the user to guess a number and compares it</div> <div> do</div> <div> {</div> <div> cout << “Please enter a character between a-z for guessing : “ ;</div> <div> cin >> c ;</div> <div> //check the entered character for equality</div> <div> if ( c == ‘z’)</div> <div> {</div> <div> cout << “Congratulations, Your guess is correct” ;</div> <div> }</div> <div> else</div> <div> {</div> <div> tryNum = tryNum + 1;</div> <div> }</div> <div> }</div> <div> while ( tryNum <= 5 && c != ‘z’ );</div> <div> }</div> <div> The output of the program is given below.</div> <div> Please enter a character between a-z for guessing : g</div> <div> Please enter a character between a-z for guessing : z</div> <div> Congratulations, Your guess is correct</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 59</div> <div> for Loop</div> <div> Let’s see what we do in a loop. In a loop, we initialize variable(s) at first. Then we set</div> <div> a condition for the continuation/termination of the loop. To meet the condition to</div> <div> terminate the loop, we affect the condition in the body of the loop. If there is a</div> <div> variable in the condition, the value of that variable is changed within the body of the</div> <div> loop. If the value of the variable is not changed, then the condition of termination of</div> <div> the loop will not meet and loop will become an infinite one. So there are three things</div> <div> in a loop structure i.e. (i) initialization, (ii) a continuation/termination condition and</div> <div> (iii) changing the value of the condition variable, usually the increment of the variable</div> <div> value.</div> <div> To implement these things, C provides a loop structure known as for loop. This is the</div> <div> most often used structure to perform repetition tasks for a known number of</div> <div> repetitions. The syntax of for loop is given below.</div> <div> for ( initialization condition ; continuation condition ; incrementing condition )</div> <div> {</div> <div> statement(s) ;</div> <div> }</div> <div> We see that a 'for statement' consists of three parts. In initialization condition, we</div> <div> initialize some variable while in continuation condition, we set a condition for the</div> <div> continuation of the loop. In third part, we increment the value of the variable for</div> <div> which the termination condition is set.</div> <div> Let's suppose, we have a variable counter of type int. We write for loop in our</div> <div> program as</div> <div> for ( counter = 0 ; counter < 10 ; counter = counter +1 )</div> <div> {</div> <div> cout << counter << endl;</div> <div> }</div> <div> This 'for loop' will print on the screen 0, 1, 2 …. 9 on separate lines (as we use endl in</div> <div> our cout statement). In for loop, at first, we initialize the variable counter to 0. And in</div> <div> the termination condition, we write counter < 10. This means that the loop will</div> <div> continue till value of counter is less than 10. In other words, the loop will terminate</div> <div> when the value of counter is equal to or greater than 10. In the third part of for</div> <div> statement, we write counter = counter + 1 this means that we add 1 to the existing</div> <div> value of counter. We call it incrementing the variable.</div> <div> Now let's see how this loop executes. When the control goes to for statement first</div> <div> time, it sets the value of variable counter to 0, tests the condition (i.e. counter < 10). If</div> <div> it is true, then executes the body of the loop. In this case, it displays the value of</div> <div> counter which is 0 for the first execution. Then it runs the incrementing statement (i.e.</div> <div> counter = counter + 1 ). Thus the value of counter becomes 1. Now, the control goes</div> <div> to for statement and tests the condition of continuation. If it is true, then the body of</div> <div> the loop is again executed which displays 1 on the screen. The increment statement is</div> <div> again executed and control goes to for statement. The same tasks are repeated. When</div> <div> the value of counter becomes 10, the condition counter < 10 becomes false. Then the</div> <div> loop is terminated and control goes out of for loop.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 60</div> <div> The point to be noted is that, the increment statement (third part of for statement) is</div> <div> executed after executing the body of the loop. Thus for structure is equivalent to a</div> <div> while structure, in which, we write explicit statement to change</div> <div> (increment/decrement) the value of the condition variable after the last statement of</div> <div> the body. The for loop does this itself according to the increment statement in the for</div> <div> structure. There may be a situation where the body of for loop, like while loop, may</div> <div> not be executed even a single time. This may happen if the initialization value of the</div> <div> variable makes the condition false. The statement in the following for loop will not be</div> <div> executed even a single time as during first checking, the condition becomes false. So</div> <div> the loop terminates without executing the body of the loop.</div> <div> for ( counter = 5 ; counter < 5 ; counter ++)</div> <div> {</div> <div> cout << “The value of counter is “ << counter ;</div> <div> }</div> <div> Sample Program 1</div> <div> Let’s take an example to explain for loop. We want to write a program that prints the</div> <div> table of 2 on the screen.</div> <div> In this program, we declare a variable counter of type int. We use this variable to</div> <div> multiply it by 2 with values 1 to 10. For writing the table of 2, we multiply 2 by 1, 2,</div> <div> 3 .. upto 10 respectively and each time display the result on screen. So we use for loop</div> <div> to perform the repeated multiplication.</div> <div> Following is the code of the program that prints the table of 2.</div> <div> //This program display the table of 2 up to multiplier 10</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> int counter;</div> <div> //the for loop</div> <div> for ( counter = 1 ; counter <= 10 ; counter = counter + 1)</div> <div> {</div> <div> cout << “2 x “ << counter << “ = “ << 2 * counter << “\n” ;</div> <div> }</div> <div> }</div> <div> This is a simple program. In the for statement, we initialize the variable counter to 1</div> <div> as we want the multiplication of 2 starting from 1. In the condition clause, we set the</div> <div> condition counter <= 10 as we want to repeat the loop for 10 times. And in the</div> <div> incrementing clause, we increment the variable counter by 1.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 61</div> <div> In the body of the for loop, we write a single statement with cout. This single</div> <div> statement involves different tasks. The portion ‘<< “2 x “’ displays the string “2 x “</div> <div> on the screen. After this, the next part ‘<< counter’ will print the value of counter.</div> <div> The ‘<< “ = ”’ will display ‘ = ‘ and then the next part ‘<< 2 * counter’ will display</div> <div> the result of 2 multiply by counter and the last <<”\n” ( the new line character) will</div> <div> start a new line. Thus in the first iteration where the value of counter is 1, the cout</div> <div> statement will display the following line</div> <div> 2 x 1 = 2</div> <div> After the execution of cout statement, the for statement will increment the counter</div> <div> variable by 1. Thus value of counter will be 2. Then condition will be checked which</div> <div> is still true. Thus the body of for loop (here the cout statement) will be executed again</div> <div> having the value of counter 2. So the following line will be printed.</div> <div> 2 x 2 = 4</div> <div> The same action will be repeated 10 times with values of counter from 1 to 10. When</div> <div> the value of counter is 11, the condition ( counter <= 10 ) will become false and the</div> <div> loop will terminate.</div> <div> The output of the above program is as the following.</div> <div> 2 x 1 = 2</div> <div> 2 x 2 = 4</div> <div> 2 x 3 = 6</div> <div> 2 x 4 = 8</div> <div> 2 x 5 = 10</div> <div> 2 x 6 = 12</div> <div> 2 x 7 = 14</div> <div> 2 x 8 = 16</div> <div> 2 x 9 = 18</div> <div> 2 x 10 = 20</div> <div> Now what will we do, if some one says us to write a table of 3, or 4 or 8 or any other</div> <div> number. Here comes the point of re-usability and that a program should be generic.</div> <div> We write a program in which a variable is used instead of a hard code number. We</div> <div> prompt the user to enter the number for which he wants a table. We store this number</div> <div> in the variable and then use it to write a table. So in our previous example, we now</div> <div> use a variable say number where we were using 2. We also can allow the user to</div> <div> enter the number of multipliers up to which he wants a table. For this, we use a</div> <div> variable maxMultiplier and execute the loop for maxMultiplier times by putting the</div> <div> condition counter <= maxMultiplier. Thus our program becomes generic which can</div> <div> display a table for any number and up to any multiplier.</div> <div> Thus, the code of our program will be as below:</div> <div> //This program takes an integer input from user and displays its table</div> <div> //The table is displayed up to the multiplier entered by the user</div> <div> # include <iostream.h></div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 62</div> <div> main ( )</div> <div> {</div> <div> int counter, number, maxMultiplier ;</div> <div> // Prompt the user for input</div> <div> cout << “Please enter the number for which you want a table : “ ;</div> <div> cin >> number ;</div> <div> cout << “Please enter the multiplier up to which you want a table : “ ;</div> <div> cin >> maxMultiplier ;</div> <div> //the for loop</div> <div> for ( counter = 1 ; counter <= maxMultiplier ; counter = counter + 1)</div> <div> {</div> <div> cout << number << “ x “ << counter << “ = “ << number * counter << “\n” ;</div> <div> }</div> <div> }</div> <div> The output of the program is shown as follows:</div> <div> Please enter the number for which you want a table : 7</div> <div> Please enter the multiplier up to which you want a table : 8</div> <div> 7 x 1 = 7</div> <div> 7 x 2 = 14</div> <div> 7 x 3 = 21</div> <div> 7 x 4 = 28</div> <div> 7 x 5 = 35</div> <div> 7 x 6 = 42</div> <div> 7 x 7 = 49</div> <div> 7 x 8 = 56</div> <div> Here is a guideline for programming style. We should avoid using constant values in</div> <div> our calculations or in long routines. The disadvantage of this is that if we want to</div> <div> change that constant value later, then we have to change every occurrence of that</div> <div> value in the program. Thus we have to do a lot of work and there may be some places</div> <div> in code where we do not change that value. To avoid such situations, we can use a</div> <div> variable at the start and assign that constant value to it and then in the program use</div> <div> that variable. Thus, if we need to change the constant value, we can assign the new</div> <div> value to that variable and the remaining code will remain the same. So in our program</div> <div> where we wrote the table of 2, we can use a variable (say number) and assign it the</div> <div> value 2. And in cout statement we use this variable instead of constant 2. If we want</div> <div> that the program should display a table of 5, then we just change the value of the</div> <div> variable. So for good programming, use variables for constant values instead of</div> <div> explicit constant values.</div> <div> Increment Decrement Operators</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 63</div> <div> We have seen that in while, do-while and for loop we write a statement to increase the</div> <div> value of a variable. For example, we used the statements like counter = counter + 1;</div> <div> which adds 1 to the variable counter. This increment statement is so common that it is</div> <div> used almost in every repetition structure (i.e. in while, do-while and for loop). The C</div> <div> language provides a unary operator that increases the value of its operator by 1. This</div> <div> operator is called increment operator and sign ++ is used for this. The statement</div> <div> counter = counter + 1; can be replaced with the statement</div> <div> counter ++ ;</div> <div> The statement counter++ adds 1 to the variable counter. Similarly the expressions i =</div> <div> i + 1 ; and j = j + 1 ; are equivalent to i++ ; and j++; respectively. There is also an</div> <div> operator -- called decrement operator. This operator decrements, the value of its</div> <div> operand by 1. So the statements counter = counter - 1; and j = j - 1; are equivalent to</div> <div> counter--; and j--; respectively.</div> <div> The increment operator is further categorized as pre-increment and post-increment.</div> <div> Similarly, the decrement operator, as pre-decrement and post-decrement.</div> <div> In pre-increment, we write the sign before the operand like ++j while in postincrement,</div> <div> the sign ++ is used after the operand like j++. If we are using only variable</div> <div> increment, pre or post increment does not matter. In this case, j++ is equivalent to</div> <div> ++j. The difference of pre and post increment matters when the variable is used in an</div> <div> expression where it is evaluated to assign a value to another variable. If we use preincrement</div> <div> ( ++j ), the value of j is first increased by 1. This new value is used in the</div> <div> expression. If we use post increment ( j++ ),the value of j is used in the expression.</div> <div> After that it is increased by 1. Same is the case in pre and post decrement.</div> <div> If j = 5, and we write the expression</div> <div> x = ++ j ;</div> <div> After the evaluation of this expression, the value of x will be 6 (as j is incremented</div> <div> first and then is assigned to x). The value of j will also be 6 as ++ operator increments</div> <div> it by 1.</div> <div> If j = 5, and we write the expression</div> <div> x = j++ ;</div> <div> Then after the evaluation of the expression, the value of x will be 5 (as the value of j</div> <div> is used before increment) and the value of j will be 6.</div> <div> The same phenomenon is true for the decrement operator with the difference that it</div> <div> decreases the value by 1. The increment and decrement operators affect the variable</div> <div> and update it to the new incremented or decremented value.</div> <div> The operators ++ and -- are used to increment or decrement the variable by 1. There</div> <div> may be cases when we are incrementing or decrementing the value of a variable by a</div> <div> number other than 1. For example, we write counter = counter + 5; or j = j – 4;. Such</div> <div> assignments are very common in loops, so C provides operators to perform this task</div> <div> in short. These operators do two things they perform an action (addition, subtraction</div> <div> etc) and do some assignment.</div> <div> These operators are +=, -=, *=, /= and %=. These operators are compound assignment</div> <div> operators. These operators assign a value to the left hand variable after performing an</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 64</div> <div> action (i.e. +, -, *, / and %). The use of these operators is explained by the following</div> <div> examples.</div> <div> Let’s say we have an expression, counter = counter + 5;. The equivalent of this</div> <div> expression is counter += 5;. The statement counter += 5; does two tasks. At first, it</div> <div> adds 5 to the value of counter and then assigns this result to counter. Similarly the</div> <div> following expressions</div> <div> x = x + 4 ;</div> <div> x = x - 3 ;</div> <div> x = x * 2 ;</div> <div> x = x / 2 ;</div> <div> x = x % 3;</div> <div> can be written in equivalent short statements using the operators ( +=, -=, *=, /=, %= )</div> <div> as follows</div> <div> x += 4 ;</div> <div> x -= 3 ;</div> <div> x *= 2;</div> <div> x /= 2;</div> <div> x %= 3 ;</div> <div> Note that there is no space between these operators. These are treated as single signs.</div> <div> Be careful about the operator %=. This operator assigns the remainder to the variable.</div> <div> These operators are alternate in short hand for an assignment statement. The use of</div> <div> these operators is not necessary. A programmer may use these or not. It is a matter of</div> <div> style.</div> <div> Example Program 2</div> <div> Let’s write a program using for loop to find the sum of the squares of the integers</div> <div> from 1 to n. Where n is a positive value entered by the user (i.e. Sum = 12 + 22 + 32 +</div> <div> ……+ n2)</div> <div> The code of the program is given below:</div> <div> //This program displays the sum of squares of integers from 1 to n</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> //declare and initialize variables</div> <div> int i, n, sum;</div> <div> sum = 0 ;</div> <div> //get input from user and construct a for loop</div> <div> cout << “Please enter a positive number for sum of squares: ” ;</div> <div> cin >> n;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 65</div> <div> for ( i = 1 ; i <= n ; i ++)</div> <div> {</div> <div> sum += i * i ;</div> <div> }</div> <div> cout << “The sum of the first ” << n << “ squares is “ << sum << endl ;</div> <div> }</div> <div> In the program declared three variables i, n and sum. We prompted the user to enter a</div> <div> positive number. We stored this number in the variable n. Then we wrote a for loop.</div> <div> In the initialization part, we initialized variable i with value 1 to start the counting</div> <div> from 1. In the condition statement we set the condition i less than or equal to n</div> <div> (number entered by the user) as we want to execute the loop n times. In the increment</div> <div> statement, we incremented the counter variable by 1. In the body of the for loop we</div> <div> wrote a single statement sum += i * i ;. This statement takes the square of the counter</div> <div> variable ( i )and adds it to the variable sum. This statement is equivalent to the</div> <div> statement sum = sum + ( i * i ) ; Thus in each iteration the square of the counter</div> <div> variable (which is increased by 1 in each iteration ) is added to the sum. Thus loop</div> <div> runs n times and the squares of numbers from 1 to n are summed up. After</div> <div> completing the for loop the cout statement is executed which displays the sum of the</div> <div> squares of number from 1 to n.</div> <div> Following is the output when the number 5 is entered.</div> <div> Please enter a positive number for sum of squares: 5</div> <div> The sum of the first 5 squares is 55</div> <div> Tips</div> <div> Comments should be meaningful, explaining the task</div> <div> Don’t forget to affect the value of loop variable in while and do-while loops</div> <div> Make sure that the loop is not an infinite loop</div> <div> Don’t affect the value of loop variable in the body of for loop, the for loop does this</div> <div> by itself in the for statement</div> <div> Use pre and post increment/decrement operators cautiously in expressions</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 66</div> <div> Lecture No. 8</div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program Chapter 2</div> <div> 2.16, 2.18</div> <div> Summary</div> <div> o Switch Statement</div> <div> o Break Statement</div> <div> o Continue Statement</div> <div> o Guide Lines</div> <div> o Rules for structured Programming/Flow Charting</div> <div> o Sample Program</div> <div> o Tips</div> <div> Switch Statement</div> <div> Sometimes, we have multiple conditions and take some action according to each</div> <div> condition. For example, in the payroll of a company, there are many conditions to</div> <div> deduct tax from the salary of an employee. If the salary is less than Rs. 10000, there is</div> <div> no deduction. But if it falls in the slab Rs. 10000 - 20000, then the income tax is</div> <div> deducted. If it exceeds the limit of Rs. 20000, some additional tax will be deducted.</div> <div> So the appropriate deduction is made according to the category or slab of the salary.</div> <div> We can also understand this from the example of grades secured by the students of a</div> <div> class. Suppose we want to print description of the grade of a student. If the student has</div> <div> grade ‘A’ we print ‘Excellent’ and 'Very good', 'good', 'poor' and 'fail' for grades B,</div> <div> C, D, and F respectively. Now we have to see how this multi-condition situation can</div> <div> be applied in a program. We have a tool for decision making i.e. 'if statement'. We can</div> <div> use 'if statement' to decide what description for a grade should be displayed. So we</div> <div> check the grade in if statement and display the appropriate description. We have five</div> <div> categories of grades-- A, B, C, D, and F. We have to write five if statements to check</div> <div> all the five possibilities (probabilities) of grade. So we write this in our program as</div> <div> underif</div> <div> ( grade == ‘A’ )</div> <div> cout << “Excellent” ;</div> <div> if ( grade == ‘B’ )</div> <div> cout << “Very Good” ;</div> <div> if ( grade == ‘C’ )</div> <div> cout << “Good” ;</div> <div> if ( grade == ‘D’ )</div> <div> cout << “Poor” ;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 67</div> <div> if ( grade == ‘F’ )</div> <div> cout << “Fail” ;</div> <div> These statements are correct and perform the required task. But the 'if statement' is</div> <div> computationally one of the most expensive statements in a program. We call it</div> <div> expensive due to the fact that the processor has to go through many cycles to execute</div> <div> an if statement to evaluate a single decision. So to make a program more efficient, try</div> <div> to use the minimum number of if statements. This will make the performance of the</div> <div> program better.</div> <div> So if we have different conditions in which only one will be true as seen in the</div> <div> example of student’s grades, the use of if statement is very expensive. To avoid this</div> <div> expensiveness, an alternate of multiple if statements can be used that is if/else</div> <div> statements. We can write an if statement in the body of an if statement which is</div> <div> known as nested if. We can write the previous code of if statements in the following</div> <div> nested if/else form.</div> <div> If ( grade == ‘A’ )</div> <div> cout << “Excellent” ;</div> <div> else if ( grade == ‘B’ )</div> <div> cout << “Very Good” ;</div> <div> else if ( grade == ‘C’ )</div> <div> cout << “Good” ;</div> <div> else if ( grade == ‘D’ )</div> <div> cout << “Poor” ;</div> <div> else if ( grade == ‘F’ )</div> <div> cout << “Fail” ;</div> <div> In the code, there is single statement with each if statement. If there are more</div> <div> statements with an if statement, then don’t forget the use of braces and make sure that</div> <div> they match (i.e. there is a corresponding closing brace for an opening brace). Proper</div> <div> indentation of the blocks should also be made.</div> <div> In the above example, we see that there are two approaches for a multi way decision.</div> <div> In the first approach, we use as many if statements as needed. This is an expensive</div> <div> approach. The second is the use of nested if statements. The second is little more</div> <div> efficient than the first one. In the 'nested if statements' the nested else is not executed</div> <div> if the first if condition is true and the control goes out of the if block.</div> <div> The C language provides us a stand-alone construct to handle these instances. This</div> <div> construct is switch structure. The switch structure is a multiple-selection construct</div> <div> that is used in such cases (multi way decisions) to make the code more efficient and</div> <div> easy to read and understand.</div> <div> The syntax of switch statement is as follows.</div> <div> switch ( variable/expression )</div> <div> {</div> <div> case constant1 : statementLlist1 ;</div> <div> case constant2 : statementLlist2 ;</div> <div> :</div> <div> :</div> <div> case constantN : statementListN ;</div> <div> default : statementList ;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 68</div> <div> }</div> <div> In the switch statement, there should be an integer variable (also include char) or an</div> <div> expression which must evaluate an integer type (whole numbers only, the decimal</div> <div> numbers 2.5, 14.3 etc are not allowed). We can’t use compound conditions (i.e. the</div> <div> conditions that use logical operators && or ||) in switch statement and in case</div> <div> statements. The constants also must be integer constants (which include char). We</div> <div> can’t use a variable name with the case key word. The default statement is optional. If</div> <div> there is no case which matches the value of the switch statement, then the statements</div> <div> of default are executed.</div> <div> The switch statement takes the value of the variable, if there is an expression then it</div> <div> evaluates the expression and after that looks for its value among the case constants. If</div> <div> the value is found among the constants listed in cases, the statements in that</div> <div> statementList are executed. Otherwise, it does nothing. However if there is a default</div> <div> (which is optional), the statements of default are executed.</div> <div> Thus our previous grade example will be written in switch statement as below.</div> <div> switch ( grade )</div> <div> {</div> <div> case ‘A’ : cout << “Excellent” ;</div> <div> case ‘B’ : cout << “Very Good” ;</div> <div> case ‘C’ : cout << “Good” ;</div> <div> case ‘D’ : cout << “Poor” ;</div> <div> case ‘F’ : cout << “Fail” ;</div> <div> }</div> <div> We know that C language is 'case sensitive'. In this language, ‘A’ is different from</div> <div> ‘a’. Every character has a numeric value which is stored by the computer.. The</div> <div> numeric value of a character is known as ASCII code of the character. The ASCII</div> <div> code of small letters (a, b, c etc ) are different from ASCII code of capital letters (A,</div> <div> B, C etc). We can use characters in switch statement as the characters are represented</div> <div> as whole numbers inside the computers.</div> <div> Now we will see how the use of ' the letter a' instead of 'A' can affect our program.</div> <div> We want our program to be user- friendly. We don’t want to restrict the user to enter</div> <div> the grade in capital letters only. So we have to handle both small and capital letters in</div> <div> our program. Here comes the limitations of switch statement. We can’t say in our</div> <div> statement like</div> <div> case ‘A’ or ‘a’ : statements ;</div> <div> We have to make two separate cases so we write</div> <div> case ‘A” :</div> <div> case ‘a’ :</div> <div> statements;</div> <div> In the switch statement, the cases fall through the case which is true. All the</div> <div> statements after that case will be executed right down to the end of the switch</div> <div> statement. This is very important to understand it. Let's suppose that the user enters</div> <div> grade ‘B’. Now the case ‘A’ is skipped. Next case ‘B’ matches and statement cout <<</div> <div> “Very Good” ; is executed. After that, all the statements will be executed. So cout <<</div> <div> “Good” ; cout << “Poor” ;and cout << “Fail” ; will be executed after one another.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 69</div> <div> We don’t want this to happen. We want that when a case matches, then after</div> <div> executing its statement, the control should jump out of the switch statement leaving</div> <div> the other cases. For this purpose we use a key word break.</div> <div> Break Statement</div> <div> The break statement interrupts the flow of control. We have seen in switch statement</div> <div> that when a true case is found, the flow of control goes through every statement down</div> <div> ward. We want that only the statements of true case should be executed and the</div> <div> remaining should be skipped. For this purpose, we use the break statement. We write</div> <div> the break statement after the statements of a case. Thus, when a true case is found and</div> <div> its statements are executed then the break statement interrupts the flow of control and</div> <div> the control jumps out of the switch statement. If we want to do the same task for two</div> <div> cases, like in previous example for ‘A’ and ‘a’, then we don't put break statement</div> <div> after the first case. We write both the cases (or the cases may be more than two) line</div> <div> by line then write the common statements to be executed for these cases. We write the</div> <div> break statement after these common statements. We should use the break statement</div> <div> necessarily after the statements of each case. The break statement is necessary in</div> <div> switch structure, without it the switch structure becomes illogic. As without it all the</div> <div> statement will execute after first match case is found.</div> <div> The above code does nothing if the grade is other than these five categories (i.e. A, B,</div> <div> C, D and F). To handle all the possibilities of grade input, we write a default</div> <div> statement after the last case. The statement in this default case is executed if no case</div> <div> matches the grade. So in our program, we can write the default statement after the</div> <div> last case as under.</div> <div> default : cout << “Please enter grade from A to D or F ” ;</div> <div> The break statement is also used in decision structures other than switch structure. We</div> <div> have seen that in while, do-while and for loops, we have to violate some condition</div> <div> explicitly to terminate the loop before its complete repetitions. As in a program of</div> <div> guessing a character, we make a variable tryNum greater than 5 to violate the while</div> <div> condition and exit the loop if the correct character is guessed before five tries. In these</div> <div> loops, we can use the break statement to exit a loop. When a break statement is</div> <div> encountered in a loop, the loop terminates immediately. The control exits the inner</div> <div> most loop if there are nested loops. The control passes to the statement after the loop.</div> <div> In the guessing character example, we want that if the character is guessed in first or</div> <div> second attempt,. then we print the message ‘Congratulations, You guess is correct’</div> <div> and exit the loop. We can do this by using a break statement with an if statement. If</div> <div> the character is guessed, we print the message. Afterwards, the break statement is</div> <div> executed and the loop terminates. So we can write this as follows.</div> <div> if ( c == ‘z’ ) // c is input from user</div> <div> {</div> <div> cout << “Great, Your guess is correct” ;</div> <div> break;</div> <div> }</div> <div> Thus, break statement can be used to jump out of a loop very quickly.</div> <div> The flow chart of the switch statement is similar to if statement and is given below.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 70</div> <div> switch</div> <div> (variable )</div> <div> Process</div> <div> Process</div> <div> case const 1</div> <div> case const2</div> <div> break</div> <div> break</div> <div> The flow chart of switch statement</div> <div> The number of case statement can vary from</div> <div> 1 to any number. Thus there are same</div> <div> number of process blocks as cases.</div> <div> Now we can write the complete code for the program that prints the description of the</div> <div> grade entered by the user.</div> <div> The flow chart of the program is given below.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 71</div> <div> Start</div> <div> switch (grade)</div> <div> Stop</div> <div> Display "Excellent"</div> <div> case 'A'</div> <div> break</div> <div> Display</div> <div> "Very Good"</div> <div> case 'B'</div> <div> break</div> <div> Display "Good"</div> <div> case 'C'</div> <div> break</div> <div> Display "Poor"</div> <div> case 'D'</div> <div> break</div> <div> Display "Fail"</div> <div> case 'F'</div> <div> break</div> <div> Display " Please</div> <div> enter grade A-D or</div> <div> F"</div> <div> default</div> <div> break</div> <div> The flow chart of a program that displays the description of a grade</div> <div> using switch statement</div> <div> The code of the program is given below.</div> <div> //This program gets a grade from user and displays a description accordingly</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 72</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> char grade ;</div> <div> cout << “Please enter the student’s grade : ” ;</div> <div> cin >> grade ;</div> <div> switch ( grade )</div> <div> {</div> <div> case ‘A’ : // grade was upper case A</div> <div> case ‘a’ : // grade was lower case a</div> <div> cout << “Excellent” ;</div> <div> break : // necessary to exit switch</div> <div> case ‘B’ : // grade was upper case B</div> <div> case ‘b’ : // grade was lower case b</div> <div> cout << “Very Good” ;</div> <div> break : // necessary to exit switch</div> <div> case ‘C’ : // grade was upper case C</div> <div> case ‘c’ : // grade was lower case c</div> <div> cout << “Good” ;</div> <div> break : // necessary to exit switch</div> <div> case ‘D’ : // grade was upper case D</div> <div> case ‘d’ : // grade was lower case d</div> <div> cout << “Poor” ;</div> <div> break : // necessary to exit switch</div> <div> case ‘F’ : // grade was upper case F</div> <div> case ‘f’ : // grade was lower case f</div> <div> cout << “Fail” ;</div> <div> break : // necessary to exit switch</div> <div> default :</div> <div> cout << “Please enter grade from A to D or F ” ;</div> <div> }</div> <div> }</div> <div> A sample out put of the program is shown here.</div> <div> Please enter the student’s grade : b</div> <div> Very Good</div> <div> continue Statement</div> <div> There is another statement relating to loops. This is the continue statement.</div> <div> Sometimes we have a lot of code in the body of a loop. The early part of this code is</div> <div> common that is to be executed every time (i.e. in every iteration of loop) and the</div> <div> remaining portion is to be executed in certain cases and may not be executed in other</div> <div> cases. But the loop should be continuous. For this purpose, we use the continue</div> <div> statement. Like the break statement, the continue statement is written in a single line.</div> <div> We write it as</div> <div> continue ;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 73</div> <div> The continue forces the immediate next iteration of the loop. So the statements of the</div> <div> loop body after continue are not executed. The loop starts from the next iteration</div> <div> when a continue statement is encountered in the body of a loop. One can witness very</div> <div> subtle things while using continue.</div> <div> Consider the while loop. In while loop, we change the value of the variable of while</div> <div> condition so that it could make the condition false to exit the loop. Otherwise, the</div> <div> loop will become an infinite one. We should be very careful about the logic of the</div> <div> program while using continue in a loop. Before the continue statement, it is necessary</div> <div> to change (increment/decrement) the value of the variable on which the while</div> <div> condition depends. Similarly it is same with the do-while loop. Be careful to</div> <div> increment or decrement the conditional variable before the continue statement.</div> <div> In for loop, there is a difference. In a while loop when continue is encountered, the</div> <div> control goes to the while statement and the condition is checked. If condition is true</div> <div> the loop is executed again else the loop exits. In a for loop, the three things i.e.</div> <div> initialization, condition and increment/decrement are enclosed together as we write</div> <div> for ( counter = 0 ; counter <= 5 ; counter ++) . In the for loop when a continue is</div> <div> encountered, the counter (i.e. loop variable) is incremented at first before the</div> <div> execution of the loop condition. Thus, in 'for loop' the increment to the loop variable</div> <div> is built in and after continue the next iteration of the loop is executed by incrementing</div> <div> the loop variable. The condition is checked with the incremented value of the loop</div> <div> variable. In while and do-while loop, it is our responsibility to increment the value of</div> <div> the loop variable to test the condition. In a for loop, the continue automatically forces</div> <div> this increment of value before going to check the condition.</div> <div> goto Statement</div> <div> Up to now we have covered the basic programming constructs. These include</div> <div> sequences, decisions and repetition structures (i.e. loops). In sequences, we use the</div> <div> simple statements in a sequence i.e. one after the other. In decisions construct we use</div> <div> the if statement, if/else statement, the multi way decision construct (i.e. the switch</div> <div> statement). And in repetition structures, we use the while, do-while and for loops.</div> <div> Sometime ago, two computer scientists Gome and Jacopi proved that any program</div> <div> can be written with the help of these three constructs (i.e. sequences, decisions and</div> <div> loops).</div> <div> There is a statement in the computer languages COBOL, FORTRON and C. This</div> <div> statement is goto statement. The goto is an unconditional branch of execution. The</div> <div> goto statement is used to jump the control anywhere (back and forth) in a program. In</div> <div> legacy programming, the programs written in COBOL and FORTRAN languages</div> <div> have many unconditional branches of execution. To understand and decode such</div> <div> programs that contain unconditional branches is almost impossible. In such programs,</div> <div> it is very difficult, for a programmer, to keep the track of execution as the control</div> <div> jumps from one place to the other and from there to anywhere else. We call this kind</div> <div> of traditional code as spagatti code. It is very difficult to trace out the way of</div> <div> execution and figure out what the program is doing. And debugging and modifying</div> <div> such programs is very difficult.</div> <div> When structured programming was started, it was urged not to use the goto statement.</div> <div> Though goto is there in C language but we will not use it in our programs. We will</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 74</div> <div> adopt the structured approach. All of our programs will consist of sequences,</div> <div> decisions and loops.</div> <div> Guide Lines</div> <div> In general, we should minimize the use of break statement in loops. The switch</div> <div> statement is an exception in this regard where it is necessary to use the break</div> <div> statement after every case. Otherwise, there may be a logical error. While writing</div> <div> loops, we should try to execute the loops with the condition test and should try to</div> <div> avoid the break statement. The same applies to the continue statement. The continue</div> <div> statement executes some statements of the loop and then exits the loop without</div> <div> executing some statements after it. We can use the if statement for this purpose</div> <div> instead of continue. So never use the goto statement and minimize the usage of break</div> <div> and continue statements in loops. This will make the code easy to understand for you</div> <div> and for others. Moreover the additions and modifications to such code will be easy, as</div> <div> the path of execution will be easy to trace.</div> <div> Make a program modular. This means that divide a large program into small parts. It</div> <div> will be easy to manage these small parts rather than a larger program. There should be</div> <div> single entry and single exit in every module or construct. The use of break statement</div> <div> in a construct violates this rule as a loop having a break statement can exit through</div> <div> break statement or can terminate when the loop condition violates. As there are two</div> <div> exit points, this should be avoided. The single entry- single exit approach makes the</div> <div> execution flow simple.</div> <div> Here is an example from daily life, which shows that single entry and single exit</div> <div> makes things easy. You would have often seen at a bus stop, especially in rush hours,</div> <div> that when a bus reaches the stop, everyone tries to jump into the bus without caring</div> <div> for others. The passengers inside the bus try to get down from the vehicle. So you see</div> <div> there a wrestling like situation at the door of the bus. Separate doors for entering or</div> <div> exiting the bus can be the solution. In this way, the passengers will easily enter or exit</div> <div> the bus.</div> <div> We have applied this single entry and single exit rule in drawing our flow charts. In</div> <div> the flow charts, we draw a vertical line from top to down. The point where the line</div> <div> starts is our entry point and downward at the same line at the end is our exit point.</div> <div> Our all other processes and loops are along or within these two points. Thus our flow</div> <div> charts resemble with the code.</div> <div> Rules for Structured Programming/Flow Charting</div> <div> There are few simple rules for drawing structured flow charts of programs. One</div> <div> should be familiar with these.</div> <div> Rule No:1-Start with the simple flow chart. This means that draw a start symbol,</div> <div> draw a rectangle and write in it whatsoever you want to do and then draw a stop</div> <div> symbol. This is the simplest flow chart.</div> <div> Rule No:2- Any rectangle ( a rectangle represents a process which could be input,</div> <div> output or any other process) can be replaced by two rectangles.</div> <div> This concept is the same as taking a complex problem and splitting it up into two</div> <div> simpler problems. So we have ‘split it up’ method to move towards a modular</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 75</div> <div> approach. So start with a block (rectangle) and then any rectangle can be replaced by</div> <div> two rectangles (blocks).</div> <div> Rule No:3- Any rectangle can be replaced with a structured flow charting construct.</div> <div> These construct include decisions, loops or multi- way decision. This means that we</div> <div> can put a structure of an if construct or switch construct in the place of a rectangle.</div> <div> Here we come to know the advantage of single entry and single exit concept. This</div> <div> single entry and single exit block can be replaced with a rectangle.</div> <div> Rule No: 4- This rule states that rule number 2 and 3 can be repeated as many times</div> <div> as you want.</div> <div> By using these rules we are splitting a problem into simpler units so that each part can</div> <div> be handled either by sequences (one rectangle, second rectangle and so on) or by a</div> <div> decision (if, if/else, switch or by a loop). Through this approach, a large problem can</div> <div> be solved easily.</div> <div> The flow charts drawn with these rules and indented to the left side will have one to</div> <div> one correspondence with our code. Thus it becomes very easy to identify the code that</div> <div> is written for a specific part of the flow chart. In this way the code can easily be</div> <div> debugged.</div> <div> Sample Program</div> <div> Let’s consider a problem. In a company, there are deductions from the salary of the</div> <div> employees for a fund. The deductions rules are as follows:</div> <div> If salary is less than 10,000 then no deduction</div> <div> If salary is more than 10,000 and less than 20,000 then deduct Rs. 1,000 as fund</div> <div> If salary is equal to or more than 20,000 then deduct 7 % of the salary for fund</div> <div> Take salary input from user and after appropriate deduction show the net payable</div> <div> amount.</div> <div> Solution</div> <div> As we see that there is multi way decision in this problem, so we use switch</div> <div> statement. The salary is the switch variable upon which the different decisions</div> <div> depend. We can use only a single constant in case statement. So we divide the salary</div> <div> by 10000 to convert it into a single case constant. As we know that in integer division</div> <div> we get the whole number as the answer. Thus if answer is 0 the salary is less than</div> <div> 10000, if answer is 1 then it is in range 10000 to 19999 ( as any amount between</div> <div> 10000 – 19999 divided by 10000 will result 1). If the answer is greater than 1, it</div> <div> means the salary is equal to or more than 20000.</div> <div> Following is the complete code of our program.</div> <div> // This program gets salary input from user and calculates and displays the net payable</div> <div> // amount after deduction according the conditions</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> int salary ;</div> <div> float deduction, netPayable ;</div> <div> cout << “Please enter the salary : “ ;</div> <div> cin >> salary ;</div> <div> // here begins the switch statement</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 76</div> <div> switch ( salary / 10000 ) // this will produce a single value</div> <div> {</div> <div> case 0 : // this means salary is less than 10,000</div> <div> deduction = 0; // as deduction is zero in this case</div> <div> netPayable = salary ;</div> <div> cout << “Net Payable (salary – deduction) = “ ;</div> <div> cout << salary << “ - ” << deduction << “ = “ << netPayable;</div> <div> break; //necessary to exit switch</div> <div> case 1 : // this means salary is in range 10,000 – 19,999</div> <div> deduction = 1000 ;</div> <div> netPayable = salary – deduction ;</div> <div> cout << “Net Payable (salary – deduction) = “ ;</div> <div> cout << salary << “ - ” << deduction << “ = “ << netPayable;</div> <div> break; //necessary to exit switch</div> <div> default : // this means the salary is 20,000 or more</div> <div> deduction = salary * 7 /100 ;</div> <div> netPayable = salary – deduction ;</div> <div> cout << “Net Payable (salary – deduction) = “ ;</div> <div> cout << salary << “ - ” << deduction << “ = “ << netPayable;</div> <div> }</div> <div> }</div> <div> Here is the out put of the program.</div> <div> Please enter the salary : 15000</div> <div> Net Payable (salary – deduction) = 15000 – 1000 = 14000</div> <div> Tips</div> <div> Try to use the switch statement instead of multiple if statements</div> <div> Missing a break statement in a switch statement may cause a logical error</div> <div> Always provide a default case in switch statements</div> <div> Never use goto statement in your programs</div> <div> Minimize the use of break and continue statements</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 77</div> <div> Lecture No. 9</div> </div> <div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program chapter 2</div> <div> 3.1, 3.2, 3.3, 3.4,</div> <div> 3.5, 3.6</div> <div> Summary</div> <div> o Introduction</div> <div> o Functions</div> <div> o Structure of a Function</div> <div> o Declaration and Definition of a Function</div> <div> o Sample Program 1</div> <div> o Sample Program 2</div> <div> o Sample Program 3</div> <div> o Summary</div> <div> o Tips</div> <div> Introduction</div> <div> Now our toolkit is almost complete. The basic constructs of programming are</div> <div> sequence, decision making and loops. You have learnt all these techniques. Now we</div> <div> can write almost all kinds of programs. There are more techniques to further refine the</div> <div> programs. One of the major programming constructs is Functions. C is a functionoriented</div> <div> language. Every program is written in different functions.</div> <div> In our daily life, we divide our tasks into sub tasks. Consider the making of a</div> <div> laboratory stool.</div> <div> It has a seat and three legs. Now we need to make a seat and three legs out of wood.</div> <div> The major task is to make a stool. Sub tasks are, make a seat and then fabricate three</div> <div> legs. The legs should be identical. We can fashion one leg and then re-using this</div> <div> prototype, we have to build two more identical legs. The last task is to assemble all</div> <div> these to make a stool. We have a slightly difficult task and have broken down it into</div> <div> simpler pieces. This is the concept of functional design or top-down designing. In top</div> <div> design, we look at the problem from top i.e. identification of the problem. What we</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 78</div> <div> have to solve? Then refine it and divide it into smaller pieces. We refine it again and</div> <div> divide it into smaller pieces. We keep on doing it as long as we get easily manageable</div> <div> task. Let's consider an example like home construction. From the top level, we have to</div> <div> construct a home. Then we say that we need design of the home according to which</div> <div> the building will be constructed. We need to construct rooms. How can we construct a</div> <div> room? We need bricks, cement, doors, windows etc. Procurement of all of these</div> <div> things is tasks. Once we come down to the level where a task is easily manageable</div> <div> and doable, we stop doing further refinement. When we break up a task into smaller</div> <div> sub tasks, we stop at a reasonable level. Top-down designing mechanism is based on</div> <div> the principle of 'divide and conquer' i.e. we divide a big task into smaller tasks and</div> <div> then accomplish them.</div> <div> Let's have a look at a simple example to understand the process of dividing big task</div> <div> into simple ones. Suppose we want to know how many students are currently logged</div> <div> in the LMS (Learning Management System) of VU. This task will be handed over to</div> <div> the network administrator to find out the number of students currently logged in LMS</div> <div> of the university. The network administrator will check the network activity or get this</div> <div> information from the database and get the list of students currently logged in. The</div> <div> number of students is counted from that list and the result is given back to us. What</div> <div> has happened in this whole process? There was a simple request to find the number of</div> <div> students currently logged in LMS. This request is delegated to the network</div> <div> administrator. The network administrator performs this task and we get the result. In</div> <div> the mean time, we can do some other task as we are not interested in the names or list</div> <div> of students. We only want the number of students. This technique is known as parallel</div> <div> processing. In terms of programming, network administrator has performed a function</div> <div> i.e. calculation of the number of students. During this process, the network</div> <div> administrator also gets the list of students which is hidden from us. So the information</div> <div> hiding is also a part of the function. Some information is given to the network</div> <div> administrator (i.e. the request to calculate the number of students currently logged in</div> <div> the LMS) while some information is provided back to us (i.e. the number of students).</div> <div> Functions</div> <div> The functions are like subtasks. They receive some information, do some process and</div> <div> provide a result. Functions are invoked through a calling program. Calling program</div> <div> does not need to know what the function is doing and how it is performing its task.</div> <div> There is a specific function-calling methodology. The calling program calls a function</div> <div> by giving it some information and receives the result.</div> <div> We have a main ( ) in every C program. ‘main ( )’ is also a function. When we write a</div> <div> function, it must start with a name, parentheses, and surrounding braces just like with</div> <div> main ( ). Functions are very important in code reusing.</div> <div> There are two categories of functions:</div> <div> 1. Functions that return a value</div> <div> 2. Functions that do not return a value</div> <div> Suppose, we have a function that calculates the square of an integer such that function</div> <div> will return the square of the integer. Similarly we may have a function which displays</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 79</div> <div> some information on the screen so this function is not supposed to return any value to</div> <div> the calling program.</div> <div> Structure of a Function</div> <div> The declaration syntax of a function is as follows:</div> <div> return-value-type function-name( argument-list )</div> <div> {</div> <div> declarations and statements</div> <div> }</div> <div> The first line is the function header and the declaration and statement part is the body</div> <div> of the function.</div> <div> return-value_type:</div> <div> Function may or may not return a value. If a function returns a value, that must be of a</div> <div> valid data type. This can only be one data type that means if a function returns an int</div> <div> data type than it can only return int and not char or float. Return type may be int,</div> <div> float, char or any other valid data type. How can we return some value from a</div> <div> function? The keyword is return which is used to return some value from the</div> <div> function. It does two things, returns some value to the calling program and also exits</div> <div> from the function. We can only return a value (a variable or an expression which</div> <div> evaluates to some value) from a function. The data type of the returning variable</div> <div> should match return_value_type data type.</div> <div> There may be some functions which do not return any value. For such functions, the</div> <div> return_value_type is void. ‘void’ is a keyword of ‘C’ language. The default</div> <div> return_value_type is of int data type i.e. if we do not mention any return_value_type</div> <div> with a function, it will return an int value.</div> <div> Function-name:</div> <div> The same rules of variable naming conventions are applied to functions name.</div> <div> Function name should be self-explanatory like square, squareRoot, circleArea etc.</div> <div> argument-list:</div> <div> Argument list contains the information which we pass to the function. Some function</div> <div> does not need any information to perform the task. In this case, the argument list for</div> <div> such functions will be empty. Arguments to a function are of valid data type like int</div> <div> number, double radius etc.</div> <div> Declarations and Statements:</div> <div> This is the body of the function. It consists of declarations and statements. The task of</div> <div> the function is performed in the body of the function.</div> <div> Example:</div> <div> //This function calculates the square of a number and returns it.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 80</div> <div> int square(int number)</div> <div> {</div> <div> int result = 0;</div> <div> result = number * number;</div> <div> return result;</div> <div> }</div> <div> Calling Mechanism:</div> <div> How a program can use a function? It is very simple. The calling program just needs</div> <div> to write the function name and provide its arguments (without data types). It is</div> <div> important to note that while calling a function, we don’t write the return value data</div> <div> type or the data types of arguments.</div> <div> Example:</div> <div> //This program calculates the square of a given number</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> int number, result;</div> <div> result = 0;</div> <div> number = 0;</div> <div> // Getting the input from the user</div> <div> cout << “ Please enter the number to calculate the square ”;</div> <div> cin >> number;</div> <div> // Calling the function square(int number)</div> <div> result = square(number);</div> <div> cout << “ The square of “ << number << “ is “ << result;</div> <div> }</div> <div> Declaration and Definition of a Function</div> <div> Declaration and definition are two different things. Declaration is the prototype of the</div> <div> function, that includes the return type, name and argument list to the function and</div> <div> definition is the actual function code. Declaration of a function is also known as</div> <div> signature of a function.</div> <div> As we declare a variable like int x; before using it in our program, similarly we need</div> <div> to declare function before using it. Declaration and definition of a function can be</div> <div> combined together if we write the complete function before the calling functions.</div> <div> Then we don’t need to declare it explicitly. If we have written all of our functions in a</div> <div> different file and we call these functions from main( ) which is written in a different</div> <div> file. In this case, the main( ) will not be compiled unless it knows about the functions</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 81</div> <div> declaration. Therefore we write the declaration of functions before the main( )</div> <div> function. Function declaration is a one line statement in which we write the return</div> <div> type, name of the function and the data type of arguments. Name of the arguments is</div> <div> not necessary. The definition of the function contains the complete code of the</div> <div> function. It starts with the declaration statement with the addition that in definition,</div> <div> we do write the names of the arguments. After this, we write an opening brace and</div> <div> then all the statements, followed by a closing brace.</div> <div> Example:</div> <div> If the function square is defined in a separate file or after the calling function, then we</div> <div> need to declare it:</div> <div> Declaration:</div> <div> int square ( int );</div> <div> Definition:</div> <div> int square ( int number)</div> <div> {</div> <div> return (number * number ) ;</div> <div> }</div> <div> Here is the complete code of the program:</div> <div> //This program calculates the square of a given number</div> <div> #include <iostream.h></div> <div> // Function declarations.</div> <div> int square(int);</div> <div> main()</div> <div> {</div> <div> int number, result;</div> <div> result = 0;</div> <div> number = 0;</div> <div> cout << “ Please enter the number to calculate the square ”;</div> <div> cin >> number;</div> <div> // Calling the function square(int number)</div> <div> result = square(number);</div> <div> cout << “ The square of “ << number << “ is “ << result;</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 82</div> <div> // function to calculate the square of a number</div> <div> int square ( int number)</div> <div> {</div> <div> return (number * number ) ;</div> <div> }</div> <div> A function in a calling program can take place as a stand-alone statement, on righthand</div> <div> side of a statement. This can be a part of an assignment expression.</div> <div> Considering the above example, here are some more ways of function calling</div> <div> mechanism.</div> <div> result = 10 + square (5);</div> <div> or</div> <div> result = square (number + 10);</div> <div> or</div> <div> result = square (number) + square (number + 1) + square (3 * number);</div> <div> or</div> <div> cout << “ The square of “ << number << “ is “ << square (number);</div> <div> In the above statements, we see that functions are used in assignment statements. In a</div> <div> statement result = square(5); The square(5) function is called and the value which is</div> <div> returned from that function (i.e. the value returned within the function using the</div> <div> return keyword) is assigned to the variable result. In this case, the square(5) will</div> <div> return 25, which will be assigned to variable result. There may be functions which do</div> <div> not return any value. These functions can't be used in assignment statements. These</div> <div> functions are written as stand-alone statements.</div> <div> Sample Program 1</div> <div> C is called function-oriented language. It is a very small language but there are lots of</div> <div> functions in it. Function can be on a single line, a page or as complex as we want.</div> <div> Problem statement:</div> <div> Calculate the integer power of some number (xn).</div> <div> Solution:</div> <div> We want to get the power of some number. There is no operator for power function in</div> <div> C. We need to write a function to calculate the power of x to n (i.e. xn). How can we</div> <div> calculate the power of some number? To get the power of some number x to n, we</div> <div> need to multiply x with x up to n times. Now what will be the input (arguments) to the</div> <div> function? A number and power, as number can be a real number so we have to declare</div> <div> number as a double date type and the power is an integer value so we will declare the</div> <div> power as an integer. The power is an integer value so we will declare power as an</div> <div> integer. The result will also be a real number so the return value type will be of double</div> <div> data type. The function name should be descriptive, we can name this function as</div> <div> raiseToPow. The declaration of the function is:</div> <div> double raiseToPow ( double x, int power ) ;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 83</div> <div> To calculate the power of x up to power times, we need a loop which will be executed</div> <div> power times. The definition of function is:</div> <div> // function to calculate the power of some number</div> <div> double raiseToPow ( double x , int power )</div> <div> {</div> <div> double result ;</div> <div> int i ;</div> <div> result = 1.0 ;</div> <div> for ( i = 1 ; i <= power ; i ++ )</div> <div> {</div> <div> result *= x ; // same as result = result * x</div> <div> }</div> <div> return ( result ) ;</div> <div> }</div> <div> Here is the program which is calling the above function.</div> <div> // This program is calling a function raiseToPow.</div> <div> #include <iostream.h></div> <div> //Function declaration</div> <div> double raiseToPow ( double , int )</div> <div> main ( )</div> <div> {</div> <div> double x ;</div> <div> int i ;</div> <div> cout << “ Please enter the number “ ;</div> <div> cin >> x ;</div> <div> cout << “ Please enter the integer power that you want this number raised to “ ;</div> <div> cin >> i ;</div> <div> cout << x << “ raise to power “ << i << “ is equal to “ << raiseToPow ( x , i ) ;</div> <div> }</div> <div> Now we have to consider what will happen to the values of arguments that are passed</div> <div> to the function? As in the above program, we are passing x and i to the raiseToPow</div> <div> function. Actually nothing is happening to the values of x and i. These values are</div> <div> unchanged. A copy of values x and i are passed to the function and the values in the</div> <div> calling program are unchanged. Such function calls are known as 'call by value'.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 84</div> <div> There is another way to call a function in which the function can change the values of</div> <div> variables that are passed as arguments, of calling program. Such function call is</div> <div> known as call by reference.</div> <div> Sample Program 2</div> <div> Problem statement:</div> <div> Calculate the area of a ring.</div> <div> Solution:</div> <div> We know that a ring consists of a small circle and a big circle. To calculate the area of</div> <div> a ring, we have to subtract the area of small circle from the area of big circle. Area of</div> <div> any circle is calculated as Pi * r2. We write a function to calculate the area of a circle</div> <div> and use this function to calculate the area of small circle and big circle.</div> <div> Following is the code of the function circleArea:</div> <div> // Definition of the circleArea function.</div> <div> double circleArea ( double radius )</div> <div> {</div> <div> // the value of Pi = 3.1415926</div> <div> return ( 3.1415926 * radius * radius ) ;</div> <div> }</div> <div> Here is the complete code of the calling program.</div> <div> // This program calculates the area of a ring</div> <div> #include <iostream.h></div> <div> // function declaration.</div> <div> double circleArea ( double);</div> <div> void main ( )</div> <div> {</div> <div> double rad1 ;</div> <div> double rad2 ;</div> <div> double ringArea ;</div> <div> cout << “ Please enter the outer radius value: ” ;</div> <div> cin >> rad1 ;</div> <div> cout << “ Please enter the radius of the inner circle: “ ;</div> <div> cin >> rad2 ;</div> <div> ringArea = circleArea ( rad1 ) – circleArea (rad2 ) ;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 85</div> <div> cout<< “ Area of the ring having inner raduis “ << rad2 << “ and the outer radius “ <<</div> <div> rad1 << “ is “ << ringArea ;</div> <div> }</div> <div> double circleArea ( double radius )</div> <div> {</div> <div> // the value of Pi = 3.1415926</div> <div> return ( 3.1415926 * radius * radius ) ;</div> <div> }</div> <div> Sample Program 3</div> <div> There are some other kinds of functions which are used to test some condition. Such</div> <div> functions return true or false. These functions are very important and used a lot in</div> <div> programming. In C condition statements, the value zero (0) is considered as false and</div> <div> any value other than zero is considered as true. So the return type of such functions is</div> <div> int. We usually return 1 when we want the function to return true and return 0 when</div> <div> we want the function to return 0. Here is a sample program to elaborate this.</div> <div> Problem statement:</div> <div> Write a function which tests that a given number is even or not? It should return true</div> <div> if the number is even, otherwise return false.</div> <div> Solution:</div> <div> We already know the method of deciding whether a number is even or not. The name</div> <div> of the function is isEven. Its return type will be int. It will take an int as an argument.</div> <div> So the declaration of the function should be as below;</div> <div> int isEven ( int ) ;</div> <div> We can also use a function in the conditional statements like:</div> <div> if ( isEven ( number ) )</div> <div> If the number is even, the function will return none zero value (i.e. usually 1) and the</div> <div> if statement will be evaluated as true. However, if the number is odd, the function will</div> <div> return a zero value and the if statement is evaluated as false.</div> <div> Here is a complete program.</div> <div> // This program is calling a function to test the given number is even or not</div> <div> #include <iostream.h></div> <div> // function declaration.</div> <div> int isEven(int);</div> <div> void main ( )</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 86</div> <div> {</div> <div> int number;</div> <div> cout << " Please enter the number: " ;</div> <div> cin >> number ;</div> <div> if ( isEven ( number ) )</div> <div> {</div> <div> cout << " The number entered is even " << endl;</div> <div> }</div> <div> else</div> <div> {</div> <div> cout << " The number entered is odd " << endl;</div> <div> }</div> <div> }</div> <div> int isEven ( int number )</div> <div> {</div> <div> if ( 2 * ( number / 2 ) == number )</div> <div> {</div> <div> return 1;</div> <div> }</div> <div> else</div> <div> {</div> <div> return 0;</div> <div> }</div> <div> }</div> <div> Summary</div> <div> Functions are very good tools for code reuse. We have seen in the above example that</div> <div> the area of two circles has been calculated without rewriting the code. This means that</div> <div> the code has been reused. We can reuse the circleArea function to find the area of any</div> <div> circle.</div> <div> A function performs a specific task. Functions also provide encapsulation. The calling</div> <div> program does not know how the function is performing its task. So we can build up</div> <div> modular form from small building blocks and build up more and more complex</div> <div> programs.</div> <div> If we are going to use a function in our program and the definition of the function is</div> <div> after the calling program. The calling program needs to know how to call the function,</div> <div> what the arguments are and what it will return. So its declaration must occur before</div> <div> usage. If we do not declare a function before using, the compiler will give an error. If</div> <div> we define a function before the calling program, then we do not need a separate</div> <div> declaration. The function declaration is also known as function prototype or function</div> <div> signature. Whenever, we need to build something, first of all we build a prototype of</div> <div> that thing and then later on we build it. Similarly the function declaration is used as a</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 87</div> <div> prototype. We are following the top- down methodology. We break the program into</div> <div> smaller modules and just declare the functions and later on we can define these.</div> <div> Exercise:</div> <div> 1. Modify the raise to power function so that it can handle negative power of x,</div> <div> zero and positive power of x.</div> <div> 2. Modify the area of ring function put in error checking mechanism.</div> <div> Tips</div> <div> • We used functions for breaking complex problems into smaller pieces,</div> <div> which is a top-down structured approach.</div> <div> • Each function should be a small module, self-contained. It should solve a well</div> <div> defined problem.</div> <div> • Variable names and function names should be self- explanatory.</div> <div> • Always comment the code.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 88</div> <div> Lecture No. 10</div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program Chapter 3</div> <div> 3.73.11, 3.12, 3.14, 3.17</div> <div> Contents</div> <div> • Header Files</div> <div> • Scope of Identifiers</div> <div> • Functions</div> <div> - Call by Value</div> <div> - Call by Reference</div> <div> Header Files</div> <div> You have already been using a header file from day-zero. You know that we used to</div> <div> write at the top before the start of the main() function <iostream.h>, with ‘.h’ as an</div> <div> extension, you might have got the idea that it is a header file.</div> <div> Now we will see why a Header file is used.</div> <div> In the previous lecture, we discussed a little bit about Function Prototypes. One thing</div> <div> is Declaration and other is Definition. Declaration can also be called as 'Prototype'.</div> <div> Normally, if we have lot of functions and want to use them in some other function or</div> <div> program, then we are left with only one way i.e. to list the prototypes of all of them</div> <div> before the body of the function or program and then use them inside the function or</div> <div> program. But for frequent functions inside a program, this technique increases the</div> <div> complexity (of a program). This problem can be overcome by putting all these</div> <div> function prototypes in one file and writing a simple line of code for including the file</div> <div> in the program. This code line will indicate that this is the file, suppose 'area.h'</div> <div> containing all the prototypes of the used functions and see the prototypes from that</div> <div> file. This is the basic concept of a header file.</div> <div> So what we can do is:</div> <div> Make our own header file which is usually a simple text file with '.h' extension ('.h'</div> <div> extension is not mandatory but it is a rule of good programming practice).</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 89</div> <div> Write function prototypes inside that file. (Recall that prototype is just a simple line of</div> <div> code containing return value, function name and an argument list of data types with</div> <div> semi-colon at the end.)</div> <div> That file can be included in your own program by using the ‘#include’ directive and</div> <div> that would be similar to explicitly writing that list of function prototypes.</div> <div> Function prototypes are not the only thing that can be put into a header file. If you</div> <div> remember that we wrote a program for calculating Area of a Circle in our previous</div> <div> lectures. We used the value of 'pi' inside that and we have written the value of 'pi' as</div> <div> 3.1415926. This kind of facts are considered as Universal Constants or Constants</div> <div> within our domain of operations . It would be nice, if we can assign meaningful</div> <div> names to them. There are two benefits of doing this. See, We could have declared a</div> <div> variable of type double inside the program and given a name like 'pi':</div> <div> double pi = 3.1415926;</div> <div> Then everywhere in the subsequent calculations we can use 'pi'.</div> <div> But it is better to pre-define the value of the constant in a header file ( one set for all)</div> <div> and simply including that header file, the constant ‘pi’, is defined. Now, this</div> <div> meaningful name ‘pi’ can be used in all calculations instead of writing the horrendous</div> <div> number 3.1415926 again and again.</div> <div> There are some preprocessor directives which we are going to cover later. At the</div> <div> moment, we will discuss about ‘#define’ only. We define the constants using this</div> <div> preprocessor directive as:</div> <div> #define pi 3.1415926</div> <div> The above line does a funny thing as it is not creating a variable. Rather it associates a</div> <div> name with a value which can be used inside the program exactly like a variable. (Why</div> <div> it is not a variable?, because you can’t use it on the left hand side of any assignment.).</div> <div> Basically, it is a short hand, what actually happens. You defined the value of the ‘pi’</div> <div> with ‘#define’ directive and then started using ‘pi’ symbol in your program. Now we</div> <div> will see what a compiler does when it is handed over the program after the writing</div> <div> process. Wherever it finds the symbol ‘pi’, replaces the symbol with the value</div> <div> 3.1415926 and finally compiles the program.</div> <div> Thus, in compilation process the symbols or constants are replaced with actual values</div> <div> of them. But for us as human beings, it is quite readable to see the symbol ‘pi’.</div> <div> Additionally, if we use meaningful names for variables and see a line ‘2 * pi * radius’,</div> <div> it becomes obvious that circumference of a circle is being calculated. Note that in the</div> <div> above statement, ‘2 * pi * radius’; 2 is used as a number as we did not define any</div> <div> constant for it. We have defined ‘pi’ and ‘radius’ but defining 2 would be over killing.</div> <div> Scope of Identifiers</div> <div> An 'Identifier' means any name that the user creates in his/her program. These names</div> <div> can be of variables, functions and labels. Here the scope of an identifier means its</div> <div> visibility. We will focus Scope of Variables in our discussion.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 90</div> <div> Suppose we write the function:</div> <div> void func1()</div> <div> {</div> <div> int i;</div> <div> . . . //Some other lines of code</div> <div> int j = i+2; //Perfectly alright</div> <div> . . .</div> <div> }</div> <div> Now this variable ‘i’ can be used in any statement inside the function func1(). But</div> <div> consider this variable being used in a different function like:</div> <div> void func2()</div> <div> {</div> <div> int k = i + 4; //Compilation error</div> <div> . . .</div> <div> }</div> <div> The variable ‘i’ belongs to func1() and is not visible outside that. In other words, ‘i’</div> <div> is local to func1().</div> <div> To understand the concept of scope further, we have to see what are Code Blocks? A</div> <div> code block begins with ‘{‘ and ends with ‘}’.Therefore, the body of a function is</div> <div> essentially a code block. Nonetheless, inside a function there can be another block of</div> <div> code like 'for loop' and 'while loop' can have their own blocks of code respectively.</div> <div> Therefore, there can be a hierarchy of code blocks.</div> <div> A variable declared inside a code block becomes the local variable for that for that</div> <div> block. It is not visible outside that block. See the code below:</div> <div> void func()</div> <div> {</div> <div> int outer; //Function level scope</div> <div> . . .</div> <div> {</div> <div> int inner; //Code block level scope</div> <div> inner = outer; //No problem</div> <div> . . .</div> <div> }</div> <div> inner ++; //Compilation error</div> <div> }</div> <div> Please note that variable ‘outer’ is declared at function level scope and variable</div> <div> ‘inner’ is declared at block level scope.</div> <div> The ‘inner’ variable declared inside the inner code block is not visible outside it . In</div> <div> other words, it is at inner code block scope level. If we want to access that variable</div> <div> outside its code block, a compilation error may occur.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 91</div> <div> What will happen if we use the same names of variables at both function level scope</div> <div> and inner block level scope? Consider the following code:</div> <div> Line</div> <div> 1. void increment()</div> <div> 2. {</div> <div> 3. int num; //Function level scope</div> <div> 4. . . .</div> <div> 5. {</div> <div> 6. int num; //Bad practice, not recommended</div> <div> 7. . . .</div> <div> 8. num ++; //inner num is incremented</div> <div> 9. . . .</div> <div> 10. }</div> <div> 11. }</div> <div> Note that there is no compilation error if the variable of the same name ‘num’ is</div> <div> declared at line 6 inside the inner code block (at block level scope). Although , there</div> <div> is no error in naming the variables this way, yet this is not recommended as this can</div> <div> create confusion and decrease readability. It is better to use different names for these</div> <div> variables.</div> <div> Which variable is being used at line 8? The answer is the ‘num’ variable declared for</div> <div> inner code block (at block level scope). Why is so? It is just due to the fact that the</div> <div> outer variable ‘num’ (at function level scope) is hidden in the inner code block as</div> <div> there is a local variable of the same name. So the local variable ‘num’ inside the inner</div> <div> code block over-rides the variable ‘num’ in the outer code block.</div> <div> Remember, the re-use of a variable is perfectly alright as we saw in the code snippet</div> <div> above while using ‘outer’ variable inside the inner code block. But re-declaring a</div> <div> variable of the same name like we did for variable ‘num’ in the inner code block, is a</div> <div> bad practice.</div> <div> Now, is there a way that we declare a variable only once and then use it inside all</div> <div> functions. We have already done a similar task when we wrote a function prototype</div> <div> outside the body of all the functions. The same thing applies to declaration of</div> <div> variables. You declare variables outside of a function body (so that variable</div> <div> declarations are not part of any function) and they become visible and accessible</div> <div> inside all functions of that file. Notice that we have just used a new word ‘file’.</div> <div> A file or a source code file with extension ‘.c’ or ‘.cpp’ can have many functions</div> <div> inside. A file will contain one main() function maximum and rest of the functions as</div> <div> many as required. If you want a variable to be accessible from within all functions,</div> <div> you declare the variable outside the body of any function like the following code</div> <div> snippet has declared such a variable ‘size’ below.</div> <div> #include <iostream.h></div> <div> . . .</div> <div> // Declare your global variables here</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 92</div> <div> int size;</div> <div> . . .</div> <div> int main( … )</div> <div> {</div> <div> . . .</div> <div> }</div> <div> Now, this ‘size’ is visible in all functions including main(). We call this as 'file scope</div> <div> variable' or a 'global variable'. There are certain benefits of using global variables. For</div> <div> example, you want to access the variable ‘size’ from anywhere in your program but it</div> <div> does have some pitfalls. You may inadvertently change the value of the variable ‘size’</div> <div> considering it a local variable of the function and cause your program to behave</div> <div> differently or affect your program logic.</div> <div> Hence, you should try to minimize the use of global variables and try to use the local</div> <div> variables as far as possible. This philosophy leads us to the concept of Encapsulation</div> <div> and Data Hiding that encourages the declaration and use of data locally.</div> <div> In essence, we should take care of three levels of scopes associated with identifiers:</div> <div> global scope, function level scope and block level scope.</div> <div> Let's take a look of very simple example of global scope:</div> <div> #include <iostream.h></div> <div> //Declare your global variables here</div> <div> int i;</div> <div> void main()</div> <div> {</div> <div> i = 10;</div> <div> cout << “\n” << “In main(), the value of i is: “ << i;</div> <div> f();</div> <div> cout << “\n” << “Back in main(), the value of i is: “ << i;</div> <div> }</div> <div> void f()</div> <div> {</div> <div> cout << “\n” << ”In f(), the value of i is: “ << i;</div> <div> i = 20;</div> <div> }</div> <div> Note the keyword ‘void’ here, which is used to indicate that this function does not</div> <div> return anything.</div> <div> The output of the program is:</div> <div> In main(), the value of i is: 10</div> <div> In f(), the value of i is: 10</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 93</div> <div> Back in main(), the value of i is: 20</div> <div> Being a global variable, ‘i’ is accessible to all functions. Function f() has changed its</div> <div> value by assigning a new value i.e. 20.</div> <div> If the programmer of function f() has changed the value of ‘i’ accidentally taking it a</div> <div> local variable, your program’s logic will be affected.</div> <div> Function Calling</div> <div> We have already discussed that the default function calling mechanism of C is a 'Call</div> <div> by Value'. What does that mean? It means that when we call a function and pass some</div> <div> arguments (variables) to it, we are passing a copy of the arguments (variables) instead</div> <div> of original variables. The copy reaches to the function that uses it in whatever way it</div> <div> wants and returns it back to the calling function. The passed copy of the variable is</div> <div> used and original variable is not touched. This can be understood by the following</div> <div> example.</div> <div> Suppose you have a letter that has some mistakes in it. For rectification, you depute</div> <div> somebody to make a copy of that letter, leave the original with you and make</div> <div> corrections in that copy. You will get the corrected copy of the letter and have the</div> <div> unchanged original one too. You have given the copy of the original letter i.e. the call</div> <div> by value part.</div> <div> But if you give the original letter to that person to make corrections in it, then that</div> <div> person will come back to you with the changes in the original letter itself instead of its</div> <div> copy. This is call by reference.</div> <div> The default of C is 'Call by Value'. It is better to use it as it saves us from unwanted</div> <div> side effects. Relatively, 'Call by Reference' is a bit complex but it may be required</div> <div> sometimes when we want the actual variable to be changed by the function being</div> <div> called.</div> <div> Let's consider another example to comprehend 'Call by Value' and how it works.</div> <div> Suppose we write a main() function and another small function f(int) to call it from</div> <div> main(). This function f( ) accepts an integer, doubles it and returns it back to the</div> <div> main() function. Our program would look like this:</div> <div> #include <iostream.h></div> <div> void f(int); //Prototype of the function</div> <div> void main()</div> <div> {</div> <div> int i;</div> <div> i = 10;</div> <div> cout << “\n” << ” In main(), the value of i is: “ << i;</div> <div> f(i);</div> <div> cout << “\n” << ” Back in main(), the value of i is: “ << i;</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 94</div> <div> void f (int i)</div> <div> {</div> <div> i *= 2;</div> <div> cout << “\n” << “ In f(), the value of i is: “ << i;</div> <div> }</div> <div> The output of this program is as under:</div> <div> In main(), the value of i is: 10</div> <div> In f(), the value of i is: 20</div> <div> Back in main(), the value of i is: 10</div> <div> As the output shows the value of the variable ‘i’ inside function main() did not</div> <div> change, it proves the point that the call was made by value.</div> <div> If there are some values we want to pass on to the function for further processing, it</div> <div> will be better to make a copy of those values , put it somewhere else and ask the</div> <div> function to take that copy to use for its processing. The original one with us will be</div> <div> secure.</div> <div> Let's take another example of call by value, which is bit more relevant. Suppose we</div> <div> want to write a function that does the square of a number. In this case, the number can</div> <div> be a double precision number as seen below:</div> <div> #include <iostream.h></div> <div> double square (double);</div> <div> void main()</div> <div> {</div> <div> double num;</div> <div> num = 123.456;</div> <div> cout << “\n” << “ The square of “ << num << “ is “ << square(num);</div> <div> cout << “\n” << “ The current value of num is “ << num;</div> <div> }</div> <div> double square (double x)</div> <div> {</div> <div> return x*x;</div> <div> }</div> <div> 'C' does not have built-in mathematical operators to perform square, square root, log</div> <div> and trigonometric functions. The C language compiler comes along a complete library</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 95</div> <div> for that. All the prototypes of those functions are inside ‘<math.h>’. In order to use</div> <div> any of the functions declared inside ‘<math.h>’, the following line will be added.</div> <div> #include <math.h></div> <div> Remember, these functions are not built-in ones but library is supplied with the Ccompiler.</div> <div> It may be of interest to you that all the functions inside ‘<math.h>’ are</div> <div> called by value. Whatever variable you will pass in as an argument to these functions,</div> <div> nothing will happen to the original value of the variable. Rather a copy is passed to</div> <div> the function and a result is returned back, based on the calculation on that copy.</div> <div> Now, we will see why Call by Reference is used.</div> <div> We would like to use 'call by reference' while using a function to change the value of</div> <div> the original variable. Let's consider the square(double) function again, this time we</div> <div> want the original variable ‘x’ to be squared. For this purpose, we passed a variable to</div> <div> the square() function and as a result, on the contrary to the ‘Call by Value’, it affected</div> <div> the calling functions original variable. So these kinds of functions are ‘Call by</div> <div> Reference’ functions.</div> <div> Let us see, what actually happens inside Call by Reference?</div> <div> As apparent from the name ‘By Reference’, we are not passing the value itself but</div> <div> some form of reference or address. To understand this, you can think in terms of</div> <div> variables which are names of memory locations. We always access a variable by its</div> <div> name (which in fact is accessing a memory location), a variable name acts as an</div> <div> address of the memory location of the variable.</div> <div> If we want the called function to change the value of a variable of the calling function,</div> <div> we must pass the address of that variable to the called function. Thus, by passing the</div> <div> address of the variable to the called function, we convey to the function that the</div> <div> number you should change is lying inside this passed memory location, square it and</div> <div> put the result again inside that memory location. When the calling function gets the</div> <div> control back after calling the called function, it gets the changed value back in the</div> <div> same memory location.</div> <div> In summary, while using the call by reference method, we can’t pass the value. We</div> <div> have to pass the memory address of the value. This introduces a new mechanism</div> <div> which is achieved by using ‘&’ (ampersand) operator in C language. This ‘&’</div> <div> operator is used to get the address of a variable. Let's look at a function, which</div> <div> actually is a modification of our previous square() function.</div> <div> #include <iostream.h></div> <div> void square(double*);</div> <div> main()</div> <div> {</div> <div> double x;</div> <div> x = 123.456;</div> <div> cout << “\n” << “ In main(), before calling square(), x = “ << x;</div> <div> square(&x); //Passing address of the variable x</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 96</div> <div> cout << “\n” << “ In main(), after calling square(), x = “ << x;</div> <div> }</div> <div> void square(double* x) //read as: x is a pointer of type double</div> <div> {</div> <div> *x = *x * *x; //Notice that there is no space in *x</div> <div> }</div> <div> Here *x means whatever the x points to and &x means address of the variable x. We</div> <div> will discuss Pointers in detail later.</div> <div> We are calling function square(double*) with the statement square(&x) that is</div> <div> actually passing the address of the variable x , not its value. In other words, we have</div> <div> told a box number to the function square(double*) and asked it to take the value</div> <div> inside that box, multiply it with itself and put the result back in the same box. This is</div> <div> the mechanism of ‘Call by Reference’.</div> <div> Notice that there is no return statement of square(double*) as we are putting the</div> <div> changed value (that could be returned) inside the same memory location that was</div> <div> passed by the calling function.</div> <div> The output of the program will be as under:</div> <div> In main(), before calling square(), x = 123.456</div> <div> In main(), after calling square(), x = 15241.4</div> <div> By and large, we try to avoid a call by reference. Why? Mainly due to the side-effects,</div> <div> its use may cause. As mentioned above, it will be risky to tell the address of some</div> <div> variables to the called function. Also, see the code above for some special</div> <div> arrangements for call by reference in C language. Only when extremely needed, like</div> <div> the size of the data to be passed as value is huge or original variable is required to be</div> <div> changed, you should go for call by reference, otherwise stick to the call by value</div> <div> convention.</div> <div> Now in terms of call by reference, we see that there are some places in ‘C’ where the</div> <div> call by reference function happens automatically. We will discuss this later in detail.</div> <div> For the moment, as a hint, consider array passing in ‘C’.</div> <div> Recursive Function</div> <div> This is the special type of function which can call itself. What kind of function it</div> <div> would be? There are many problems and specific areas where you can see the</div> <div> repetitive behavior (pattern) or you can find a thing, which can be modeled in such a</div> <div> way that it repeats itself.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 97</div> <div> Let us take simple example of x10, how will we calculate it? There are many ways of</div> <div> doing it. But from a simple perspective, we can say that by definition x10 = x * x9. So</div> <div> what is x9? It is x9 = x * x8 and so on.</div> <div> We can see the pattern in it:</div> <div> xn = x * xn-1</div> <div> To compute it, we can always write a program to take the power of some number.</div> <div> How to do it? The power function itself is making recursive call to itself. As a</div> <div> recursive function writer, you should know where to stop the recursive call (base</div> <div> case). Like in this case, you can stop when the power of x i.e. n is 1 or 0.</div> <div> Similarly, you can see lot of similar problems like Factorials. A factorial of a positive</div> <div> integer ‘n’ is defined as:</div> <div> n! = (n) * (n-1) * (n-2) * ….. * 2 * 1</div> <div> Note that</div> <div> n! = (n) * (n-1)!</div> <div> and (n-1)! = (n-1) * (n-2)!</div> <div> This is a clearly a recursive behavior. While writing a factorial function, we can stop</div> <div> recursive calling when n is 2 or 1.</div> <div> long fact(long n)</div> <div> {</div> <div> if (n <= 1)</div> <div> return 1;</div> <div> else</div> <div> return n * fact(n-1);</div> <div> }</div> <div> Note that there are two parts (branches) of the function: one is the base case ( which</div> <div> indicates when the function will terminate) and other is recursively calling part.</div> <div> All the problems can be solved using the iterative functions and constructs we have</div> <div> studied until now. So the question is: do we need to use recursive functions? Yes, it</div> <div> adds little elegance to the code of the function but there is a huge price to pay for this.</div> <div> Its use may lead to the problems of having memory overhead. There may also be</div> <div> stacking overhead as lots of function calls are made. A lot of functions can be written</div> <div> without recursion (iteratively) and more efficiently.</div> <div> So as a programmer, you have an option to go for elegant code or efficient code,</div> <div> sometimes there is a trade-off. As a general rule, when you have to make a choice out</div> <div> of elegance and efficiency, where the price or resources is not an issue, go for</div> <div> elegance but if the price is high enough then go for efficiency.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 98</div> <div> ‘C’ language facilitates us for recursive functions like lot of other languages but not</div> <div> all computer languages support recursive functions. Also, all the problems can not be</div> <div> solved by recursion but only those, which can be separated out for base case, not</div> <div> iterative ones.</div> <div> Tips</div> <div> Header file is a nice mechanism to put function prototypes and define constants</div> <div> (global constants) in a single file. That file can be included simply with a single line</div> <div> of code.</div> <div> There are three levels of scopes to be taken care of, associated with identifiers: global</div> <div> scope, function level scope and block level scope.</div> <div> For Function calling mechanism, go for ‘Call by Value’ unless there is a need of ‘Call</div> <div> by Reference’.</div> <div> Apply the recursive function where there is a repetitive pattern, elegance is required</div> <div> and there is no resource problem.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 99</div> <div> Lecture No. 11</div> <div> Reading Material</div> <div> Deitel & Deitel - C++ How to Program chapter 4</div> <div> 4.2, 4.3, 4.4</div> <div> Summary</div> <div> • Introduction</div> <div> • Arrays</div> <div> • Initialization of Arrays</div> <div> • Sample Program 1</div> <div> • Copying Arrays</div> <div> • Linear Search</div> <div> • The Keyword ‘const’</div> <div> • Tips</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 100</div> <div> Introduction</div> <div> We have started writing functions, which will become a part of our every program. As</div> <div> C language is a function-oriented language, so we will be dealing with too many</div> <div> functions. Our programming toolkit is almost complete but still a very important</div> <div> component is missing. We are going to discuss this component i.e. Arrays in this</div> <div> lecture.</div> <div> Let us consider an example about calculation of average age of 10 students. At first,</div> <div> we will declare 10 variables to store the age of each student and then sum up all the</div> <div> ages and divide this with 10 to get the average age. Suppose, we have 100 students</div> <div> instead of 10, we have to declare 100 variables i.e. one for each student’s age. Is there</div> <div> any other way to deal with this problem? Arrays are possible solution to the problem.</div> <div> Array is a special data-type. If we have a collection of data of same type as in the case</div> <div> of storage of ages of 100 students, arrays can be used. Arrays are data structure in</div> <div> which identical data types are stored. The concept of arrays is being explained further</div> <div> in the following parts of the lecture.</div> <div> Arrays</div> <div> In C language, every array has a data type i.e. name and size. Data type can be any</div> <div> valid data type. The rules of variable naming convention apply to array names. The</div> <div> size of the array tells how many elements are there in the array. The size of the array</div> <div> should be a precise number. The arrays occupy the memory depending upon their size</div> <div> and have contiguous area of memory. We can access the arrays using the array index.</div> <div> Declaration:</div> <div> The declaration of arrays is as follows:</div> <div> data_type array_name [size] ;</div> <div> for example:</div> <div> int ages[10];</div> <div> Let's consider an array int C[10]; This is an array of integer and has a name ’C'. It has</div> <div> a size ten which depicts that the array ‘C’ can contain ten elements of int data type. In</div> <div> the memory, the array occupies the contiguous area, in this case it will occupy forty</div> <div> bytes (one int = 4 bytes). The elements of the array are manipulated using the index.</div> <div> In C language, the index of array starts from zero and is one less than array's size.</div> <div> Index of array is also called subscript.</div> <div> Memory image of an array:</div> <div> Name</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 101</div> <div> In the above figure, the memory chunk containing the array C is shown. On the first</div> <div> line, C[0] is written while on the 2nd line, C[1] is written and so on. The number in the</div> <div> [ ] is the index of the array. C[0] is used for the first element, followed by C[1] for</div> <div> the second element and so on. It is important to note that in an array the index 6 ([6])</div> <div> means the seventh element of the array and thus the eighth element will have an index</div> <div> 7. Thus, the index of the last element of the array will be 1 less than the size of the</div> <div> array. On the right hand side, the values of the elements are shown in the memory i.e.</div> <div> the value of the element at zero position ( C[0] ) is 24 while that of the element at</div> <div> first position ( C[1] ) is 59 and so on. The important thing to be noted here is that the</div> <div> indexing of the array starts from zero, not from one. So in the above example, the</div> <div> index of the array C will be from C[0] to C[9]. If we have an array of size 25, its</div> <div> index will be from 0 to 24.</div> <div> Usage of Arrays</div> <div> To declare arrays, we have to give their data type, name and size. These are fixed-size</div> <div> arrays. In the coming lectures, we will discuss arrays without using size at declaration</div> <div> time. Arrays may be declared with simple variables in a single line.</div> <div> int i, age [10];</div> <div> int height [10], length [10] ;</div> <div> To access array, we can’t use the whole array at a time. We access arrays element by</div> <div> element. An index (subscript) may be used to access the first element of the array. In</div> <div> this case, to access first element we write like age[0]. To access the 5th element, we</div> <div> will write age[4] and so on. Using the index mechanism, we can use the array</div> <div> elements as simple variables. Their use can be anywhere where there we can use a</div> <div> simple variable i.e. in assignment statements, expressions etc. Please do not confuse</div> <div> the usage of array and declaration of array. When we write int age [10], it means we</div> <div> are declaring an array of type int, its name is age and its size is 10. When we write</div> <div> C[0]</div> <div> C[1]</div> <div> C[2]</div> <div> C[3]</div> <div> Index</div> <div> C[7]</div> <div> C[8]</div> <div> C[9]</div> <div> ...</div> <div> 35</div> <div> 59</div> <div> 24</div> <div> ..</div> <div> ..</div> <div> ..</div> <div> ..</div> <div> ...</div> <div> …</div> <div> Memory</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 102</div> <div> age[5], it means we are referring to the single element of the array not the whole</div> <div> array.</div> <div> Consider the example of student’s ages again. Is there a way to calculate the average</div> <div> age of all the students in an array?</div> <div> As we know that arrays can be accessed with indexing. So we can use a 'for loop' as</div> <div> under;</div> <div> for (i = 0 ; i < 10 ; i++ )</div> <div> {</div> <div> cout << “Please enter the age of the student “;</div> <div> cin >> age [i];</div> <div> }</div> <div> In the above 'for loop' the value of i is changing from 0 to 9. Here the loop condition</div> <div> is i<10. This means that the cin and cout statements will be executed 10 times. We</div> <div> have used i as the index of the array. The index we are referring to the array needs to</div> <div> be an integer. It can be 4, 5 or an integer variable like i. In the first repetition, the</div> <div> value of i is 0, i.e. age[0] so the value of first element of the age will be read. In the</div> <div> second repetition, the value of i becomes 1 i.e. age[1] so the value of 2nd element of</div> <div> the age will be read and so on. We get all the 10 values from the user which will be</div> <div> stored in the array age.</div> <div> Now we will calculate the total of ages. We can use another 'for loop' to add up all the</div> <div> elements of the array age.</div> <div> int totalAge = 0;</div> <div> for (i = 0 ; i < 10 ; i++ )</div> <div> {</div> <div> totalAge += age [i];</div> <div> }</div> <div> In the above loop, all the elements of the array age will be added to the variable</div> <div> totalAge. When the value of i is 0 i.e. age[0] the value of first element will be added</div> <div> to the totalAge. As the value of i is changing from 0 to 9 so all the 10 elements of the</div> <div> array will be added to the totalAge. By dividing this totalAge by 10 we will get the</div> <div> average age.</div> <div> Initialization of Arrays</div> <div> There are many ways to initialize an array. Don't use the default initialization of</div> <div> arrays. Compiler may assign some value to each declared array. Always initialize the</div> <div> array in such a manner that the process is clear.</div> <div> We can initialize an array using a 'loop' while assigning some value.</div> <div> int i, age [10];</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 103</div> <div> for ( i = 0; i < 10 ; i++ )</div> <div> {</div> <div> age[i] = 0;</div> <div> }</div> <div> With the help of this simple loop, we have initialized all the elements of array age to</div> <div> zero. In the loop condition, we have used the condition i < 10, where the size of the</div> <div> array is ten. As we know, the array index is one less than the size of the array. Here</div> <div> we are using i as the index of array and its values are from 0 to 9.</div> <div> We can also initialize the array at the time of declaration as:</div> <div> int age [10] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };</div> <div> The above statement creates an array age of integers and initializes all the elements</div> <div> with zero. We can use any value to initialize the array by using any other number</div> <div> instead of zero. However, generally, zero is used to initialize the integer variables.</div> <div> We can do it by using the following shortcut.</div> <div> int age [10] = { 0 };</div> <div> The above statement has also initialized all the elements of the array to zero.</div> <div> We have different ways of initializing the arrays. Initialization through the use of loop</div> <div> is a better choice. If the size of the array gets larger, it is tedious to initialize at the</div> <div> declaration time.</div> <div> Consider the following statement:</div> <div> int age [ ] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };</div> <div> Here we have not mentioned the size of the array. The compiler is quite intelligent as</div> <div> it detects the initialization list which consists of ten 0’s. Therefore, it creates an array</div> <div> of 10 integers and initializes all the elements with zero.</div> <div> The index of the arrays starts from the index 0 and is up to one less than the size of</div> <div> the array. So if the size of the array is ten, the index will be from 0 to 9. Similarly, if</div> <div> the size of the array is 253, the index will be from 0 to 252.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 104</div> <div> Sample Program 1</div> <div> Problem Statement:</div> <div> Write a program which reads integers from the user and</div> <div> stores these ones in an array. User can enter a maximum of 100</div> <div> numbers. Stop taking input when user enters -1.</div> <div> Solution:</div> <div> We have to declare an integer array of size 100 to be used to store the integers. We</div> <div> used a loop to get the input from the users. There are two conditions to terminate the</div> <div> loop i.e. either user has entered 100 numbers or user entered -1. 'For' and 'while' loops</div> <div> can execute zero or more times whereas ‘do-while’ may execute one or more times.</div> <div> By analyzing the problem, the loop will be executed at least once so do-while loop</div> <div> logically fits in this problem. We take an integer z to get the input from the user and i</div> <div> as the counter so the condition will be as ( z != -1 && i < 100 ). && is used to</div> <div> enforce that both the conditions are true. If any of the two conditions becomes false,</div> <div> the loop will be terminated. The loop counter is less than 100 because the index of the</div> <div> array will be from 0 to 99.</div> <div> We will read a number from the user and store it at some particular location of the</div> <div> array unless user enters -1 or 100 numbers are entered. In the loop, we will use the if</div> <div> statement whether the number entered by user is -1 or not. If the number entered is</div> <div> not -1, then we will store it in the array. The index of the array will also be</div> <div> incremented in each repetition. We can assign some value to array element as:</div> <div> c[ 3 ] = 33;</div> <div> In an assignment statement, we cannot use expression on the left hand side. Here c[3]</div> <div> is used as a variable which represents the 4th element of the array.</div> <div> The complete code of the program as under:</div> <div> // This program reads the input from user and store it into an array and stop at -1.</div> <div> #include <iostream.h></div> <div> main( )</div> <div> {</div> <div> int c [ 100 ] ;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 105</div> <div> int z , i = 0 ;</div> <div> do</div> <div> {</div> <div> cout << “Please enter the number (-1 to end input) “ << endl;</div> <div> cin >> z ;</div> <div> if ( z != -1 )</div> <div> {</div> <div> c[ i ] = z ;</div> <div> }</div> <div> i ++ ;</div> <div> } while ( z != -1 && i < 100 ) ;</div> <div> cout << “ The total number of positive integers entered by user is “ << i -1;</div> <div> }</div> <div> The above code shows that the assignment statement of the array is inside the if block.</div> <div> Here the numbers will be assigned to the array elements when the 'if statement'</div> <div> evaluates to true. When the user enters -1, the if statement will evaluate it false. So the</div> <div> assignment statement will not be executed and next i will be incremented. The</div> <div> condition in the 'while loop' will be tested. As the value of z is -1, the loop will be</div> <div> terminated.</div> <div> Now we have to calculate how many positive numbers, the user has entered. In the</div> <div> end, we have incremented i so the actual positive integers entered by the users is i -1.</div> <div> The above example is very useful in terms of its practical usage. Suppose we have to</div> <div> calculate the ages of students of the class. If we don’t know the exact number of</div> <div> students in the class, we can declare an array of integers of larger size and get the ages</div> <div> from the user and use -1 to end the input from the user.</div> <div> A sample out put of the program is as follow.</div> <div> Please enter the number (-1 to end input) 1</div> <div> 2</div> <div> 3</div> <div> 4</div> <div> 5</div> <div> 6</div> <div> -1</div> <div> The total number of positive integers entered by user is 6</div> <div> Copying Arrays</div> <div> Sometimes, we need to copy an array. That means after copying, both the arrays will</div> <div> contain elements with same values. For being copy able, both arrays need to be of</div> <div> same data type and same size. Suppose, we have two arrays a and b and want to copy</div> <div> array a into array b. Both arrays are of type int and of size 10.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 106</div> <div> int array a[10];</div> <div> int array b[10];</div> <div> We know that a value can be assigned to an element of array using the index. So we</div> <div> can write assignment statements to copy these arrays as:</div> <div> b[0] = a[0] ;</div> <div> b[1] = a[1] ;</div> <div> b[2] = a[2] ;</div> <div> ……</div> <div> ……</div> <div> ……</div> <div> b[9] = a[9] ;</div> <div> As the size of array is 10, its index will be from 0 to 9. Using the above technique, we</div> <div> can copy one array to another. Now if the array size is 100 or 1000, this method can</div> <div> be used. Is there some other way to do things in a better way? We can use the loop</div> <div> construct to deal with this easily in the following way.</div> <div> for (i = 0; i < 10 ; i ++)</div> <div> {</div> <div> b[i] = a[i];</div> <div> }</div> <div> With the help of loop, it becomes very simple. We are no more worried about the size</div> <div> of the array. The same loop will work by just changing the condition. We are</div> <div> assigning the corresponding values of array a into array b. The value of first element</div> <div> of array a is assigned to the first element of array b and so on.</div> <div> Example:</div> <div> Take the sum of squares of 10 different numbers stored in an array.</div> <div> Here is the code of the program:</div> <div> // This program calculates the sum of squares of numbers stored in an array.</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> int a[10];</div> <div> int sumOfSquares = 0 ;</div> <div> int i =0;</div> <div> cout << "Please enter the ten numbers one by one " << endl;</div> <div> // Getting the input from the user.</div> <div> for (i = 0 ; i < 10 ; i++ )</div> <div> {</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 107</div> <div> cin >> a [i];</div> <div> }</div> <div> // Calculating the sum of squares.</div> <div> for ( i = 0 ; i < 10 ; i ++ )</div> <div> {</div> <div> sumOfSquares = sumOfSquares + a[ i ] * a[ i ] ;</div> <div> }</div> <div> cout << “The sum of squares is “ << sumOfSquares << endl;</div> <div> }</div> <div> A sample out put of the program is given below.</div> <div> Please enter the ten numbers one by one</div> <div> 1</div> <div> 2</div> <div> 3</div> <div> 4</div> <div> 5</div> <div> 6</div> <div> 7</div> <div> 8</div> <div> 9</div> <div> 10</div> <div> The sum of squares is 385</div> <div> Linear Search</div> <div> Arrays are used to solve many problems. As we have seen that loops are used along</div> <div> with the arrays, so these two constructs are very important. Suppose, we are given a</div> <div> list of numbers to find out a specific number out of them. Is the number in the list or</div> <div> not? Let's suppose that there are 100 numbers in the list. We take an array of size 100</div> <div> as int a [100]. For populating it, , we can request the user to enter the numbers. Either</div> <div> these numbers can be stored into the array or we can just populate it with numbers</div> <div> from 0 to 99. We can write a simple loop and assign the values as a[i] = i. This means</div> <div> that at ith position, the value is i i.e. ( a[5] = 5 ), at 5th position the value is 5 and so</div> <div> on. Then we can request the user to enter any number and store this number into an int</div> <div> variable. To search this number in the array, we write a loop and compare all the</div> <div> elements with the number. The loop will be terminated, if we found the number or we</div> <div> have compared all the elements of the array, which means that number is not found.</div> <div> We used a flag to show that we have found the number or not. If the value of found is</div> <div> zero, the number is not found while the value 1 will mean that number has been</div> <div> found. When we find the number, is there a need to compare it with other elements of</div> <div> the array? May be not, so when we found the number, we just jumped out of the loop.</div> <div> In the end, we check the variable found. If the value is 1, it means number has been</div> <div> found. Otherwise number stands unfound.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 108</div> <div> Here is the complete code of the program.</div> <div> // This program is used to find a number from the array.</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> int z, i ;</div> <div> int a [ 100 ] ;</div> <div> // Initializing the array.</div> <div> for ( i =0 ; i < 100 ; i ++ )</div> <div> {</div> <div> a [ i ] = i ;</div> <div> }</div> <div> cout << “ Please enter a positive integer “ ;</div> <div> cin >> z ;</div> <div> int found = 0 ;</div> <div> // loop to search the number.</div> <div> for ( i = 0 ; i < 100 ; i ++ )</div> <div> {</div> <div> if ( z == a [ i ] )</div> <div> {</div> <div> found = 1 ;</div> <div> break ;</div> <div> }</div> <div> }</div> <div> if ( found == 1 )</div> <div> cout << “ We found the integer at index ” << i ;</div> <div> else</div> <div> cout << “ The number was not found ” ;</div> <div> }</div> <div> The following is an output of the program.</div> <div> Please enter a positive integer 34</div> <div> We found the integer at index 34</div> <div> The loop in the above program may run 100 times or less. The loop will terminate if</div> <div> the number is found before the 100th repetition. Therefore, in the linear search the</div> <div> maximum limit of the loop execution is the size of the list. If the size of list is 100,</div> <div> then the loop can execute a maximum of 100 times.</div> <div> Using random function (Guessing Game):</div> <div> We can turn this problem into an interesting game. If we as programmers do not</div> <div> know, which number is stored in the array? We can make this a guessing game. How</div> <div> can we do that? We need some mechanism by which the computer generates some</div> <div> number. In all the C compilers, a random number generation function is provided. The</div> <div> function is rand() and is in the standard library. To access this function, we need to</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 109</div> <div> include <stdlib.h> library in our program. This function will return a random number.</div> <div> The number can be between 0 and 32767. We can use this function as:</div> <div> x = rand ( );</div> <div> The random function generates an integer which is assigned to variable x. Let's</div> <div> consider the function-calling mechanism. The program starts its execution in the main</div> <div> function. When the control goes to the statement containing a function call, the main</div> <div> program stops here and the control goes inside the function called. When the function</div> <div> completes or returns some value, the control comes back to the main program.</div> <div> Here is the complete code of the program using rand().</div> <div> // This program is used to find a number from the array.</div> <div> #include <iostream.h></div> <div> #include <stdlib.h></div> <div> main()</div> <div> {</div> <div> int z, i ;</div> <div> int a [ 100 ] ;</div> <div> // Initializing the array.</div> <div> for ( i =0 ; i < 100 ; i ++ )</div> <div> {</div> <div> a [i] = rand() ;</div> <div> }</div> <div> cout << “ Please enter a positive integer “ ;</div> <div> cin >> z ;</div> <div> int found = 0 ;</div> <div> // loop to search the number.</div> <div> for ( i = 0 ; i < 100 ; i ++ )</div> <div> {</div> <div> if ( z == a [ i ] )</div> <div> {</div> <div> found = 1 ;</div> <div> break ;</div> <div> }</div> <div> }</div> <div> if ( found == 1 )</div> <div> cout << “ We found the integer at position ” << i ;</div> <div> else</div> <div> cout << “ The number was not found ” ;</div> <div> }</div> <div> The following is an output of the program.</div> <div> Please enter a positive integer 34</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 110</div> <div> The number was not found</div> <div> The function rand ( ) returns a value between 0 and 32767. Can we limit the generated</div> <div> random number in a smaller range? Suppose we have a die with six faces marked with</div> <div> 1, 2, 3, 4, 5 and 6. We want to generate random die number i.e. the number should be</div> <div> between 1 and 6 inclusive. Here we can use the modulus operator to achieve this.</div> <div> Modulus operator returns the remainder. What will be the result of the statement?</div> <div> rand ( ) % 6</div> <div> When 6 divides any number, the remainder will always be less than 6. Therefore, the</div> <div> result will be between 0 and 5 inclusive. We want the number between 1 and 6,</div> <div> therefore we will add 1.</div> <div> 1 + rand ( ) % 6;</div> <div> The above statement will give us the desired result. We need to know whether this is a</div> <div> fair die or not. A fair die is a die when it is rolled 10 or 100 million of times. Then on</div> <div> average, equal number of 1’s, equal number of 2’s, equal number of 3’s etc. will be</div> <div> generated. Can we test our die i.e. it is fair or not? That is there are equal numbers of</div> <div> chances of 1 or 2 etc. Think about generating a test for our random number generator.</div> <div> Does it produce a fair die?</div> <div> The random function is very useful. It can be used to guess the tossing of the coin.</div> <div> There can be only two possibilities of tossing a coin. Therefore we can use rand ( ) %</div> <div> 2 which will give 0 or 1.</div> <div> The Keyword ‘const’:</div> <div> To declare an array, we need its data type, name and size. We use simple integer for</div> <div> the size like 10 or 100. While using arrays in loops, we use the size a lot. Suppose if</div> <div> we have to change the size of the array from 10 to 100, it will have to be changed at</div> <div> all the places. Missing a place will lead to unexpected results. There is another way to</div> <div> deal this situation i.e. keyword construct. The keyword const can be used with any</div> <div> data type and is written before the data type as:</div> <div> const int arraySize = 100;</div> <div> This statement creates an identifier arraySize and assigns it the value 100. Now the</div> <div> arraySize is called integer constant. It is not a variable. We cannot change its value in</div> <div> the program. In the array declaration, we can use this as:</div> <div> int age [arraySize];</div> <div> Now in the loop condition, we can write like this:</div> <div> for ( i = 0; i < arraySize ; i ++)</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 111</div> <div> If we have to change the size of the array, we only have to change the value of</div> <div> arraySize where it is declared. The program will work fine in this case. This is a good</div> <div> programming practice to use const for array size.</div> <div> Tips</div> <div> Initialize the array explicitly</div> <div> Array index (subscript) starts from 0 and ends one less than the array size</div> <div> To copy an array, the size and data type of both arrays should be same</div> <div> Array subscript may be an integer or an integer expression</div> <div> Assigning another value to a const is a syntax error</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 112</div> <div> Lecture No. 12</div> <div> Reading Material</div> <div> Deitel & Deitel - C++ How to Program chapter 4</div> <div> 4.4, 4.5, 4.6, 4.8, 4.9</div> <div> Summary</div> <div> • Character Arrays</div> <div> • Initialization Of Character Arrays</div> <div> • Arrays Comparison</div> <div> • Sorting Arrays</div> <div> • Searching arrays</div> <div> • Functions And arrays</div> <div> • Example 1</div> <div> • Multidimensional Arrays</div> <div> • Example 2</div> <div> • Tips</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 113</div> <div> Character Arrays</div> <div> While dealing with words and sentences, we actually make use of character arrays.</div> <div> Up to now, we were dealing with integer arrays and storing integer values. Here we</div> <div> have to see what needs to be done for storing a name. A simple variable can't be used</div> <div> to store a name (which is a string of characters) as a variable stores only a single</div> <div> character. We need a character array to grab a name. A character array is not different</div> <div> from an integer array. To declare a character array, we will write as under:</div> <div> char name [100] ;</div> <div> In this way, we declare a string or character array. There are some special properties</div> <div> of character arrays. Suppose that we declare an array of 100 characters. We enter a</div> <div> name with 15-20 characters. These characters in the array occupy 15-20 character</div> <div> spaces. Now we have to see what has happened to the remaining character spaces in</div> <div> the array. Similarly, a question arises, will an array displayed on the screen, show 100</div> <div> characters with a name in 15-20 spaces and blanks for the remaining. Here C has a</div> <div> character handling capability i.e. the notion of strings. When we place a string in a</div> <div> character array, the computer keeps a mark to identify that the array was of this size</div> <div> while the string stored in it is of the other size. That marker is a special character,</div> <div> called null character. The ASCII code of null character is all zeros. In C language, we</div> <div> represent the null character as “\0”. C uses this character to terminate a string. All</div> <div> strings are terminated with the null character.</div> <div> Now, we will see how the character arrays are stored in memory. While declaring a</div> <div> character array, we normally declare its size larger than the required one. By using a</div> <div> character array, it becomes easy to store a string. We declare a character array as</div> <div> under.</div> </div> <div> <div> char name [100] ;</div> <div> Now we can store a string in this array simply by using the cin statement in the</div> <div> following way.</div> <div> cin >> name ;</div> <div> In the above statement, there is an array on right hand side of cin instead of a simple</div> <div> variable. The cin stream has a built-in intelligence that allows the compiler (program)</div> <div> to read whole string at a time rather than a single character as in case of simple</div> <div> variable of type char. The compiler determines that the name is not a simple variable.</div> <div> Rather it is a string or character array. Thus cin reads a character array until the user</div> <div> presses the enter key. When enter key is pressed, cin takes the whole input (i.e. string)</div> <div> and stores it into the array name. The C language, by itself, attaches a null character at</div> <div> the end of the string. In this way, the total number of spaces occupied in the array by</div> <div> the string is the number of characters entered by the user plus 1 (this one character is</div> <div> the null character inserted at the end of the string by C automatically). The null</div> <div> character is used to determine where the populated area of the array has ended. If we</div> <div> put a string larger than the size of the array in absence of a null character in it, then it</div> <div> is not possible to determine where a string is terminated in the memory. This can</div> <div> cause severe logical error. So, one should be careful while declaring a character array.</div> <div> The size of array should be one more than the number of characters you want to store.</div> <div> Initialization Of Character Arrays</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 114</div> <div> Now we will look into integer array initialization process that can provide a list of</div> <div> integer values separated by commas and enclosed in curly braces. Following is the</div> <div> statement through which we initialize an integer array.</div> <div> int age [5] = {12, 13, 16, 13, 14};</div> <div> If we don’t mention the size of the array and assign a list of values to the array, the</div> <div> compiler itself generates an array of the size according the number of values in the</div> <div> list. Thus, the statement int age [] = {14, 15, 13}; will allocate a memory to the</div> <div> array of size 3 integers. These things also apply to character arrays as well. We can</div> <div> initialize an array by giving a list of characters of the string, the way we assign integer</div> <div> values in integer array. We write the characters of this string one by one in single</div> <div> quotes (as we write a single character in single quotes), separated by commas and</div> <div> enclosed in curly braces. So the initialization line will be as under</div> <div> char name [100] = {‘i’, ‘m’, ‘r’, ‘a’, ‘n’};</div> <div> we can also write the string on right hand side in double quotes as</div> <div> char name [100] = “imran” ;</div> <div> The easy way to initialize a character array is to assign it a string in double quotes.</div> <div> We can skip the size of the array in the square brackets. We know that the compiler</div> <div> allocates the memory at the declaration time, which is used during the execution of</div> <div> the program. In this case, the compiler will allocate the memory to the array of size</div> <div> equal to the number of characters in the provided string plus 1 (1 is for the null</div> <div> character that is inserted at the end of string). Thus it is a better to initialize an array in</div> <div> the following way.</div> <div> char name [] = “Hello World” ;</div> <div> In the above statement, a memory of 12 characters will be allocated to the array name</div> <div> as there are 11 characters in double quotes (space character after Hello is also</div> <div> considered and counted) while the twelfth is the null character inserted automatically</div> <div> at the end of the string.</div> <div> We can do many interesting things with arrays. Let’s start with reading a string (for</div> <div> example your name) from keyboard and displaying it on the screen. For this purpose,</div> <div> we can write the following code segment</div> <div> char name [100] ;</div> <div> cout << “Please enter your name : “ ;</div> <div> cin >> name ;</div> <div> In the cin statement, when the user presses the enter key the previous characters</div> <div> entered, that is a string will be stored in the array name. Now we have a string in the</div> <div> array name. We can display it with cout statement. To display the string, we have</div> <div> stored in name. We can write as under</div> <div> cout << name ;</div> <div> This will display the string. Alternatively, we can use a loop to display the string. As</div> <div> the string is an array of characters, we can display these characters one by one in a 'for</div> <div> loop'. We can write a loop as under</div> <div> for ( i = 0 ; i < 100 ; i ++ )</div> <div> cout << name [ i ] ;</div> <div> Thus this loop will display the characters in the array one by one in each iteration.</div> <div> First, it will display the character at name [0], followed by that at name [1] and so on.</div> <div> Here we know that the string in the array is terminated by a null character and after</div> <div> this null character, there are random values that may not be characters (some garbage</div> <div> data) in the array. We don’t want to display the garbage data that is in the array after</div> <div> this null character. While using the statement cout << name; the cout stream takes</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 115</div> <div> the characters of the array name up to the null character and the remaining part of the</div> <div> array is ignored. When we are displaying the characters one by one, it is necessary to</div> <div> stop the displaying process at the end of a string (which means when null character is</div> <div> reached). For this purpose, we may put a condition in the loop to terminate the loop</div> <div> when the null character is reached. So we can use if statement in the loop to check the</div> <div> null character. We can modify the above for loop so that it could terminate when null</div> <div> character reaches in the array.</div> <div> for ( i = 0 ; i < 100 ; i ++ )</div> <div> { if (name [ i ] == ‘\0’)</div> <div> break ;</div> <div> cout << name [ i ] ;</div> <div> }</div> <div> Here a while loop can also be used instead of a 'for loop'.</div> <div> Arrays Comparison</div> <div> We can use this character-by-character manipulation of the array to compare the</div> <div> characters of two arrays of the same size. Two arrays can be equal only when first of</div> <div> all their sizes are equal. Afterwards, we compare the values of the two arrays with one</div> <div> to one correspondence. If all the values in the first array are equal to the</div> <div> corresponding values of the second array, then both the arrays will be equal. Suppose,</div> <div> we have two integer arrays num1 and num2 of size 100 each and want to find</div> <div> whether both arrays are equal. For this purpose, we will declare a flag and set it to</div> <div> zero, that means that arrays are not equal this time. For this flag, we write int equals</div> <div> = 0 ;</div> <div> To compare the values of the arrays one by one, we write a for loop i.e. for ( i = 0 ; i</div> <div> < 100 ; i ++ ). In the body of the for loop, we use an if statement to check the values.</div> <div> In the if statement, we use the not equal operator ( != ). The advantage of using notequal</div> <div> operator is that in case if the values at some position are not equal to each other,</div> <div> then we need not to compare the remaining values. We terminate the loop here and</div> <div> say that the arrays are not equal. If the values at a position are equal, we continue to</div> <div> compare the next values. If all the values are found same, we set the flag equal to 1</div> <div> and display the results that both the arrays are identical. The same criterion applies to</div> <div> character arrays. The comparison of character arrays is very common. While finding a</div> <div> name in a database, we will compare two character arrays (strings). The comparison</div> <div> of two strings is so common in programming that C has a function in its library to</div> <div> manipulate it. We will discuss it later in the lecture on string handling. For the time</div> <div> being, we will write our own function to find the equality of two strings.</div> <div> Following is the code of a program, which takes two arrays of 5 numbers from the</div> <div> user and compares them for equality.</div> <div> // This program takes two arrays of 5 integers from user</div> <div> //displays them and after comparing them displays the result</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 116</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> int num1 [5], num2 [5], i, equals = 0 ;</div> <div> // input of 5 integers of first array</div> <div> cout << “Please enter five integers for the first array” << endl ;</div> <div> for ( i = 0 ; i < 5 ; i ++)</div> <div> cin >> num1 [ i ] ;</div> <div> // input of 5 integers of 2nd array</div> <div> cout << “Please enter five integers for the second array” << endl ;</div> <div> for ( i = 0 ; i < 5 ; i ++)</div> <div> cin >> num2 [ i ] ;</div> <div> //display the elements of two arrays</div> <div> cout << “\n The values in the first array are : “ ;</div> <div> for ( i = 0 ; i < 5 ; i ++)</div> <div> cout << “\t” << num1 [ i ] ;</div> <div> cout << “\n The values in the second array are : “ ;</div> <div> for ( i = 0 ; i < 5 ; i ++)</div> <div> cout << “\t” << num2 [ i ];</div> <div> // compare the two arrays</div> <div> for ( i = 0 ; i < 5 ; i ++ )</div> <div> { if ( num1 [ i ] != num2 [ i ] )</div> <div> {</div> <div> cout << “\n The arrays are not equal “ ;</div> <div> equals = 0 ; //set the flag to false</div> <div> break ;</div> <div> }</div> <div> equals = 1; //set flag to true</div> <div> }</div> <div> if (equals)</div> <div> cout << “\n Both arrays are equal” ;</div> <div> }</div> <div> Similarly, we can write a program that compares two strings (character arrays) of the</div> <div> same size. While comparing strings, a point to remember is that C language is casesensitive.</div> <div> In C-language ‘A’ is not equal to ‘a’. Similarly, the string “AZMAT” is not</div> <div> equal to the string “azmat” or “Azmat”.</div> <div> A sample out-put of the program is given below.</div> <div> Please enter five integers for the first array</div> <div> 1</div> <div> 3</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 117</div> <div> 5</div> <div> 7</div> <div> 9</div> <div> Please enter five integers for the second array</div> <div> 1</div> <div> 3</div> <div> 4</div> <div> 5</div> <div> 6</div> <div> The values in the first array are : 1 3 5 7 9</div> <div> The values in the first array are : 1 3 4 5 6</div> <div> The arrays are not equal</div> <div> Sorting</div> <div> We want to sort an array in ascending order. There may be many ways to sort an</div> <div> array. Suppose we have an array of 100 numbers. To sort it in ascending order, we</div> <div> start from the first number (number at zero index ) and find the smallest number in the</div> <div> array. Suppose, we find it at sixteenth position (index 15). If we assign this number</div> <div> directly to the first position, the number already placed at first position will be over</div> <div> written. But we want that number should exist in the array. For this purpose, we use a</div> <div> technique called swapping. In this technique, we swap two values with each other. For</div> <div> this purpose, we declare a variable and assign the value of first variable to this</div> <div> variable before assigning the second number (i.e. to be swapped) to the first variable.</div> <div> Then we assign the value of second variable to the first variable. Afterwards, the</div> <div> number, which we have stored in a separate third variable (that is actually the value of</div> <div> first variable) is assigned to the second variable. In arrays, the single element of an</div> <div> array is treated as a single variable so we can swap two numbers of an array with each</div> <div> other with this technique.</div> <div> In our sorting process, we declare a variable x and assign it the number at the first</div> <div> position. Then assign the number at sixteenth position to the first position. After this,</div> <div> we assign the number in x (that is actually the number that was at first position in the</div> <div> array) to the sixteenth position. In programming, this can be done in the following</div> <div> fashion.</div> <div> x = num [0] ; // assign number at first position to x</div> <div> num [0] = num [15] ; // assign number at sixteenth position to first position</div> <div> num [15] = x ; // assign number in x to sixteenth position</div> <div> We have the smallest number at the first position. Now we start reading the array</div> <div> from second position (index 1) and find the smallest number. We swap this number</div> <div> with the second position before starting from index 2. The same process can be</div> <div> repeated later. We continue this process of finding smallest number and swapping it</div> <div> till we reach the last number of the array. The sorting of array in this way is a brute</div> <div> force and a very tedious work. The computer will do fine with small arrays. The large</div> <div> arrays may slow it down.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 118</div> <div> Searching</div> <div> The same applies to the search algorithms. For finding out a particular number in an</div> <div> array, we can use technique of linear search. In this technique, there may be as many</div> <div> comparisons as numbers in the array. We make comparison of the number to be found</div> <div> with each number in the array and find it out if it matches any number in the array.</div> <div> However, we can perform even better by using a binary search algorithm.</div> <div> Binary Search Algorithm</div> <div> In binary search algorithm, the ‘divide and conquer’ strategy is applied. This</div> <div> algorithm applies only to sorted arrays in ascending or descending order. Suppose that</div> <div> we want to search a number in an ascending array. For this purpose, we divide the</div> <div> array into two parts (say left and right). We compare the target value with the value at</div> <div> middle location of the array. If it does not match, we see whether it is greater or less</div> <div> than the middle value. If it is greater than the middle value, we discard the left part of</div> <div> the array. Being an ascending array, the left part contains the smaller numbers than</div> <div> the middle. Our target number is greater than the middle number. Therefore, it will be</div> <div> in the right part of the array. Now we have a sub-array, which is the half of the actual</div> <div> array (right side portion of main array). Now we divide this array into two parts and</div> <div> check the target value. If target value is not found, we discard a portion of the array</div> <div> according to the result whether target value is greater or less than the middle value. In</div> <div> each iteration of testing the target value, we get an array that is half of the previous</div> <div> array. Thus, we find the target value.</div> <div> The binary search is more efficient than the linear search. In binary search, each</div> <div> iteration reduces the search by a factor of two (as we reduce to half array in each</div> <div> iteration). For example, if we have an array of 1000 elements, the linear search could</div> <div> require 1000 iterations. The binary search would not require more than 10. If an array</div> <div> has elements 2n, then the maximum number of iterations required by binary search</div> <div> will be n. If there are 1000 elements (i.e. 210, actually it will 1024), the number of</div> <div> iterations would not be more than 10.</div> <div> Functions and Arrays</div> <div> In C language, the default mechanism of calling a function is ‘call by value’. When</div> <div> we call a function, say fn, and pass it a parameter x (argument value) by writing</div> <div> statement fn(x), the calling mechanism puts the value of x at some other place. Then</div> <div> calls the function and gives this value to it. This means a copy of the value is sent to</div> <div> the program. The original x remains untouched and unchanged at its place. The</div> <div> function uses the passed value (that has placed at some other place) and manipulates it</div> <div> in its own way. When the control goes back to the calling program, the value of</div> <div> original x is found intact. This is the call by value mechanism.</div> <div> Now let’s see what happens when we pass an array to a function. To pass an array to a</div> <div> function, we will tell the function two things about the array i.e. the name of the array</div> <div> and the size. The size of the array is necessary to pass to the function. As the array is</div> <div> declared in the calling function, it is visible there. The calling function knows its size</div> <div> but the function being called does not know the size of the array. So it is necessary to</div> <div> pass the size of the array along with its name. Suppose we have declared a character</div> <div> array in the program by the following statement:</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 119</div> <div> char name[50] ;</div> <div> We have a function (say reverse, you should write it as an exercise) that reverses the</div> <div> array elements and displays them.</div> <div> Firstly, we need to write the prototype of the function reverse. We say that this</div> <div> function returns nothing so we use the keyword void in its return type. Secondly, we</div> <div> have to write the parameters this function will get. We write these parameters with</div> <div> their type.</div> <div> Now the prototype of this function will be written as</div> <div> void reverse ( char [], int ) ;</div> <div> In the above statement, the brackets [] are necessary. These brackets indicate that an</div> <div> array of type char will be passed to the function. If we skip these brackets and simply</div> <div> write char, it will mean that a single character will be passed to the function. In</div> <div> addition, the second parameter i.e. of type int, is of array's size. Note that in the</div> <div> prototype of the function we have not written the names of the parameters. It is not</div> <div> necessary to write the names of the parameters in function prototype. However, if we</div> <div> write the names, it is not an error. The compiler will simply ignore these names.</div> <div> Now we will define the function reverse. In the function's definition, we will use the</div> <div> array and variable names. These names are local to this function so we can give these</div> <div> variables a name other than the one used in declaration in the calling program. We</div> <div> write this as below.</div> <div> void reverse ( char characters [], int arraySize )</div> <div> {</div> <div> // The body of the function.</div> <div> }</div> <div> Here, the body of the function is left over for an exercise.</div> <div> Let’s say we have a character array name and a name ‘adnan’ is stored in it. We call</div> <div> the reverse function by passing the array name to it. For this we write reverse (</div> <div> name, 100 );</div> <div> In this function call, we are sending the name of the array to the function i.e. name</div> <div> and the size of the array that is 100. When this call of the function is executed the</div> <div> control goes to the function reverse. The statements in this function are executed</div> <div> which reverses the array and displays it. After this, the control comes back to the main</div> <div> function to the statement next to the function call statement. The return type of the</div> <div> function is void so it does not return any thing. Now in the main, we write the</div> <div> statement cout << name; What will be displayed by this statement? Whether it will be</div> <div> the original name ‘adnan’ or something else. It will display the reversed array. In this</div> <div> instance, we see that whatever the function reverse did to the array ( that was passed</div> <div> to it) is appearing in the calling function. It means that the original array in the calling</div> <div> program has been changed. Here we change (reverse) the order of the characters of</div> <div> array in the function and find that the characters of the array in the calling function are</div> <div> reversed. This means that the called function has not a copy of the array but has the</div> <div> original array itself. Whereas in case of simple variables, a called function uses a copy</div> <div> of variables passed to it in a 'call by value' mechanism, which is by default in case of</div> <div> simple variables. In arrays, the by default mechanism is ‘call by reference’. While</div> <div> passing arrays to a function, we don’t need to use & and * operators, as we use for</div> <div> variables in call by reference mechanism.</div> <div> Thus if we pass an array to a function, the array itself is passed to the function. This is</div> <div> due to the fact that when we declare an array, the name of the array has the address of</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 120</div> <div> the memory location from where the array starts. In other words, it is the address of</div> <div> the first element of the array. Thus the name of the array actually represents the</div> <div> address of the first location of the array. Passing the name of array to a function</div> <div> means the passing of the address of the array which is exactly the same as we do in</div> <div> call by reference. So whatever the function does to the array, it is happening in the</div> <div> same memory locations where the array originally resides. In this way, any</div> <div> modifications that the function does to the contents of the array are taking place in the</div> <div> contents of the original array too. This means that any change to the array made by</div> <div> the function will be reflected in the calling program. Thus an important point to</div> <div> remember is that whenever we pass simple variables to a function, the default</div> <div> mechanism is call by value and whenever we pass an array to a function, the default</div> <div> mechanism is call by reference. We know that when we talk about a single element of</div> <div> an array like x [3] (which means the fourth element of the array x), it is treated as</div> <div> simple variable. So if we pass a single element of an array to a function (let’s say like</div> <div> fn ( x [3] ); ), it is just like a simple variable whose copy is passed to the function (as</div> <div> it is a call by value). The original value of the element in the array remains the same.</div> <div> So be careful while passing arrays and a single element of array to functions. This can</div> <div> be well understood from the following examples.</div> <div> Example 1</div> <div> Suppose we declare an array in the main program and pass this array to a function,</div> <div> which populates it with values. After the function call, we display the elements of the</div> <div> array and see that it contains the values that were given in the function call. This</div> <div> demonstrates that the called function changes the original array passed to it.</div> <div> Following is the code of the program.</div> <div> //This program demonstrates that when an array is passed to a function then it is a call by</div> <div> //reference and the changes made by the function effects the original array</div> <div> # include <iostream.h></div> <div> void getvalues( int [], int) ;</div> <div> main ( )</div> <div> {</div> <div> int num [10], i ;</div> <div> getvalues ( num, 10) ; //function call, passing array num</div> <div> //display the values of the array</div> <div> cout << “\n The array is populated with values \n” ;</div> <div> for ( i = 0 ; i < 10 ; i ++)</div> <div> cout << " num[" << i << "] = " << num[i]<< endl ;</div> <div> }</div> <div> void getvalues ( int num[], int arraysize)</div> <div> {</div> <div> int i ;</div> <div> for ( i = 0 ; i < arraysize ; i ++)</div> <div> num[i] = i ;</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 121</div> <div> Here in the function getvalues, we can get the values of the array from user by using</div> <div> the cin statement.</div> <div> Following is the output of the execution of the program.</div> <div> The array is populated with values</div> <div> num[0] = 0</div> <div> num[1] = 1</div> <div> num[2] = 2</div> <div> num[3] = 3</div> <div> num[4] = 4</div> <div> num[5] = 5</div> <div> num[6] = 6</div> <div> num[7] = 7</div> <div> num[8] = 8</div> <div> num[9] = 9</div> <div> Multidimensional Arrays</div> <div> There may be many applications of arrays in daily life. In mathematics, there are</div> <div> many applications of arrays. Let’s talk about vectors. A vector is a set of values which</div> <div> have independent coordinates. There may be two-dimensional vector or threedimensional</div> <div> vector. There are dot and cross products of vectors besides many other</div> <div> manipulations. We do all the manipulations using arrays. We manipulate the arrays</div> <div> with loops. Then there is a mathematical structure matrix, which is in rows and</div> <div> columns. These rows and columns are manipulated in two-dimensional arrays. To</div> <div> work with rows and columns, C provides a structure i.e. a two-dimensional array. A</div> <div> two dimensional array can be declared by putting two sets of brackets [] with the</div> <div> name of array. The first bracket represents the number of rows while the second one</div> <div> depicts the number of columns. So we can declare an array numbers of two rows and</div> <div> three columns as follows.</div> <div> int numbers [2] [3] ;</div> <div> Using two-dimensional arrays, we can do the addition, multiplication and other</div> <div> manipulations of matrices. A value in a two-dimensional array is accessed by using</div> <div> the row number and column number. To put values in a two-dimensional array is</div> <div> different from the one-dimensional array. In one-dimensional array, we use a single</div> <div> 'for loop' to populate the array while nested loops are used to populate the twodimensional</div> <div> array.</div> <div> We can do addition, multiplication and other manipulations of two-dimensional</div> <div> arrays. In C language, we can declare arrays of any number of dimensions (i.e. 1, 2, 3</div> <div> … n ). We declare a n-dimensional array by putting n pair of brackets [] after the</div> <div> name of the array. So a three-dimensional array with values of dimensions 3, 5 and 7</div> <div> respectively, will be declared as int num [3] [5] [7] ;</div> <div> Example 2</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 122</div> <div> Let’s have a matrix (two-dimensional array) of two rows and three columns. We want</div> <div> to fill it with values from the user and to display them in two rows and three columns.</div> <div> Solution</div> <div> To solve this problem, we use a two-dimensional array of two rows and three</div> <div> columns. First, we will declare the array by writing</div> <div> int matrix [2] [3] ;</div> <div> We declare different variables in our program. To put the values in the array, we use</div> <div> two nested for loops, which can be written as under.</div> <div> for ( row = 0 ; row < maxrows ; row ++ )</div> <div> {</div> <div> for ( col = 0 ; col < maxcols ; col ++)</div> <div> {</div> <div> cout << “Please enter a value for position [“ << row << “, ” << col << ”]” ;</div> <div> cin >> matrix [row] [col] ;</div> <div> }</div> <div> }</div> <div> The inner for loop totals the elements of the array one row at a time. It fills all the</div> <div> columns of a row. The outer for loop increments the row after each iteration. In the</div> <div> above code segment, the inner loop executes for each iteration of the outer loop. Thus,</div> <div> when the outer loop starts with the value of row 0, the inner loop is executed for a</div> <div> number of iterations equal to the number of columns i.e. 3 in our program. Thus the</div> <div> first row is completed for the three columns with positions [0,0], [0,1] and [0,2]. Then</div> <div> the outer loop increments the row variable to 1 and the inner loop is again executed</div> <div> which completes the second row (i.e. the positions [1,0], [1,1] and [1,2] ). All the</div> <div> values of matrix having two rows and three columns are found.</div> <div> Similarly, to display these values one by one, we again use nested loops.</div> <div> Following is the code of the program.</div> <div> //This program takes values from user to fill a two-dimensional array (matrix) having two</div> <div> //rows and three columns. And then displays these values in row column format.</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> int matrix [2] [3], row, col, maxrows = 2, maxcols = 3 ;</div> <div> // get values for the matrix</div> <div> for ( row = 0 ; row < maxrows ; row ++)</div> <div> {</div> <div> for (col = 0 ; col < maxcols ; col ++)</div> <div> {</div> <div> cout << “Please enter a value for position [“ << row << “, ” << col << ”] ” ;</div> <div> cin >> matrix [row] [col] ;</div> <div> }</div> <div> }</div> <div> // Display the values of matrix</div> <div> cout << “The values entered for the matrix are “ << endl ;</div> <div> for ( row = 0 ; row < maxrows ; row ++)</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 123</div> <div> {</div> <div> for (col = 0 ; col < maxcols ; col ++)</div> <div> {</div> <div> cout << “\t” << matrix [row] [col] ;</div> <div> }</div> <div> cout << endl ; //to start a new line for the next row</div> <div> }</div> <div> }</div> <div> A sample output of the program is given below.</div> <div> Please enter a value for position [0,0] 1</div> <div> Please enter a value for position [0,1] 2</div> <div> Please enter a value for position [0,2] 3</div> <div> Please enter a value for position [1,0] 4</div> <div> Please enter a value for position [1,1] 5</div> <div> Please enter a value for position [1,2] 6</div> <div> The values entered for the matrix are</div> <div> 1 2 3</div> <div> 4 5 6</div> <div> Tips</div> <div> A character array can be initialized using a string literal</div> <div> Individual characters in a string stored in an array can be accessed directly</div> <div> using array subscript</div> <div> Arrays are passed to functions by reference</div> <div> To pass an array to a function, the name of the array(without any brackets) is passed</div> <div> along with its size</div> <div> To receive an array, the function’s parameter list must specify that an array will be</div> <div> received</div> <div> Including variable names in function prototype is unnecessary. The compiler ignores</div> <div> these names.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 124</div> <div> Lecture No. 13</div> <div> Reading Material</div> <div> Deitel & Deitel – C++ How to Program Chapter 4</div> <div> 4.5, 4.9</div> <div> Summary</div> <div> Array Manipulation</div> <div> Real World Problem and Design Recipe</div> <div> Exercises</div> <div> Array Manipulation</div> <div> We have already discussed what an array is. Identical or similar values are stored in</div> <div> an array. The identical and similar terms here are related to the context of the problem</div> <div> we try to solve. For example, height or age of an individual is a number. We don't</div> <div> store height and age in one array as, in contextual terms, they are different things.</div> <div> These can not be mixed in one array. So the height of individuals will be stored in one</div> <div> array and the age in some other one. The idea behind the array is that whenever you</div> <div> have similar data with multiple values, it is easier and more elegant to store them in</div> <div> an array.</div> <div> Let's try to find out, how to process arrays. What is the easiest way and what are the</div> <div> issues related to this process.</div> <div> As discussed in previous lectures, whenever we come across an array, we start</div> <div> thinking in terms of loops. We pick up the first element of the array and process it.</div> <div> Then the second array element is processed and so on. Naturally that falls into an</div> <div> iterative structure.</div> <div> Let's try to understand how to process a two dimensional array. The following</div> <div> example can help us comprehend it effectively.</div> <div> Suppose we have a two-dimensional array of numbers. While dealing with a twodimensional</div> <div> array of numbers, we should try to understand it in terms of a matrix.</div> <div> Matrices in mathematics have rows and column and there is always a number at each</div> <div> row and column intersection. Suppose we have a matrix of dimension 3 * 3 i.e. a</div> <div> simple two-dimensional array. We want to input some numbers to that array first.</div> <div> After reading these numbers, we want to output them in such a fashion that the last</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 125</div> <div> row is printed first, followed by second last and so on till the first row that is printed</div> <div> at the bottom. We don't want to change the column numbers with this output. It is not</div> <div> a difficult task. As it is a two-dimensional array so there is a row subscript and a</div> <div> column subscript. Following example will make the matter further clear.</div> <div> Suppose we have the following array:</div> <div> int a[3][3];</div> <div> We will access elements of it as: a[row index][column index] e.g. a[1][2]. This is a</div> <div> single element at row 1 and column 2 of array a.</div> <div> The flow chart to read in numbers into the two-dimensional array is given on the next</div> <div> page. See the code snippet below:</div> <div> const int maxRows = 3;</div> <div> const int maxCols = 3;</div> <div> int row, col;</div> <div> int a[maxRows][maxCols];</div> <div> // To input numbers in the array</div> <div> for (row = 0; row < maxRows; row ++)</div> <div> {</div> <div> for(col=0; col < maxCols; col ++)</div> <div> {</div> <div> cout << "\n" << "Enter " << row << "," << col << "element: ";</div> <div> cin >> a[row][col];</div> <div> }</div> <div> }</div> <div> Now let's see what this nested loop structure is doing. The outer loop takes the first</div> <div> row i.e. row 0, then instantly inner loop begins which reads col 0, 1 and 2 elements of</div> <div> the row 0 into the array. Afterwards, control goes back to the outer loop. The row</div> <div> counter is incremented and becomes 1 i.e. row 1 or second row is taken for</div> <div> processing. Again, the inner loop reads all the elements of second row into the array.</div> <div> This process goes on until all the elements for three rows and three columns array are</div> <div> read and stored in the array called a.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 126</div> <div> No</div> <div> maxRows = n</div> <div> maxCols = n</div> <div> a[maxRows][maxCol</div> <div> s]</div> <div> while</div> <div> row <</div> <div> maxRows</div> <div> Yes</div> <div> No</div> <div> col = 0</div> <div> while</div> <div> col <</div> <div> maxCols</div> <div> Read</div> <div> a[row][col]</div> <div> Yes</div> <div> col++</div> <div> row++</div> <div> Flow Chart to Input Two-dimensional Array</div> <div> Exit</div> <div> Exit</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 127</div> <div> Now we want to reverse the rows of the matrix (flip the matrix) and display it. There</div> <div> are several ways of doing it. You might have already started thinking of how can we</div> <div> flip the matrix. We may declare a new matrix and copy the array elements into this</div> <div> matrix while flipping the elements at the same time. But we should keep in mind the</div> <div> problem statement. The problem statement is 'to read the array elements and then</div> <div> simply display it in the reverse row order'. It does not state anything about storing the</div> <div> elements inside the memory.</div> <div> Please see the flow chart to display the flipped matrix on the next page.</div> <div> Normally, we start our loops from zero and keep incrementing the counter until a</div> <div> certain bigger value is attained. But this is not mandatory. We can start from a bigger</div> <div> number and keep on decrementing the counter every time. To display the rows in</div> <div> reverse order, we can start from the last row and go to the first row by decrementing</div> <div> the row counter every time. It is very simple programming trick. However, we have to</div> <div> take care of the value of the index.</div> <div> We can write our code inside nested loops for flipping the elements as under-</div> <div> // To flip the elements of the matrix</div> <div> cout << '\n' << "The flipped matrix is: " << '\n';</div> <div> for ( row = maxRows-1; row >= 0; row --)</div> <div> {</div> <div> for ( col = 0; col < maxCols; col ++)</div> <div> {</div> <div> cout << a [row][col] << '\t';</div> <div> }</div> <div> cout << '\n';</div> <div> }</div> <div> Note the '\t' character in the above code. It is a tab character that displays tab (spaces)</div> <div> at the cursor position on the screen. Similary '\n' as told in previous lectures is newline</div> <div> character which takes the cursor to the new line.</div> <div> It is better to print the original matrix elements before showing the flipped matrix</div> <div> elements so that you can really see whether your function has flipped the matrix or</div> <div> not.</div> <div> To run this function for the big-sized arrays, adjust the values of the maxRows and</div> <div> maxCols constants as the rest of the program remains the same..</div> <div> Whenever we work with arrays, normally the loops are there. If the array is single</div> <div> dimensional, there will be one loop. A two-dimensional arrays is going to have pair of</div> <div> nested loops and so on.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 128</div> <div> No</div> <div> Flow Chart to Display Array Elements in the Reverse</div> <div> Row Order</div> <div> maxRows = n</div> <div> maxCols = n</div> <div> a[maxRows][maxCols]</div> <div> while</div> <div> row >= 0</div> <div> Yes</div> <div> No</div> <div> col = 0</div> <div> while</div> <div> col <</div> <div> maxCols</div> <div> Print</div> <div> a[row][col]</div> <div> Yes</div> <div> col++</div> <div> row--</div> <div> Input the array ‘a’</div> <div> elements</div> <div> row = maxRows - 1</div> <div> Exit</div> <div> Exit</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 129</div> <div> /* Array Manipulation - Flipping of a Matrix (reversing the row order): This program reads a</div> <div> matrix (two-dimensional array), displays its contents and also displays the flipped matrix</div> <div> */</div> <div> #include <iostream.h></div> <div> const int maxRows = 3;</div> <div> const int maxCols = 3;</div> <div> void readMatrix(int arr[][maxCols]);</div> <div> void displayMatrix(int a[][maxCols]);</div> <div> void displayFlippedMatrix(int a[][maxCols]);</div> <div> void main(void)</div> <div> {</div> <div> int a[maxRows][maxCols];</div> <div> // Read the matrix elements into the array</div> <div> readMatrix(a);</div> <div> // Display the original matrix</div> <div> cout << "\n\n" << "The original matrix is: " << '\n';</div> <div> displayMatrix(a);</div> <div> // Display the flipped matrix</div> <div> cout << "\n\n" << "The flipped matrix is: " << '\n';</div> <div> displayFlippedMatrix(a);</div> <div> }</div> <div> void readMatrix(int arr[][maxCols])</div> <div> {</div> <div> int row, col;</div> <div> for (row = 0; row < maxRows; row ++)</div> <div> {</div> <div> for(col=0; col < maxCols; col ++)</div> <div> {</div> <div> cout << "\n" << "Enter " << row << ", " << col << " element: ";</div> <div> cin >> arr[row][col];</div> <div> }</div> <div> cout << '\n';</div> <div> }</div> <div> }</div> <div> void displayMatrix(int a[][maxCols])</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 130</div> <div> {</div> <div> int row, col;</div> <div> for (row = 0; row < maxRows; row ++)</div> <div> {</div> <div> for(col = 0; col < maxCols; col ++)</div> <div> {</div> <div> cout << a[row][col] << '\t';</div> <div> }</div> <div> cout << '\n';</div> <div> }</div> <div> }</div> <div> void displayFlippedMatrix(int a[][maxCols])</div> <div> {</div> <div> int row, col;</div> <div> for (row = maxRows - 1; row >= 0; row --)</div> <div> {</div> <div> for(col = 0; col < maxCols; col ++)</div> <div> {</div> <div> cout << a[row][col] << '\t';</div> <div> }</div> <div> cout << '\n';</div> <div> }</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 131</div> <div> Till now, we have only solved very simple problems to understand processing of</div> <div> arrays. You can test your capability of doing so through an exercise by inputting</div> <div> (reading in) a matrix and print it in reverse column order. Here, the rows remain the</div> <div> same.</div> <div> Let's move on to slightly more practical problem. Before going ahead, we need to</div> <div> understand the concept of Transpose of a Matrix. Transpose of a matrix means that</div> <div> when we interchange rows and columns, the first row becomes the first column,</div> <div> second row becomes the second column and so on. Mathematically, the transpose can</div> <div> be written as:</div> <div> A(i,j) should be replaced with A(j,i) where i and j are row and column indexes.</div> <div> For this purpose, we take a square matrix (a matrix with equal number of rows and</div> <div> columns) to transpose. Here, if you are thinking in terms of loops, you are absolutely</div> <div> right. Let's say the array is 'a', with dimension as ‘arraySize’. Please see the flow chart</div> <div> for this problem on the next page.</div> <div> We write a pair of nested loops:</div> <div> int temp;</div> <div> for (row = 0; row < arraySize; row ++)</div> <div> {</div> <div> for (col = 0; col < arraySize; col ++)</div> <div> {</div> <div> // Interchange the values here using the swapping mechanism</div> <div> temp = a[row][col]; // Save the original value in the temp variable</div> <div> a[row][col] = a[col][row];</div> <div> a[col][row] = temp; //Take out the original value</div> <div> }</div> <div> }</div> <div> While interchanging values, we should be careful. We can't simply write: a[row][col]</div> <div> = a[col][row]. We will lose information this way. We need a swapping mechanism</div> <div> here to interchange the elements properly.</div> <div> We have yet to do more to get the problem solved. You are strongly recommended to</div> <div> write this program and run it to see the problem area.</div> <div> It is something interesting that we are interchanging the value of first row, first</div> <div> column with itself, which means nothing. When we are doing transpose of a matrix,</div> <div> the diagonal elements will remain unchanged as the row and column indexes are the</div> <div> same. Then we interchange the row 0, col 1 element with row 1, col 0. The row 0, col</div> <div> 2 element with row 2, col 0. What will happen when we process second row i.e. row</div> <div> 1. The row 1, col 0 will be swapped with row 0, col 1 but these are the same elements,</div> <div> already swapped in the above iteration. Therefore, this is the problem area that</div> <div> elements swapped once are swapped again to their original positions if the loops are</div> <div> run in all the rows and columns. As a result, the resultant matrix remains unchanged.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 132</div> <div> No</div> <div> arraySize = n</div> <div> a[arraySize][arraySize]</div> <div> while</div> <div> row <</div> <div> arraySize</div> <div> Yes</div> <div> No</div> <div> col = row</div> <div> while</div> <div> col <</div> <div> arraySize</div> <div> temp = a[row][col]</div> <div> a[row][col] = a[col][row]</div> <div> a[col][row] = temp</div> <div> Yes</div> <div> col++</div> <div> row ++</div> <div> Flow Chart of Transpose of a Square Matrix</div> <div> Input the array ‘a’</div> <div> elements</div> <div> row = 0</div> <div> Exit</div> <div> Exit</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 133</div> <div> Then what is the solution of the problem?</div> <div> Now draw a matrix on the paper and cut it diagonally. We will get two triangles i.e.</div> <div> upper triangle and lower triangle. We only need to interchange one triangle with the</div> <div> other and not the whole of the matrix. Now the question is, how we can determine the</div> <div> limits of triangles? By looking at a triangle, let's say upper triangle, we can see that all</div> <div> the rows are being processed as the triangle crosses every row. Similarly all the</div> <div> columns are being processed because the first row in the upper triangle covers all the</div> <div> columns. The only difference is that we will not process the beginning element before</div> <div> starting each row. That means that we will not start the inner loop (columns loop)</div> <div> with index 0. Rather we start with the current row number. Therefore, for first row i.e.</div> <div> row 0, we will process from row 0, col 0 to row 0, col arraySize-1. For second row i.e.</div> <div> row 1, we will process from row 1, col 1 to row 1, col arraySize-1 while in case of</div> <div> third row i.e. row 2, we will go from row 2, col 2 to row 2 , col arraySize-1. If you</div> <div> structure the loops in this manner, the correct behavior of matrix transposition will be</div> <div> found.</div> <div> The full source code to solve this problem by taking the upper triangle and swapping</div> <div> it with the lower triangle is given below:</div> <div> /* Array Manipulation - Transpose of a Square Matrix: This program reads a matrix (twodimensional</div> <div> array), displays its contents, transposes it and then displays the transposed matrix.</div> <div> */</div> <div> #include <iostream.h></div> <div> const int arraySize = 3;</div> <div> void readMatrix(int arr[][arraySize]);</div> <div> void displayMatrix(int a[][arraySize]);</div> <div> void transposeMatrix(int a[][arraySize]);</div> <div> void main(void)</div> <div> {</div> <div> int a[arraySize][arraySize];</div> <div> // Read the matrix elements into the array</div> <div> readMatrix(a);</div> <div> // Display the matrix</div> <div> cout << "\n\n" << "The original matrix is: " << '\n';</div> <div> displayMatrix(a);</div> <div> //Transpose the matrix</div> <div> transposeMatrix(a);</div> <div> //Display the transposed matrix</div> <div> cout << "\n\n" << "The transposed matrix is: " << '\n';</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 134</div> <div> displayMatrix(a);</div> <div> }</div> <div> void readMatrix(int arr[][arraySize])</div> <div> {</div> <div> int row, col;</div> <div> for (row = 0; row < arraySize; row ++)</div> <div> {</div> <div> for(col=0; col < arraySize; col ++)</div> <div> {</div> <div> cout << "\n" << "Enter " << row << ", " << col << " element: ";</div> <div> cin >> arr[row][col];</div> <div> }</div> <div> cout << '\n';</div> <div> }</div> <div> }</div> <div> void displayMatrix(int a[][arraySize])</div> <div> {</div> <div> int row, col;</div> <div> for (row = 0; row < arraySize; row ++)</div> <div> {</div> <div> for(col = 0; col < arraySize; col ++)</div> <div> {</div> <div> cout << a[row][col] << '\t';</div> <div> }</div> <div> cout << '\n';</div> <div> }</div> <div> }</div> <div> void transposeMatrix(int a[][arraySize])</div> <div> {</div> <div> int row, col;</div> <div> int temp;</div> <div> for (row = 0; row < arraySize; row ++)</div> <div> {</div> <div> for (col = row; col < arraySize; col ++)</div> <div> {</div> <div> /* Interchange the values here using the swapping mechanism */</div> <div> temp = a[row][col]; // Save the original value in the temp variable</div> <div> a[row][col] = a[col][row];</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 135</div> <div> a[col][row] = temp; //Take out the original value</div> <div> }</div> <div> }</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 136</div> <div> Real Word Problem and Design Recipe</div> <div> We will take one problem that is not very complex but will follow it rigorously for all</div> <div> steps of design recipe.</div> <div> In practical life, the employees get salaries and pay taxes honestly. Sometimes, the</div> <div> process of drawing salaries and payment of taxes may lead to some interesting</div> <div> situation. Suppose, a person draws salary of Rs. 10,000 per month. A certain</div> <div> percentage of tax is charged on that amount, which is deducted every month. But if</div> <div> the salary of the person is more than Rs. 10,000 per month, then the tax rate is</div> <div> different. Similarly if a person is getting Rs. 20,000 per month, he/she would be</div> <div> charged more under a different tax rate slab. The interesting situation develops if there</div> <div> is an anomaly in the tax rates i.e. a person who is getting higher salary takes home</div> <div> lesser money as compared to the other person with less gross salary.</div> <div> To further elaborate it, we suppose that there is company 'C' where 100 or less than</div> <div> 100 persons are employed. The salaries of the employees and their tax rates are</div> <div> known to us. We are required to list those unlucky persons, who are getting lesser</div> <div> take-home salary (net salary) than their colleagues with less gross salaries but lower</div> <div> tax rates.</div> <div> As per our design recipe, let's see what steps we need to follow.</div> <div> A design recipe asks us to analyze the problem first and write it in a precise statement</div> <div> that what actual the problem is. Also by formulating the precise statement, we need to</div> <div> provide some examples to illustrate. At the design phase, we try to break up the</div> <div> problem into functional units and resort to a detailed designing. Then we move to</div> <div> implementation stage where the pseudo code is translated into the computer language</div> <div> and then the program is compiled and run to ensure that it works as expected.</div> <div> At the first step i.e Analysis, we try to have a precise problem statement. Once it is</div> <div> established, we try to determine what are the inputs of this program. What data should</div> <div> be provided to this program. We will also try to determine if there are some constants</div> <div> required for calculation or manipulation. We list down all the constants. Then we split</div> <div> it up into functions and modules.</div> <div> Let's try to make a precise statement of the above problem. The precise problem</div> <div> statement is:</div> <div> "Given tax brackets and given employees gross salaries, determine those employees</div> <div> who actually get less take-home salary than others with lower initial income."</div> <div> Suppose the tax deduction law states that:</div> <div> No tax will be deducted for persons with salaries ranging from Rs. 0 to Rs. 5,000 per</div> <div> month or in other words tax deduction rate is 0%.</div> <div> 5% tax deduction will be made from the persons with salaries ranging from Rs. 5,001</div> <div> to Rs. 10,000 per month.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 137</div> <div> For persons with salaries ranging from Rs. 10,001 to Rs. 20,000, a 10% tax deduction</div> <div> rate would be employed.</div> <div> For persons with salaries ranging from Rs. 20,001 and higher, 15% tax deduction</div> <div> would be made.</div> <div> Taking these rules, let's formulate the problem.</div> <div> Consider the example of a person with a salary of Rs. 10,000 per month. As per rules,</div> <div> he/she would be charged by 5% of tax rate. 5% of 10,000 is 500 rupees. So the take</div> <div> home salary of the person is Rs. 9500.</div> <div> Now the unfortunate individual, whose gross salary is Rs, 10,001 falls in the next</div> <div> bracket of tax rate of 10%. He will have to pay tax worth Rs 1000.1. That means the</div> <div> take home salary of this person is Rs. 9000.9, which is lesser than the person with</div> <div> lower gross salary of Rs. 10,000. This is the problem.</div> <div> We can calculate the net salaries of all individuals, determining all the unlucky ones.</div> <div> Now we will carry out the analysis of the requirements. For looking into the</div> <div> requirements, we have to see, how to input the salaries of these people.</div> <div> As stated in the problem, the number of employees of the company 'C is at most 100.</div> <div> So we know the size of the array. But for some other company, suppose company 'D',</div> <div> we don't know the number of employees. Therefore, it makes sense to take input from</div> <div> the user for the number of employees. Once we have determined the number of</div> <div> employees, we will input the gross salary of each of employees. But where will we</div> <div> store the gross salary? For this purpose, we will use the two-dimensional array. In the</div> <div> first column, we will store the gross salary. Our program after calculating the net</div> <div> salary for each employee will write (store) it in the second column of the array.</div> <div> At the next stage, we will find out the unlucky individuals. This will be based on the</div> <div> analysis of algorithms. At the higher level design, we assume that there would be a</div> <div> way to determine the unlucky individuals. Finally, a list of unlucky employees would</div> <div> be prepared. For that, we will simply output the employee numbers.</div> <div> We want to workout the space and storage requirements of this problem. As earlier</div> <div> mentioned, we will use a two dimensional array to store the gross and net salaries and</div> <div> output the list of unlucky employees. That means we need a storage to store that list.</div> <div> For this, we will take a single dimensional array of 'int' type. We will initialize the</div> <div> array with zero. '0' means the individual is lucky. Therefore, by default, all</div> <div> individuals are lucky. Whenever, we will find an unlucky individual by using the two</div> <div> dimensional array, we will write '1' in single dimensional array for that individual. So</div> <div> this is the storage requirement of the program.</div> <div> Afterwards, we will discuss the interface issues. The interface guidelines are the same</div> <div> i.e. be polite and try to explain what is required from the user. When the program runs</div> <div> the user will know what is required from him/her. So there would be prompts in the</div> <div> program where the user will key in the data. All the input data will be coming from</div> <div> keyboard and displayed on the screen. This is a rudimentary interface analysis.</div> <div> We have distributed the program into four major parts:</div> <div> Input</div> <div> Salary calculation</div> <div> Identification of unlucky individuals and</div> <div> Output</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 138</div> <div> Let's start the coding or detailed design phase of this program.</div> <div> In a true tradition, all the four parts of the program should be function calls. The main</div> <div> program is very simple as it contains functions:</div> <div> Get input</div> <div> Calculate salary</div> <div> Locate unlucky individuals</div> <div> Display output</div> <div> The arrays will be declared inside the main function. As we already know the</div> <div> maximum number of employees is 100, so we can declare it as a constant:</div> <div> const int arraySize=100;</div> <div> double sal[arraySize][2];</div> <div> int lucky[arraySize] = {0}; //Notice the array initialization</div> <div> Once this is done inside main, we want to run the input function to read the salaries of</div> <div> the employees. Now, inside the input data function, we will get value for number of</div> <div> employees from the user. We have already set the upper limit as 100 but the actual</div> <div> number of employees will be entered by the user of the program. If we take that input</div> <div> inside the input data function, what can be the problem. Well, there is no problem in</div> <div> taking the input within that function but the problem is the declaration of the variable</div> <div> 'numEmps', which contains the current number of employees. If the 'numEmps'</div> <div> variable is declared inside the input data function, it will be local to that function.</div> <div> After the input data function returns, the 'numEmps' will no longer be there because it</div> <div> was local to input data function and not visible in any other function. So it is better to</div> <div> declare the variables inside the main function. But the problem arises: how the input</div> <div> data function will get information about it, if we declare it inside main function. We</div> <div> will have to send it to input data function, either through call by reference or we can</div> <div> declare 'numEmp' as a global variable so that it is visible in all the functions. Global</div> <div> variables are useful but tricky. They exist when we need them but they exist even</div> <div> when we don’t need them. Therefore, it might be good to declare this variable</div> <div> 'numEmps' inside main function and then pass by reference to the input data function.</div> <div> While passing one-dimensional array to the function, we write in the function</div> <div> prototype as:</div> <div> f(int a[]);</div> <div> However, when we pass two-dimensional array to a function, we must specify the</div> <div> number of columns because this depends on how a computer stores the two</div> <div> dimensional array in the memory. The computer stores the rows in a contiguous (row</div> <div> after row) fashion inside memory. Therefore , in order to locate where the first row</div> <div> has finished or the second row starts, it should know the number of columns.</div> <div> Whenever, we pass two-dimensional array to a function, the number of columns</div> <div> inside that array should be specified. We will pass two dimensional array 'sal' to input</div> <div> data function getInput() in the same manner. We also want to pass 'numEmps'</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 139</div> <div> variable by reference using the '&' sign to this function. This will ensure that whatever</div> <div> the user inputs inside this function getInput(), will be available in the main function.</div> <div> There is another way that we get input from the user inside the main function and then</div> <div> pass this by value to the getInput() function. We are going to do the same in our</div> <div> function.</div> <div> getInput(double sal[][2], int numEmps);</div> <div> {</div> <div> for (int i = 0; i < numEmps; i ++) //Note that this numEmps is local to this</div> <div> function</div> <div> {</div> <div> cin >> sal[i][0]; // Get the gross salary for each employee</div> <div> }</div> <div> }</div> <div> To calculate tax, we will write a function. This function will be passed in similar</div> <div> parameters as getInput function to calculate the taxes for all the employees. There is</div> <div> one important point to reiterate here i.e. by default, arrays are passed by reference.</div> <div> That means if getInput() function puts some values in the 'sal' array, these are written</div> <div> in the 'sal' array and are available inside main function. The 'numEmps' variable on</div> <div> the other hand is passed by value to getInput() function. Therefore, any changes done</div> <div> by geInput() function will not affect the original value of 'numEmps' inside the main</div> <div> function.</div> <div> We will continue with this problem to determine algorithm that what is the precise</div> <div> sequence of steps to determine the unlucky employees. For this, we need to analyze a</div> <div> bit more because it contains a complex 'if' condition. The function to calculate net</div> <div> salary also has interesting issues which will be explained in the next lecture.</div> <div> Here is the source code of the first cut solution for real world problem:</div> <div> * This is the first cut of the program to solve the real world problem of</div> <div> 'Unlucky Employees' */</div> <div> #include <iostream.h></div> <div> void getInput(double sal[][2], int numEmps);</div> <div> void calcNetSal(double sal[][2], int numEmps);</div> <div> void findUnluckies(double sal[][2], int numEmps, int lucky[]);</div> <div> void markIfUnlucky(double sal[][2], int numEmps, int lucky[], int upperBound, int empNbr);</div> <div> void printUnluckies(int lucky[], int numEmps);</div> <div> void main(void)</div> <div> {</div> <div> const int arraySize=100;</div> <div> double sal[arraySize][2];</div> <div> int lucky[arraySize] = {0};</div> <div> int numEmps;</div> <div> /* Read the actual number of employees in the company */</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 140</div> <div> cout << "\n Please enter the total number of employees in your company: ";</div> <div> cin >> numEmps;</div> <div> cout << '\n';</div> <div> /* Read the gross salaries of the employees into the array 'sal' */</div> <div> getInput(sal, numEmps);</div> <div> /* Calculate net salaries of the employees and store them in the array */</div> <div> cout << "\n\n Calculating the net salaries ... ";</div> <div> calcNetSal(sal, numEmps);</div> <div> /* Find the unlucky employees */</div> <div> cout << "\n\n Locating the unlucky employees ... ";</div> <div> findUnluckies(sal, numEmps, lucky);</div> <div> /* Print the unlucky employee numbers */</div> <div> cout << "\n\n Printing the unlucky employees ... ";</div> <div> printUnluckies(lucky, numEmps);</div> <div> }</div> <div> void getInput(double sal[][2], int numEmps)</div> <div> {</div> <div> for (int i = 0; i < numEmps; i++) //Note that this numEmps is local to this function</div> <div> {</div> <div> cout << "\n Please enter the gross salary for employee no." << i << ": ";</div> <div> cin >> sal[i][0]; // Store the gross salary for each employee</div> <div> }</div> <div> }</div> <div> void calcNetSal(double sal[][2], int numEmps)</div> <div> {</div> <div> for (int i = 0; i < numEmps; i++) //Note that this numEmps is local to this function</div> <div> {</div> <div> if(sal[i][0] >= 0 && sal[i][0] <= 5000)</div> <div> {</div> <div> /* There is no tax deduction */</div> <div> sal[i][1] = sal[i][0];</div> <div> }</div> <div> else if(sal[i][0] >= 5001 && sal[i][0] <= 10000)</div> <div> {</div> <div> /* Tax deduction is 5% */</div> <div> sal[i][1] = sal[i][0] - (.05 * sal[i][0]);</div> <div> }</div> <div> else if (sal[i][0] >= 10001 && sal[i][0] <= 20000)</div> <div> {</div> <div> /* Tax deduction is 10% */</div> <div> sal[i][1] = sal[i][0] - (.10 * sal[i][0]);</div> <div> }</div> <div> else if (sal[i][0] >= 20001)</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 141</div> <div> {</div> <div> /* Tax deduction is 15% */</div> <div> sal[i][1] = sal[i][0] - (.15 * sal[i][0]);</div> <div> }</div> <div> else</div> <div> {</div> <div> /* No need to do anything here */</div> <div> }</div> <div> }</div> <div> }</div> <div> void findUnluckies(double sal[][2], int numEmps, int lucky[])</div> <div> {</div> <div> for (int i = 0; i < numEmps; i++) //Note that this numEmps is local to this function</div> <div> {</div> <div> if(sal[i][0] >= 0 && sal[i][0] <= 5000)</div> <div> {</div> <div> /* No need to check for unlucky employees for this tax bracket */</div> <div> ;</div> <div> }</div> <div> else if(sal[i][0] >= 5001 && sal[i][0] <= 10000)</div> <div> {</div> <div> markIfUnlucky(sal, numEmps, lucky, 5001, i);</div> <div> }</div> <div> else if (sal[i][0] >= 10001 && sal[i][0] <= 20000)</div> <div> {</div> <div> markIfUnlucky(sal, numEmps, lucky, 10001, i);</div> <div> }</div> <div> else if (sal[i][0] >= 20001)</div> <div> {</div> <div> markIfUnlucky(sal, numEmps, lucky, 20001, i);</div> <div> }</div> <div> }</div> <div> }</div> <div> void markIfUnlucky(double sal[][2], int numEmps, int lucky[], int upperBound, int empNbr)</div> <div> {</div> <div> for (int i = 0; i < numEmps; i++)</div> <div> {</div> <div> /*</div> <div> See the if the condition below, it will mark the employee</div> <div> unlucky even if an employee in the higher tax bracket is getting</div> <div> the same amount of net salary as that of a person in the lower</div> <div> tax bracket</div> <div> */</div> <div> if (sal[i][0] < upperBound && sal[i][1] >= sal[empNbr][1])</div> <div> {</div> <div> lucky[empNbr] = 1; //Employee marked as unlucky</div> <div> break;</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 142</div> <div> }</div> <div> }</div> <div> void printUnluckies(int lucky[], int numEmps)</div> <div> {</div> <div> for (int i = 0; i < numEmps; i++)</div> <div> {</div> <div> if(lucky[i] == 1)</div> <div> {</div> <div> cout <<"\n Employee No.: " << i;</div> <div> }</div> <div> }</div> <div> }</div> <div> Exercises</div> <div> Suppose you have a Square matrix of order 5 * 5. Draw flow chart and write a</div> <div> program to input (read in) a matrix and print it in reverse column order, the rows</div> <div> remain the same.</div> <div> Suppose you have a Square matrix of order 5 * 5. Draw flow chart and write a</div> <div> program to transpose the matrix, take lower triangle and swap it with upper triangle.</div> <div> An Identity matrix is a square matrix whose diagonal elements are '1' and remaining</div> <div> elements are '0'. Suppose you are given a square matrix of size n * n. Write a program</div> <div> to determine if this is an Identity matrix.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 143</div> <div> Lecture No. 14</div> <div> Reading Material</div> <div> Deitel & Deitel - C++ How to Program</div> <div> Chapter 5</div> <div> 5.1, 5.2, 5.3, 5.4,</div> <div> 5.5, 5.6</div> <div> Summary</div> <div> 1) Pointers</div> <div> 2) Declaration of Pointers</div> <div> 3) Example 1 (Bubble Sort)</div> <div> 4) Pointers and Call By Reference</div> <div> 5) Example 2</div> <div> Pointers</div> <div> In the earlier lectures, we had briefly referred to the concept of pointers.</div> <div> Let’s see what a pointer is and how it can be useful.</div> <div> Pointers are a special type of variables in which a memory address is</div> <div> stored. They contain a memory address, not the value of the variable.</div> <div> The concept of the pointers can be well understood from the following</div> <div> example.</div> <div> Suppose, we request someone to take a parcel to the house of a</div> <div> person, named Ahmad. Here the point of reference is a name.</div> <div> However, if we specifically tell him the number of house and the street</div> <div> number. Then this is a reference by the address of the house. It means</div> <div> that we have two ways to locate an address. To understand further the</div> <div> concept of memory address, the example of the computers can be</div> <div> helpful. In computers, one can have a name x which is associated with</div> <div> a memory location. We can have the memory address of x, say 6000 or</div> <div> whatever it is. So the simple variable names are those of specific</div> <div> locations in memory. But in terms of addresses, these are the</div> <div> addresses of those memory locations. We can use these names and</div> <div> addresses interchangeably to refer to memory locations. When a value</div> <div> is referred by a normal variable is known as direct reference. While the</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 144</div> <div> value referred through the use of memory address may be known as</div> <div> indirect reference.</div> <div> To understand further the terms of direct reference and indirect</div> <div> reference, suppose that we want to assign a value 10 to x. This can be</div> <div> done by writing x = 10. In this statement, the value 10 will be</div> <div> assigned to the memory location which has label (name) x. The second</div> <div> way to assign a value to a memory location is with reference to the</div> <div> address of that memory location. In other words, ‘assign a value to the</div> <div> memory location whose address is contained in the variable (that is a</div> <div> pointer) on right hand side of the assignment operator’. Operators are</div> <div> used to refer the address of memory locations and to refer the values at</div> <div> those addresses.</div> <div> Following figure shows directly and indirectly referencing a variable.</div> <div> x directly references</div> <div> a variable whose value is 10</div> <div> x</div> <div> xptr indirectly references a variable</div> <div> whose value is 10 xptr x</div> <div> Now we will try to comprehend the concept with another daily life</div> <div> example. Suppose, hundreds of people are sitting in an auditorium. The</div> <div> host is going to announce a prize for a person amongst the audience.</div> <div> There are two methods to call the prizewinner to dais. The host can</div> <div> either call the name of the person or the number of the seat. These are</div> <div> equivalent to ‘call by name’ and ‘call by address’ methods. In both</div> <div> cases, the prize will be delivered to a person whether he is called by</div> <div> name or referred by address (seat number in this case). In</div> <div> programming, pointers are used to refer by the addresses.</div> <div> Declaration of Pointers</div> <div> Pointers work by pointing to a particular data type. We can have pointer</div> <div> to an integer, pointer to a double, pointer to a character and so on. It</div> <div> 10</div> <div> 10</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 145</div> <div> means that a type is associated to a pointer. Pointer, being a variable,</div> <div> needs a name. The rules for naming a pointer are the same as for the</div> <div> simple variable names. The pointers are declared in a specific way. The</div> <div> syntax of declaring a pointer is:</div> <div> data type *name ;</div> <div> Here ‘name’ is the name of the pointer and data type is the type of the</div> <div> data to which the pointer (name) points. There is no space between</div> <div> asterisk (*) and the name. Each variable being declared as a pointer</div> <div> must be preceded by *. The * is associated with the name of the</div> <div> variable, not with the data type. To associate the * (asterisk) with data</div> <div> type (like int* ) may confuse the declaration statement. Suppose, we</div> <div> want to declare a pointer to an integer. We will write as:</div> <div> int *myptr;</div> <div> Here myptr is the name of the pointer. The easiest way to understand</div> <div> the pointer declaration line is the reading the statement from right to left.</div> <div> For the above statement, we say that myptr is a pointer to an integer</div> <div> (int). Similarly for the declaration double *x , x is a pointer to a data of</div> <div> type double. The declaration of char *c shows that c is a pointer to a</div> <div> data of type character. The declaration of multiple pointers requires the</div> <div> use of * with each variable name. This is evident from the following</div> <div> example which declares three pointers.</div> <div> int *ptr1, *ptr2, *ptr3 ;</div> <div> Moreover, we can mix the pointers declaration with simple variables on</div> <div> one line.</div> <div> int *ptr, x, a [10] ;</div> <div> In this declaration ptr is a pointer to data of type int, x is a simple</div> <div> variable of type int and a is an array of integers.</div> <div> Whenever used, these pointers hold memory addresses.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 146</div> <div> Now we will try to understand what address a pointer holds. Suppose,</div> <div> we declare a pointer variable ptr and a variable x and assign a value 10</div> <div> to it. We write this as under.</div> <div> int *ptr ;</div> <div> int x ;</div> <div> x = 10 ;</div> <div> Here x is a name of a memory location where a value 10 is stored. We</div> <div> want to store the address of this memory location (which is labeled as x)</div> <div> into the pointer ptr. To get the address of x, we use address operator</div> <div> i.e. &. (it is & not &&, the && is logical AND). To assign the address of x</div> <div> to pointer ptr, we write</div> <div> ptr = &x ;</div> <div> This statement assigns the memory address of the location x to the</div> <div> pointer ptr. The following figure shows a schematic representation of</div> <div> memory after the preceding assignment is executed.</div> <div> x</div> <div> ptr</div> <div> The pointers contain whole numbers as they contain memory</div> <div> addresses. An address can be represented only in whole numbers.</div> <div> Therefore, a pointer is a whole number, sufficient enough, to hold any</div> <div> memory address of the computer. The pointers have no specific data</div> <div> type.</div> <div> 1</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 147</div> <div> In the above assignment statement, we have a pointer to a memory</div> <div> location. Now, it can be ascertained what value is stored in that memory</div> <div> location. To get the value stored at a memory address, we use the</div> <div> dereferencing operator, represented by asterisk (*). The * is used with</div> <div> the name of the pointer to get the value stored at that address. To get</div> <div> the value stored at the memory address ptr, we write *ptr which is read</div> <div> as the value of whatever ptr points to. Thus the line z = *ptr; means, z</div> <div> has the value of whatever ptr points to.</div> <div> The following example can explain the representation of the pointer in</div> <div> memory. Assume that variable x is stored at location 400000 and</div> <div> pointer variable ptr is stored at location 500000.</div> <div> ptr</div> <div> x</div> <div> Address: 500000</div> <div> 400000</div> <div> We can use this operator (*) to get the value and can do any arithmetic</div> <div> operation with it. The following statements make it further clear.</div> <div> z = *ptr + 2 ;</div> <div> z = *ptr * 2 ;</div> <div> z = *ptr – 2 ;</div> <div> Here *ptr gives the value stored at memory address where the pointer</div> <div> ptr points to.</div> <div> We know that it is a good programming practice to initialize a variable</div> <div> when we declare it. This will ensure that there will be no unknown value</div> <div> in the variable at some later stage.</div> <div> 400000 10</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 148</div> <div> Similarly, we should assign an initial value to a pointer after declaring it.</div> <div> Taking the address of a variable and assigning it to the pointer is one</div> <div> way of initializing a pointer. A pointer can be initialized by assigning</div> <div> either value 0 or the word NULL. The NULL is a global variable declared</div> <div> in many header files that we include at the start of the program. The</div> <div> pointer initialized by NULL as ptr = NULL; is called null pointer which</div> <div> points to nothing. Similarly, when we assign a zero to a pointer like ptr =</div> <div> 0; it means that the pointer is pointing to nothing at the moment. Here</div> <div> zero is not considered as a valid address for a memory location.</div> <div> However, at some later stage, we use the pointer in an assignment</div> <div> statement either on left hand side to assign a value to it or as a part of</div> <div> an expression on right hand side. The pointer must have a valid</div> <div> memory address where a value should have stored. We get the address</div> <div> of a variable by putting & operator before the name of the variable and</div> <div> assign it to a pointer as in the following statement ptr = &x;</div> <div> We know that in C language, the default mechanism of function call is</div> <div> ‘call by value’. Sometimes we want to make a call by reference. In call</div> <div> by reference, we pass the address of the variable to a function by using</div> <div> & operator.</div> <div> One of the major usages of pointers is to simulate call by reference</div> <div> while using it with function calls. In the calling function, we pass the</div> <div> address of the variable to a function being called by using & operator.</div> <div> We write a function call as fn( &x ) where &x indicates that the address</div> <div> of variable x is being passed to the function fn. In the receiving function,</div> <div> the function must know that the parameter passed to it is an address.</div> <div> So the declaration of the receiving function will be as</div> <div> void fn ( int *num)</div> <div> {</div> <div> statement(s) ;</div> <div> }</div> <div> The int *num in the function declaration indicates that the receiving</div> <div> variable is a pointer to a memory address. In the body of the function,</div> <div> we will use this variable as:</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 149</div> <div> cin >> *num ;</div> <div> This statement describes that the value entered through the keyboard</div> <div> (as cin is used) will be stored at the memory address wherever the</div> <div> pointer num is pointing to.</div> <div> While using value associated with the pointer, we write *num and &num</div> <div> in case of using the address. This thing can be summarized as follows</div> <div> “*num means the value of whatever the num points to and</div> <div> &num means the address of the variable num”</div> <div> The pointers can appear on the left hand side exactly like ordinary</div> <div> variables. In this case, you would have an address statement on the</div> <div> right hand side. The address (operator (&) ) cannot be of an expression.</div> <div> Rather, it is always of a simple variable. We cannot write &(x+y). The</div> <div> address (&) would be either of x (&x) or of y (&y). The address operator</div> <div> (&) operates on a simple variable. Precisely speaking, whenever we</div> <div> have a pointer on left hand side, the right hand side should have an</div> <div> address. If a pointer appears on the right hand side of an expression, it</div> <div> can participate in any expression. In this case, we use the operator *</div> <div> with the pointer name and get the value stored where the pointer points</div> <div> to. Obviously we can do any calculation with this value (i.e. it can be</div> <div> used in any expression).</div> <div> Example (Bubble Sort)</div> <div> You might be knowing the technique of bubble sorting. Its application</div> <div> helps us compare two values each time and interchange the larger and</div> <div> smaller values. In this way, we sort the arrays. To interchange the</div> <div> position of larger and smaller value, the technique of swapping is used.</div> <div> Swapping is very common in programming. While using this technique,</div> <div> we put value of one variable in a temporary location to preserve it and</div> <div> assign the value of second variable to the first. Then the temporary</div> <div> value is assigned to the second variable.</div> <div> Suppose, we want to swap the values of two variables x and y. For this</div> <div> purpose, a third variable temp is used in the following fashion.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 150</div> <div> temp = x ;</div> <div> x = y ;</div> <div> y = temp ;</div> <div> We can write the above three statements in a program to swap the</div> <div> value of x and y. Now the question arises, can we call a function swap</div> <div> (x, y) which has a code to swap the values of x and y. We call the</div> <div> function swap by passing x and y. When the control comes back to the</div> <div> calling function, the values of x and y are the same as before. These</div> <div> are not swapped. This is mainly due to the fact that passing value to</div> <div> function swap is a call by value. It does not change the values in the</div> <div> calling function. The swap function receives a copy of the values and</div> <div> interchanges the values in that copy. The original values remain the</div> <div> same.</div> <div> To interchange two values in a function, we make a call by reference to</div> <div> the function. Here comes the use of pointers. To write the swap function</div> <div> to interchange two values always use pointers in the function to get the</div> <div> swapped values in the calling function. The code fragment in our main</div> <div> program will be written as follows:</div> <div> yptr = &y ; // address of y is stored in</div> <div> yptr</div> <div> xptr = &x ; // address of x is stored in</div> <div> xptr</div> <div> swap (yptr, xptr) ; // addresses are passed</div> <div> The receiving function must know that addresses are being passed to it.</div> <div> So the declaration of swap function will be:</div> <div> swap (int *yptr, int *xptr)</div> <div> {</div> <div> … … …</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 151</div> <div> This use of pointers implements a call by reference. We can use this</div> <div> technique in bubble sort. The swap function can switch the elements of</div> <div> the array by using pointers and * operator.</div> <div> The code of the program that sorts an array by bubble sort and use the</div> <div> swap function to interchange the elements of the array is given here.</div> <div> /* This program uses bubble sorting to sort a given array.</div> <div> * We use swap function to interchange the values by using pointers</div> <div> */</div> <div> #include <iostream.h></div> <div> #include <stdlib.h></div> <div> /* Prototye of function swap used to swap two values */</div> <div> void swap(int *, int *) ;</div> <div> main()</div> <div> {</div> <div> int x [] = {1,3,5,7,9,2,4,6,8,10};</div> <div> int i, j, tmp, swaps;</div> <div> for(i = 0; i < 10; i ++)</div> <div> {</div> <div> swaps = 0;</div> <div> for(j = 0; j < 10; j ++)</div> <div> {</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 152</div> <div> if ( x[j] > x[j+1]) // compare two values and interchange if</div> <div> needed</div> <div> {</div> <div> swaps++;</div> <div> swap(&x[j],&x[j+1]);</div> <div> }</div> <div> }</div> <div> //display the array’s elements after each comparison</div> <div> for (j=0; j<10; j++)</div> <div> cout << x[j] << '\t';</div> <div> cout << endl;</div> <div> if (swaps == 0)</div> <div> break;</div> <div> }</div> <div> }</div> <div> void swap(int *x, int *y) //function using pointers to interchange the values</div> <div> {</div> <div> int tmp;</div> <div> if(*x > *y)</div> <div> {</div> <div> tmp = *x;</div> <div> *x = *y;</div> <div> *y = tmp;</div> <div> }</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 153</div> <div> Following is the output of the program of bubble sort.</div> <div> 1 3 5 7 2 4 6 8</div> <div> 9 10</div> <div> 1 3 5 2 4 6 7 8</div> <div> 9 10</div> <div> 1 3 2 4 5 6 7 8</div> <div> 9 10</div> <div> 1 2 3 4 5 6 7 8</div> <div> 9 10</div> <div> 1 2 3 4 5 6 7 8</div> <div> 9 10</div> <div> Pointers and Call By Reference</div> <div> Suppose, we have a function that performs a specific task again and</div> <div> again but with different variables each time. One way to do this is to</div> <div> pass a different variable to the function, each time, by reference. We</div> <div> can also write the function with pointers. In this case, before calling the</div> <div> function, put the address of the simple variable in the pointer variable</div> <div> and pass it to the function. This is a call by reference. Thus the same</div> <div> pointer variable can be used each time by assigning it the address of a</div> <div> different variable.</div> <div> The mechanism behind calling a function is that, when we call a</div> <div> function we pass it some variables. The values of these variables are</div> <div> used with in the function. In call by value mechanism, the values of</div> <div> these variables are written somewhere else in the memory. That means</div> <div> a copy of these values is made. Then control goes to the called function</div> <div> and this copy of values is used in the function. If we have to pass a</div> <div> huge number of values to a function, it is not advisable to copy these</div> <div> huge numbers of values. In such cases, it is better to pass the reference</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 154</div> <div> of the variables, which is a call by reference phenomenon. We perform</div> <div> a similar function in case of an array, where we can pass, say, 100</div> <div> values (size of the array) to the called function, by only passing the</div> <div> name of the array. When we pass an array to a function, actually the</div> <div> starting address of the array is passed to the function. Thus the default</div> <div> calling mechanism to call a function while passing an array to it is a call</div> <div> by reference.</div> <div> The problem with call by reference is that ‘we are letting the function to</div> <div> change the values at their actual storage place in the memory’.</div> <div> Sometimes, we want to do this according to the requirement of the logic</div> <div> of the program. At some other occasion, we may pass the addresses for</div> <div> efficiency while not affecting the values at that addresses. The use of</div> <div> const can be helpful in overcoming this problem..</div> <div> Let’s look at the use of const. Consider the following line of declaration:</div> <div> int *const myptr = &x ;</div> <div> The right hand side of this assignment statement could be read as,</div> <div> myptr is a constant pointer to an integer. Whenever we use the keyword</div> <div> const with a variable, the value of that variable becomes constant and</div> <div> no other value can be assigned to it later on. We know that when we</div> <div> declare a constant variable like const int x ; it is necessary to assign a</div> <div> value to x and we write const int x = 10 . After this, we cannot assign</div> <div> some other value to x. The value of x can not be changed as it is</div> <div> declared as a constant.</div> <div> Now consider the previous statement</div> <div> int *const myptr = &x ;</div> <div> Here we declare a constant pointer to an integer.Being a constant</div> <div> pointer, it should immediately point to something. Therefore, we assign</div> <div> this pointer an address of a variable x at the time of declaration. Now</div> <div> this pointer cannot be changed. The pointer myptr will hold the address</div> <div> of variable x throughout the program. This way, it becomes just another</div> <div> name for the variable x. The use of constant pointers is not much</div> <div> useful.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 155</div> <div> The use of keyword const in declaration statement is a little tricky. The</div> <div> statement</div> <div> int *const myptr = &x ;</div> <div> means myptr is a constant pointer to an integer. But if we change the</div> <div> place of const in this statement and write</div> <div> const int *myptr = &x ;</div> <div> This statement describes that myptr is a pointer to a constant integer.</div> <div> This means that the value of pointer myptr can be changed but the</div> <div> value stored at that location cannot be changed. This declaration is</div> <div> useful. It has a common use in call by reference mechanism. When we</div> <div> want to pass the arguments to a function by reference without changing</div> <div> the values stored at that addresses. Then we use this construct of</div> <div> declaration (i.e. const int *myptr) in the called function declaration. We</div> <div> write the declaration of the function like</div> <div> fn ( const int *myptr)</div> <div> {</div> <div> ….</div> <div> }</div> <div> This declaration informs the function that the receiving value is a</div> <div> constant integer. The function cannot change this value. Thus we can</div> <div> use the address of that value for manipulations but cannot change the</div> <div> value stored at that location.</div> <div> Example 2</div> <div> Let’s consider an example in which we use the pointers to make a call by reference.</div> <div> We want to convert the lowercase letters of a string (character array), to their</div> <div> corresponding uppercase letters.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 156</div> <div> We write a function convertToUppercase, which processes the string s one character</div> <div> at a time using pointer arithmetic. In the body of the function, we pass the character to</div> <div> a function islower. This function returns true if the character is a lowercase letter and</div> <div> false otherwise. The characters in the range ‘a’ through ‘z’ are converted to their</div> <div> corresponding uppercase letters by function toupper. Function toupper takes one</div> <div> character as an argument. If the character is a lowercase letter, the corresponding</div> <div> uppercase letter is returned, otherwise the original character is returned. The functions</div> <div> toupper and islower are part of the character handling library <ctype.h>. So we have</div> <div> to include this header file in our program. We include it in the same way, as we</div> <div> include <iostream.h>.</div> <div> The complete code of the program is given below.</div> <div> //This program converts a string into an uppercase string</div> <div> # include <iostream.h></div> <div> # include <ctype.h></div> <div> # include <stdlib.h></div> <div> //declare the functions prototype</div> <div> void convertToUppercase (char *)</div> <div> main ()</div> <div> {</div> <div> char s [30] = “Welcome To Virtual University” ;</div> <div> cout << “The string before conversion is: “ << s << endl ;</div> <div> convertToUppercase ( s) ; //function call</div> <div> cout << “The string after conversion is: “ << s ;</div> <div> }</div> <div> void convertToUppercase (char *sptr)</div> <div> {</div> <div> while ( *sptr != ‘\0’ )</div> <div> {</div> <div> if ( islower ( *sptr) )</div> <div> *sptr = toupper ( *sptr ); //convert to uppercase</div> <div> ++ sptr; // move sptr to the next</div> <div> character</div> <div> }</div> <div> }</div> <div> Following is the output of the program.</div> <div> The string before conversion is : Welcome To Virtual University</div> <div> The string after conversion is : WELCOME TO VIRTUAL UNIVERSITY</div> <div> Exercise</div> <div> 1. Modify the above program so that it gets a string from user and converts it into</div> <div> lowercase.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 157</div> <div> 2. Write a program, which converts a string of uppercase letters into its</div> <div> corresponding lowercase letters string.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 158</div> <div> Lecture No. 15</div> <div> Reading Material</div> <div> Deitel & Deitel - C++ How to Program Chapter 5</div> <div> 5.7, 5.8</div> <div> Summary</div> <div> 6) Introduction</div> <div> 7) Relationship between Pointers and Arrays</div> <div> 8) Pointer Expressions and Arithmetic</div> <div> 9) Pointers Comparison</div> <div> 10) Pointer, String and Arrays</div> <div> 11) Tips</div> <div> Introduction</div> <div> In the previous lecture, we had just started the discussion on the topic of pointers.</div> <div> This topic is little complicated, yet the power we get with the pointers is very</div> <div> interesting. We can do many interesting things with pointers. When other languages</div> <div> like Java evolve with the passage of time, pointers are explicitly excluded. In today’s</div> <div> lecture, we will discuss pointers, the relationship between pointers and arrays, pointer</div> <div> expressions, arithmetic with pointers, relationship between arrays and pointer, strings</div> <div> etc.</div> <div> Relationship between Pointers and Arrays</div> <div> When we write int x, it means that we have attached a symbolic name x, at some</div> <div> memory location. Now we can use x = 10 which replaces the value at that memory</div> <div> location with 10. Similarly while talking about arrays, suppose an array as int y[10].</div> <div> This means that we have reserved memory spaces for ten integers and named it</div> <div> collectively as y. Now we will see what actually y is? 'y' represents the memory</div> <div> address of the beginning of this collective memory space. The first element of the</div> <div> array can be accessed as y[0]. Remember arrays index starts from 0 in C language, so</div> <div> the memory address of first element i.e. y[0] is stored in y.</div> <div> “The name of the array is a constant pointer which contains the memory</div> <div> address of the first element of the array”</div> <div> The difference between this and an ordinary pointer is that the array name is a</div> <div> constant pointer. It means that the array name will always point to the start of the</div> <div> array. In other words, it always contains the memory address of the first element of</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 159</div> <div> the array and cannot be reassigned any other address. Let's elaborate the point with</div> <div> the help of following example.</div> <div> int y[10];</div> <div> int *yptr;</div> <div> In the above statements, we declare an array y of ten integers and a pointer to an</div> <div> integer i.e. yptr. This pointer may contain a memory address of an integer.</div> <div> yptr = y;</div> <div> This is an assignment statement. The value of y i.e. the address of the first element of</div> <div> the array is assigned to yptr. Now we have two things pointing to the same place, y</div> <div> and yptr. Both are pointing to the first element of the array. However, y is a constant</div> <div> pointer and always points to the same location whereas yptr is a pointer variable that</div> <div> can also point to any other memory address.</div> <div> Pointer Expressions and Arithmetic</div> <div> Suppose we have an array y and yptr, a pointer to array. We can manipulate arrays</div> <div> with both y and yptr. To access the fourth element of the array using y, we can say</div> <div> y[3]; with yptr, we can write as *(yptr + 4). Now we have to see what happens when</div> <div> we increment or add something to a pointer. We know that y is a constant pointer and</div> <div> it can not be incremented. We can write y[0], y[1] etc. On the other hand, yptr is a</div> <div> pointer variable and can be written as the statement yptr = y. It means that yptr</div> <div> contains the address of the first element of the array. However, when we say yptr++,</div> <div> the value of yptr is incremented. But how much? To explain it further, we increment a</div> <div> normal integer variable like x++. If x contains 10, it will be incremented by 1 and</div> <div> become 11. The increment of a pointer depends on its data type. The data type, the</div> <div> pointer points to, determines the amount of increment. In this case, yptr is an integer</div> <div> pointer. Therefore, when we increment the yptr, it points to the next integer in the</div> <div> memory. If an integer occupies four bytes in the memory, then the yptr++; will</div> <div> increment its value by four. This can be understood from the following example.</div> <div> // This program will print the memory address of a pointer and its incremented address.</div> <div> #include<iostream.h></div> <div> main()</div> <div> {</div> <div> int y[10]; // an array of 10 integers</div> <div> int *yptr; // an integer pointer</div> <div> yptr = y; // assigning the start of array address to pointer</div> <div> // printing the memory address</div> <div> cout << “The memory address of yptr = “ << yptr << endl ;</div> <div> yptr++; // incrementing the pointer</div> <div> // printing the incremented memory address</div> <div> cout << “The memory address after incrementing yptr = ” << yptr << endl;</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 160</div> <div> In the above program, the statement cout << yptr will show the memory address the</div> <div> yptr points to. You will notice the difference between the two printed addresses. By</div> <div> default, the memory address is printed in hexadecimal by the C output system.</div> <div> Therefore, the printed address will be in hexadecimal notation. The difference</div> <div> between the two addresses will be four as integer occupies four bytes and yptr is a</div> <div> pointer to an integer.</div> <div> “When a pointer is incremented, it actually jumps the number of memory</div> <div> spaces according to the data type that it points to”</div> <div> The sample out put of the program is:</div> <div> The memory address of yptr = 0x22ff50</div> <div> The memory address after incrementing yptr = 0x22ff54</div> <div> yptr which was pointing to the start of the array y, starts pointing to the next integer in</div> <div> memory after incrementing it. In other words, yptr is pointing to the 2nd element of</div> <div> the array. On being incremented again, the yptr will be pointing to the next element of</div> <div> the array i.e. y[2], and so on. We know that & is address operator which can be used</div> <div> to get the memory address. Therefore, we can also get the address of the first element</div> <div> of the array in yptr as:</div> <div> yptr = &y[0] ;</div> <div> y[0] is a single element and its address can be got with the use of. the address</div> <div> operator (&). Similarly we can get the address of 2nd or 3rd element as &y[1], &y[2]</div> <div> respectfully. We can get the address of any array element and assign it to yptr.</div> <div> Suppose the yptr is pointing to the first element of the array y. What will happen if we</div> <div> increment it too much? Say, the array size is 10. Can we increment the yptr up to 12</div> <div> times? And what will happen? Obviously, we can increment it up to 12 times. In this</div> <div> case, yptr will be pointing to some memory location containing garbage (i.e. there</div> <div> may be some value but is useless for us). To display the contents where the yptr is</div> <div> pointing we can use cout with dereference pointer as:</div> <div> cout << *yptr ;</div> <div> The above statement will display the contents where yptr is pointing. If the yptr is</div> <div> pointing to the first element of the array, cout << *yptr will display the contents of</div> <div> the first element of the array (i.e. y[0]). While incrementing the yptr as yptr ++, the</div> <div> statement cout << * yptr will display the contents of the 2nd element of the array(i.e.</div> <div> y[1]) and so on.</div> <div> Here is an example describing different methods to access array elements.</div> <div> /* This program contains different ways to access array elements */</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 161</div> <div> #include <iostream.h></div> <div> main ()</div> <div> {</div> <div> int y[10] = {0,5,10,15,20,25,30,35,40,45};</div> <div> int *yptr;</div> <div> yptr = y; // Assigning the address of first element of array.</div> <div> cout << “Accessing 6th element of array as y[5] = ” << y[5] << endl;</div> <div> cout << “Accessing 6th element of array as *(yptr + 5) = ” << *(yptr + 5) << endl;</div> <div> cout << “Accessing 6th element of array as yptr[5] = “ << yptr[5] << endl;</div> <div> }</div> <div> The output of the program is:</div> <div> Accessing 6th element of array as y[5] = 25</div> <div> Accessing 6th element of array as *(yptr + 5) = 25</div> <div> Accessing 6th element of array as yptr[5] = 25</div> <div> In the above example, there are two new expressions i.e. *(yptr+5) and yptr[5]. In the</div> <div> statement *(yptr+5), yptr is incremented first by 5 (parenthesis are must here).</div> <div> Resultantly, it points to the 6th element of the array. The dereference pointer gives the</div> <div> value at that address. As yptr is a pointer to an integer, so it can be used as array</div> <div> name. So the expression yptr[5] gives us the 6th element of the array.</div> <div> The following example can explain how we can step through an entire array using</div> <div> pointer.</div> <div> /* This program steps through an array using pointer */</div> <div> #include <iostream.h></div> <div> main ()</div> <div> {</div> <div> int y[10] = {10,20,30,40,50,60,70,80,90,100};</div> <div> int *yptr, i;</div> <div> yptr = y; // Assigning the address of first element of array.</div> <div> for (i = 0; i < 10 ; i ++)</div> <div> {</div> <div> cout << “\n The value of the element at position ” << i << “ is “ << *yptr;</div> <div> yptr ++ ;</div> <div> }</div> <div> }</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 162</div> <div> The output of the program is:</div> <div> The value of the element at position 0 is 10</div> <div> The value of the element at position 1 is 20</div> <div> The value of the element at position 2 is 30</div> <div> The value of the element at position 3 is 40</div> <div> The value of the element at position 4 is 50</div> <div> The value of the element at position 5 is 60</div> <div> The value of the element at position 6 is 70</div> <div> The value of the element at position 7 is 80</div> <div> The value of the element at position 8 is 90</div> <div> The value of the element at position 9 is 100</div> <div> Consider another example to elaborate the pointer arithmetic.</div> <div> /* Program using pointer arithmetic */</div> <div> #include <iostream.h></div> <div> main()</div> <div> {</div> <div> int x =10;</div> <div> int *yptr;</div> <div> yptr = &x;</div> <div> cout << “The address yptr points to = ” << yptr << endl ;</div> <div> cout << “The contents yptr points to = ” << *yptr << endl;</div> <div> (*yptr) ++;</div> <div> cout << “After increment, the contents are ” << *yptr << endl;</div> <div> cout << “The value of x is = ” << x << endl;</div> <div> }</div> <div> The output of the program is:</div> <div> The address yptr points to = 0x22ff7c</div> <div> The contents yptr points to = 10</div> <div> After increment, the contents are 11</div> <div> The value of x is = 11</div> <div> Here the statement (*yptr) ++ is read as “increment whatever yptr points to”. This</div> <div> will increment the value of the variable. As yptr and x both are pointing to the same</div> <div> location, the contents at that location becomes 11. Consider the statement *yptr + 3 ;</div> <div> This is an expression and there is no assignment so the value of x will not be changed</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 163</div> <div> where as the statement *yptr += 3; will increment the value of x by 3. If we want to</div> <div> increment the pointer and not the contents where it points to, we can do this as yptr</div> <div> ++; Now where yptr is pointing? The yptr will be now pointing four bytes away from</div> <div> the memory location of x. The memory location of x is a part of program, yet after</div> <div> incrementing yptr, it is pointing to some memory area, which is not part of the</div> <div> program. Take this as an exercise. Print the value of yptr and *yptr and see what is</div> <div> displayed? Be sure, it is not illegal and the compiler does not complain. The error will</div> <div> be displayed if we try to write some value at that memory address.</div> <div> “When a pointer is used to hold the memory address of a simple variable, do</div> <div> not increment or decrement the pointer. When a pointer is used to hold the</div> <div> address of an array, it makes sense to increment or decrement the pointer “</div> <div> Be careful while using pointers, as no warning will be given, in case of any problem.</div> <div> As pointers can point at any memory location, so one can easily get the computers</div> <div> crashed by using pointers.</div> <div> Remember that incrementing the pointer and incrementing the value where the pointer</div> <div> points to are two different things. When we want to increment the pointer, to make it</div> <div> point to next element in the memory, we write as (yptr++); Use parenthesis when</div> <div> incrementing the address. If we want to increment the value where the pointer points</div> <div> to, it can be written as (*yptr) ++; Keep in mind the precedence of operator. Write a</div> <div> program to test this.</div> <div> The decrement of the pointer is also the same. yptr --; yptr -= 3 ; will decrement the</div> <div> yptr. Whereas the statement (*yptr) --; will decrement the value where the yptr is</div> <div> pointing. So if the yptr is pointing to x the value of x will be decremented by 1.</div> <div> Pointers are associated to some data type as pointer to integer, pointer to float and</div> <div> pointer to char etc. When a pointer is incremented or decremented, it changes the</div> <div> address by the number of bytes occupied by the data type that the pointer points to.</div> <div> For example, if we have a pointer to an integer, by incrementing the pointer the</div> <div> address will be incremented by four bytes, provided the integer occupies four bytes on</div> <div> that machine. If it is a pointer to float and float occupies eight bytes, then by</div> <div> incrementing this pointer, its address will be incremented by eight bytes. Similarly, in</div> <div> case of a pointer to a char, which normally takes one byte, incrementing a pointer to</div> <div> char will change the address by one. If we move to some other architecture like</div> <div> Macintosh, write a simple program to check how many bytes integer, float or char is</div> <div> taking with the use of simple pointer arithmetic. In the modern operating systems like</div> <div> windows XP, windows 2000, calculator is provided under tools menu. Under the view</div> <div> option, select scientific view. Here we can do hexadecimal calculations. So we can</div> <div> key in the addresses our programs are displaying on the screen and by subtracting, we</div> <div> can see the difference between the two addresses. Try to write different programs and</div> <div> experiment with these.</div> <div> We have seen that we can do different arithmetic operations with pointers. Let's see</div> <div> can two pointers be added? Suppose we have two pointers yptr1 and yptr2 to integer</div> <div> and written as yptr1 + yptr2 ; The compiler will show an error in this statement.</div> <div> Think logically what we can obtain by adding the two memory addresses. Therefore,</div> <div> normally compiler will not allow this operation. Can we subtract the pointers? Yes,</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 164</div> <div> we can. Suppose we have two pointers pointing to the same memory address. When</div> <div> we subtract these, the answer will be zero. Similarly, if a pointer is pointing to the</div> <div> first element of an integer array while another pointer pointing to the second element</div> <div> of the array. We can subtract the first pointer from second one. Here the answer will</div> <div> be one, i.e. how many array elements are these two pointers apart.</div> <div> Consider the following sample program:</div> <div> /* Program using the pointer subtraction */</div> <div> #include <iostream.h></div> <div> main ()</div> <div> {</div> <div> int y[10], *yptr1, *yptr2;</div> <div> yptr1 = &y[0];</div> <div> yptr2 = &y[3];</div> <div> cout << “ The difference = “ << yptr2 - yptr1;</div> <div> }</div> <div> The output of the program is:</div> <div> The difference = 3</div> <div> In the above program, we have taken two integer pointers yptr1 and yptr2 and an</div> <div> integer array y[10]. The pointer yptr1 is pointing to the address of the first element of</div> <div> the array while yptr2 is pointing to the 4th element of the array. The difference</div> <div> between these two pointers can be shown by using cout statement. Here the result</div> <div> should be twelve. But the program will show the result as three. When we increment</div> <div> an integer pointer by 1, we have seen that the address is changed by four. When we</div> <div> subtract pointers, it tells us the distance between the two elements that the pointers</div> <div> pointed to. It will tell us how many array elements are between these two pointers. As</div> <div> the yptr1 is pointing to y[0] and the yptr2 is pointing to y[3], so the answer is three. In</div> <div> a way, it tells how many units of data type (pointers data type) are between the two</div> <div> pointers. Pointer addition is not allowed, however, pointer subtraction is allowed as it</div> <div> gives the distance between the two pointers in units, which are the same as the data</div> <div> type of the pointer.</div> <div> A memory image of an array with a pointer.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 165</div> <div> This diagram shows how an array occupies space in the memory. Suppose, we have</div> <div> an integer array named y and yptr is a pointer to an integer and is assigned the address</div> <div> of the first element of the array. As this is an integer array, so the difference between</div> <div> each element of the array is of four bytes. When the yptr is incremented, it starts</div> <div> pointing to the next element in the array.</div> <div> Pointer Comparison</div> <div> We have seen pointers in different expressions and arithmetic operations. Can we</div> <div> compare pointers? Yes, two pointers can be compared. Pointers can be used in</div> <div> conditional statements as usual variables. All the comparison operators can be used</div> <div> with pointers i.e. less than, greater than, equal to, etc. Suppose in sorting an array we</div> <div> are using two pointers. To test which pointer is at higher address, we can compare</div> <div> them and take decision depending on the result.</div> <div> Again consider the two pointers to integer i.e. yptr1 and yptr2. Can we compare</div> <div> *yptr1 and *yptr2? Obviously *yptr1 and *yptr2 are simple values. It is the value of</div> <div> integer yptr1, yptr2 points to. When we say *yptr1 > *yptr2, this is a comparison of</div> <div> simple two integer values. Whenever we are using the dereference pointer (pointers</div> <div> with *), all normal arithmetic and manipulation is valid. Whenever we are using</div> <div> pointers themselves, then certain type of operations are allowed and restrictions on</div> <div> other. Make a list what can we do with a pointer and what we cannot.</div> <div> Consider a sample program as follows:</div> <div> /* Program using the dereference pointer comparison */</div> <div> #include <iostream.h></div> <div> main ()</div> <div> {</div> <div> int x, y, *xptr, *yptr;</div> <div> cout << “ \n Please enter the value of x = “ ;</div> <div> cin >> x ;</div> <div> yptr</div> <div> y[0] y[1] y[2] y[3] y[4]</div> <div> Addresses: 3000 3004 3008 3012 3016</div> <div> yptr++ yptr</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 166</div> <div> cout << “ \n Please enter the value of y = “;</div> <div> cin >> y ;</div> <div> xptr = &x;</div> <div> yptr = &y;</div> <div> if (*xptr > *yptr )</div> <div> {</div> <div> cout << “ \n x is greater than y “;</div> <div> }</div> <div> else</div> <div> {</div> <div> cout << “\n y is greater than x “;</div> <div> }</div> <div> }</div> <div> The output of the program is;</div> <div> Please enter the value of x = 6</div> <div> Please enter the value of y = 9</div> <div> y is greater than x</div> <div> Pointer, String and Arrays</div> <div> We have four basic data types i.e. char, int, float and double. Character strings are</div> <div> arrays of characters. Suppose, there is a word or name like Amir to store in one entity.</div> <div> We cannot store it into a char variable because it can store only one character. For this</div> <div> purpose, a character array is used. We can write it as:</div> <div> char name [20];</div> <div> We have declared an array name of 20 characters .It can be initialized as:</div> <div> name[0] = ‘A’ ;</div> <div> name[1] = ‘m’ ;</div> <div> name[2] = ‘i’ ;</div> <div> name[3] = ‘r’ ;</div> <div> Each array element is initialized with a single character enclosed in single quote. We</div> <div> cannot use more than one character in single quotes, as it is a syntax error. Is the</div> <div> initialization of the array complete? No, the character strings are always terminated by</div> <div> null character ‘\0’. Therefore, we have to put the null character in the end of the array.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 167</div> <div> name[4] = ‘\0’ ;</div> <div> Here we are using two characters in single quotes. But it is a special case. Whenever</div> <div> back slash ( \ ) is used, the compiler considers both the characters as single (also</div> <div> known as escape characters). So ‘\n’ is new line character, ‘\t’ a tab character and ‘\0’</div> <div> a null character. All of these are considered as single characters. What is the benefit of</div> <div> having this null character at the end of the string? Write a program, do not use the null</div> <div> character in the string and try to print the character array using cout and see what</div> <div> happens? cout uses the null character as the string terminating point. So if cout does</div> <div> not find the null character it will keep on printing. Remember, if we want to store</div> <div> fifteen characters in an array, the array size should be at least sixteen i.e. fifteen for</div> <div> the data and one for the null character. Do we always need to write the null character</div> <div> at the end of the char array by ourselves? Not always, there is a short hand provided in</div> <div> C, i.e. while declaring we can initialize the arrays as:</div> <div> char name[20] = “Amir”;</div> <div> When we use double quotes to initialize the character array, the compiler appends null</div> <div> character at the end of the string.</div> <div> “Arrays must be at least one character space larger than the number of</div> <div> printable characters which are to be stored”</div> <div> Example:</div> <div> Write a program which copies a character array into given array.</div> <div> Solution:</div> <div> Here is the complete code of the program:</div> <div> /* This program copies a character array into a given array */</div> <div> #include <iostream.h></div> <div> main( )</div> <div> {</div> <div> char strA[80] = "A test string";</div> <div> char strB[80];</div> <div> char *ptrA; /* a pointer to type character */</div> <div> char *ptrB; /* another pointer to type character */</div> <div> ptrA = strA; /* point ptrA at string A */</div> <div> ptrB = strB; /* point ptrB at string B */</div> <div> while(*ptrA != '\0')</div> <div> {</div> <div> *ptrB++ = *ptrA++; // copying character by character</div> <div> }</div> <div> *ptrB = '\0';</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 168</div> <div> cout << “String in strA = ” << strA << endl; /* show strA on screen */</div> <div> cout << “String in strB = ” << strB << endl; /* show strB on screen */</div> <div> }</div> <div> The output of the program is:</div> <div> String in strA = A test string</div> <div> String in strB = A test string</div> <div> Explanation:</div> <div> Suppose, we have declared a char array named strA of size 80 and initialized it with</div> <div> some value say “A test String” using the double quotes. Here we don’t need to put a</div> <div> null character. The compiler will automatically insert it. But while declaring another</div> <div> array strB of the same size, we declare two char pointers *ptrA and *ptrB. The</div> <div> objective of this exercise is to copy one array into another array. We have assigned</div> <div> the starting address of array strA to ptrA and strB to ptrB. Now we have to run a loop</div> <div> to copy all the characters from one array to other. To terminate the loop, we have to</div> <div> know about the actual number of characters or have to use the string termination</div> <div> character. As we know, null character is used to terminate a string, so we are using the</div> <div> condition in 'while loop' as: *ptrA != ‘\0’ , simply checking that whatever ptrA is</div> <div> pointing to is not equal to ‘\0’. Look at the statement *ptrB++ = *ptrA++. What has</div> <div> happened in this statement? First of all, whatever ptrA is pointing to will be assigned</div> <div> to the location where ptrB is pointing to. When the loop starts, these pointers are</div> <div> pointing to the start of the array. So the first character of strA will be copied to the</div> <div> first character of strB. Afterwards, the pointers will be incremented, not the values</div> <div> they are pointing to. Therefore, ptrA is pointing to the 2nd element of the array strA</div> <div> and ptrB is pointing to the 2nd element of the array strB. In the 2nd repetition, the loop</div> <div> condition will be tested. If ptrA is not pointing to a null character the assignment for</div> <div> the 2nd element of the array takes place and so on till the null character is reached. So</div> <div> all the characters of array strA are copied to array strB. Is this program complete? No,</div> <div> the array strB is not containing the null character at the end of the string. Therefore,</div> <div> we have explicitly assigned the null character to strB. Do we need to increment the</div> <div> array pointer? No, simply due to the fact that in the assignment statement ( *ptrA++ =</div> <div> *ptrB++;), the pointers are incremented after the assignment. This program now</div> <div> successfully copies one string to other using only pointers. We can also write a</div> <div> function for the string copy. The prototype of the function will be as:</div> <div> void myStringCopy (char *destination, const char *source) ;</div> <div> This function takes two arguments. The first one is a pointer to a char while second</div> <div> argument is a const pointer to char. The destination array will be changed and all the</div> <div> characters from source array are copied to destination. At the same time, we do not</div> <div> want that the contents of source should be changed. So we used the keyword const</div> <div> with it. The keyword const makes it read only and it can not be changed accidentally.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 169</div> <div> If we try to change the contents of source array, the compiler will give an error. The</div> <div> body is same, as we have seen in the above program.</div> <div> This function will not return anything as we are using pointers. It is automatically call</div> <div> by reference. Whenever arrays are passed to functions, a reference of the original</div> <div> array is passed. Therefore, any change in the array elements in the function will</div> <div> change the actual array. The values will be written to the original array. If these are</div> <div> simple variables, we will have to send the address and get the called program to</div> <div> change it. Therefore, we do not need to return anything from this function after</div> <div> successfully copying an array into the other.</div> <div> Here is the code of the function. Write a program to test this function.</div> <div> void myStringCopy (char *destination, const char *source)</div> <div> {</div> <div> while(*source != ‘\0’)</div> <div> {</div> <div> *destination++ = *source++;</div> <div> }</div> <div> *destination = ‘\0’;</div> <div> }</div> <div> We can also write the string copy function using arrays. Here is the code of the</div> <div> myStringCopy function using arrays notation.</div> <div> void myStringCopy(char dest[], char source[])</div> <div> {</div> <div> int i = 0;</div> <div> while (source[i] != '\0')</div> <div> {</div> <div> dest[i] = source[i];</div> <div> i++;</div> <div> }</div> <div> dest[i] = '\0';</div> <div> }</div> <div> Exercise:</div> <div> 1) Print out the address and the value of a character pointer pointing to some</div> <div> character.</div> <div> 2) Write a function which copies an array of integers from one array to other</div> <div> Tips</div> <div> • While incrementing the pointers, use the parenthesis</div> <div> • Increment and decrement the pointers while using arrays</div> <div> • When a pointer is incremented or decremented, it changes the address by the</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 170</div> <div> number of bytes occupied by the data type that the pointer points to</div> <div> • Use key word const with pointers to avoid unwanted changes</div> <div> • The name of array is a constant pointer. It cannot be reassigned</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 171</div> <div> Lecture No. 16</div> <div> Reading Material</div> <div> Deitel & Deitel - C++ How to Program Chapter 5, 18</div> <div> 5.9, 5.10, 18.4</div> <div> Summary</div> <div> • Pointers (continued)</div> <div> • Multi-dimensional Arrays</div> <div> • Pointers to Pointers</div> <div> • Command-line Arguments</div> <div> • Exercises</div> <div> • Tips</div> <div> Pointers (continued)</div> <div> We will continue with the elaboration of the concept of pointers in this lecture. To</div> <div> further understand pointers, let's consider the following statement.</div> <div> char myName[] = "Full Name";</div> <div> This statement creates a 'char' type array and populates it with a string. Remember the</div> <div> character strings are null ( '\0' ) terminated. We can achieve the same thing with the</div> <div> use of pointer as under:</div> <div> char * myNamePtr = "Full Name";</div> <div> Let's see what's the difference between these two approaches?</div> <div> When we create an array, the array name, 'myName' in this case, is a constant pointer.</div> <div> The starting address of the memory allocated to string "FullName" becomes the</div> <div> Full Name\0</div> <div> myName Full Name\0</div> <div> myNamePtr</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 172</div> <div> contents of the array name 'myName' and the array name 'myName' can not be</div> <div> assigned any other value. In other words, the location to which array names points to</div> <div> can not be changed. In the second statement, the 'myNamePtr' is a pointer to a string</div> <div> "FullName", which can always be changed to point to some other string.</div> <div> Hence, the array names can be used as pointers but only as constant ones.</div> <div> Multi-dimensional Arrays</div> <div> Now we will see what is the relationship between the name of the array and the</div> <div> pointer. Suppose we have a two-dimensional array:</div> <div> char multi[5][10];</div> <div> In the above statement, we have declared a 'char' type array of 5 rows and 10</div> <div> columns.</div> <div> As discussed above, the array name points to the starting memory location of the</div> <div> memory allocated for the array elements. Here the question arises where the 'multi'</div> <div> will be pointing if we add 1 to ‘multi’.</div> <div> We know that a pointer is incremented by its type number of bytes. In this case,</div> <div> 'multi' is an array of 'char' type that takes 1 byte. Therefore, ‘muti+1’ should take us to</div> <div> the second element of the first row (row 0). But this time, it is behaving differently. It</div> <div> is pointing to the first element (col 0) of the second row (row 1). So by adding '1' in</div> <div> the array name, it has jumped the whole row or jumped over as many memory</div> <div> locations as number of columns in the array. The width of the columns depends upon</div> <div> the type of the data inside columns. Here, the data type is 'char', which is of 1 byte. As</div> <div> the number of columns for this array 'multi' is 10, it has jumped 10 bytes.</div> <div> Remember, whenever some number is added in an array name, it will jump as many</div> <div> rows as the added number. If we want to go to the second row (row 1) and third</div> <div> column (col 2) using the same technique, it is given ahead but it is not as that straight</div> <div> forward. Remember, if the array is to be accessed in random order, then the pointer</div> <div> approach may not be better than array indexing.</div> <div> We already know how to dereference array elements using indexing. So the element</div> <div> at second row and third column can be accessed as 'multi[1][2]'.</div> <div> 75</div> <div> [0]</div> <div> 72</div> <div> [1]</div> <div> 68</div> <div> [2]</div> <div> 82</div> <div> [3]</div> <div> 80</div> <div> [4]</div> <div> 79</div> <div> [5]</div> <div> 69</div> <div> [6]</div> <div> 67</div> <div> [7]</div> <div> 73</div> <div> [8]</div> <div> 77</div> <div> [9]</div> <div> 83</div> <div> [0]</div> <div> 1st row 1st col 2nd row 1st col</div> <div> Multi-dimensional array in the memory</div> <div> 80</div> <div> [1]</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 173</div> <div> To do dereferencing using pointers we use '*' operator. In case of one-dimensional</div> <div> array, '*multi' means 'the value at the address, pointed to by the name of the array'.</div> <div> But for two-dimensional array '*multi' still contains an address of the first element of</div> <div> the first row of the array or starting address of the array 'multi'. See the code snippet</div> <div> to prove it.</div> <div> /* This program uses the multi-dimensional array name as pointer */</div> <div> #include <iostream.h></div> <div> void main(void)</div> <div> {</div> <div> //To avoid any confusion, we have used ‘int’ type below</div> <div> int multi[5][10];</div> <div> cout << "\n The value of multi is: " << multi;</div> <div> cout << "\n The value of *multi is: " << *multi;</div> <div> }</div> <div> Now, look at the output below:</div> <div> The value of multi is: 0x22feb0</div> <div> The value of *multi is: 0x22feb0</div> <div> It is pertinent to note that in the above code, the array ‘multi’ has been changed to</div> <div> ‘int’ from ‘char’ type to avoid any confusion.</div> <div> To access the elements of the two-dimensional array, we do double dereferencing like</div> <div> '**multi'. If we want to go to, say, 4th row (row 3), it is achieved as 'multi + 3' . Once</div> <div> reached in the desired row, we can dereference to go to the desired column. Let's say</div> <div> we want to go to the 4th column (col 3). It can be done in the following manner.</div> <div> *(*(multi+3)+3)</div> <div> This is an alternative way of manipulating arrays. So 'multi[3][3]' element can also be</div> <div> accessed by '*(*(multi+3)+3)'.</div> <div> There is another alternative of doing this by using the normal pointer. Following code</div> <div> reflects it.</div> <div> /* This program uses array manipulation using indexing */</div> <div> #include <iostream.h></div> <div> void main(void)</div> <div> {</div> <div> int multi [5][10];</div> <div> int *ptr; // A normal ‘int’ pointer</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 174</div> <div> ptr = *multi; // ‘ptr’ is assigned the starting address of the first row</div> <div> /* Initialize the array elements */</div> <div> for(int i=0; i < 5; i++)</div> <div> {</div> <div> for (int j=0; j < 10; j++)</div> <div> {</div> <div> multi[i][j] = i * j;</div> <div> }</div> <div> }</div> <div> /* Array manipulation using indexing */</div> <div> cout << "\n Array manipulated using indexing is: \n";</div> <div> for(int i=0; i < 5; i++)</div> <div> {</div> <div> for (int j=0; j < 10; j++)</div> <div> {</div> <div> cout << multi[i][j] << '\t';</div> <div> }</div> <div> cout << '\n';</div> <div> }</div> <div> /* Array manipulation using pointer */</div> <div> cout << "\n Array manipulated using pointer is: \n";</div> <div> for(int k=0; k < 50; k++, ptr ++) // 5 * 10 = 50</div> <div> {</div> <div> cout << *ptr << '\t';</div> <div> }</div> <div> }</div> <div> The output of this program is:</div> <div> Array manipulated using indexing is:</div> <div> 0 0 0 0 0 0 0 0 0 0</div> <div> 0 1 2 3 4 5 6 7 8 9</div> <div> 0 2 4 6 8 10 12 14 16 18</div> <div> 0 3 6 9 12 15 18 21 24 27</div> <div> 0 4 8 12 16 20 24 28 32 36</div> <div> Array manipulated using pointer is:</div> <div> 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5</div> <div> 6 7 8 9 0 2 4 6 8 10 12 14 16 18 0 3</div> <div> 6 9 12 15 18 21 24 27 0 4 8 12 16 20 24</div> <div> 28 32 36</div> <div> The above line of output of array manipulation is wrapped because of the fixed width</div> <div> of the table. Actually, it is a single line.</div> <div> Why it is a single line? As discussed in the previous lectures, computer stores array in</div> <div> straight line (contiguous memory locations). This straight line is just due to the fact</div> <div> that a function accepting a multi-dimensional array as an argument, needs to know all</div> <div> the dimensions of the array except the leftmost one. In case of two-dimensional array,</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 175</div> <div> the function needs to know the number of columns so that it has much information</div> <div> about the end and start of rows within an array.</div> <div> It is recommended to write programs to understand and practice the concepts of</div> <div> double dereferencing, single dereferencing, incrementing the name of the array to</div> <div> access different rows and columns etc. Only hands on practice will help understand</div> <div> the concept thoroughly.</div> <div> Pointers to Pointers</div> <div> What we have been talking about, now we will introduce a new terminology, is</div> <div> actually a case of ‘Pointer to Pointer’. We were doing double dereferencing to access</div> <div> the elements of a two-dimensional array by using array name (a pointer) to access a</div> <div> row (another pointer) and further to access a column element (of ‘int’ data type).</div> <div> In case of single dereference, the value of the pointer is the address of the variable</div> <div> that contains the value desired as shown in the following figure. In the case of pointer</div> <div> to pointer or double dereference, the first pointer contains the address of the second</div> <div> pointer, which contains the address of the variable, which contains the desired value.</div> <div> Pointers to Pointers are very useful. But you need to be very careful while using the</div> <div> technique to avoid any problem.</div> <div> Earlier, we used arrays and pointers interchangeably. We can think that a pointer to</div> <div> pointer is like a pointer to a group of arrays because a pointer itself can be considered</div> <div> as an array. We can elaborate with the following example by declaring character</div> <div> strings.</div> <div> While using an array, we at first decide about the length of the array. For example,</div> <div> you are asked to calculate the average age of your class using the array. What would</div> <div> be the dimension of the array? Normally, you will look around, count the students of</div> <div> the class and keep the same size of the array as the number of students, say 53. Being</div> <div> a good programmer, you will look ahead and think about the maximum size of the</div> <div> class in the future and decide to take the size of the array as 100. Here, you have taken</div> <div> care of the future requirements and made the program flexible. But the best thing</div> <div> could be: to get the size of the array from the user at runtime and set it in the program</div> <div> address value</div> <div> Si l I di i ( i l d f )</div> <div> ( f )</div> <div> address</div> <div> P i</div> <div> address</div> <div> P i</div> <div> value</div> <div> V i bl</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 176</div> <div> instead of declaring the array of maximum size. We will cover this topic at some later</div> <div> stage.</div> <div> When we initialize an array with a character string, the number of characters in the</div> <div> character string determines the length of array (plus one character to include the ‘\0’</div> <div> character). eg. it is a single-dimensional array:</div> <div> char name[] = “My full name”;</div> <div> The size of the ‘name’ array is 13.</div> <div> Suppose, we have a group of character strings and we want to store them in a twodimensional</div> <div> array. As we already discussed, an array has same number of columns in</div> <div> each row, e.g. a[5][10] array has 10 columns in each row. Now if we store character</div> <div> strings of variable length in a two-dimensional array, it is necessary to set the number</div> <div> of columns of the array as the length of the longest character string in the group (plus</div> <div> 1 byte for ‘\0’ character). But the space within rows of the array would be wasted for</div> <div> all character strings with shorter length as compared to the number of columns. We</div> <div> don’t want to waste this space and want to occupy the minimum space required to</div> <div> store a character string in the memory.</div> <div> If we use the conventional two-dimensional array like a [5] [10], there is no way of</div> <div> using variable space for rows. All the rows will have fixed ’10’ number of columns in</div> <div> this case. But in case of an Array of Pointers, we can allocate variable space. An array</div> <div> of pointers is used to store pointers in it. Now we will try to understand how do we</div> <div> declare an array of pointers. The following statement can help us in comprehending it</div> <div> properly.</div> <div> char * myarray[10];</div> <div> We read it as: ‘myarray is an array of 10 pointers to character’. If we take out the size</div> <div> of the array, it will become variable as:</div> <div> char * myarray[] = {“Amir”, “Jehangir”};</div> <div> For first pointer myarray[0], 5 bytes (4 bytes for ‘Amir’ plus 1 byte for ‘\0’) of</div> <div> memory has been allocated. For second pointer myarray[1], 9 bytes of memory is</div> <div> allocated. So this is variable allocation depending on the length of character string.</div> <div> What this construct has done for us? If we use normal two-dimensional array, it will</div> <div> require fixed space for rows and columns. Therefore, we have used array of pointers</div> <div> here. We declared an array of pointers and initialized it with variable length character</div> <div> strings. The compiler allocates the same space as required for the character string to</div> <div> fit in. Therefore, no space goes waste. This approach has huge advantage.</div> <div> Amir\0</div> <div> Jehangir\0</div> <div> myarray</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 177</div> <div> We will know more about Pointers to Pointers within next topic of Command-line</div> <div> Arguments and also in the case study given at the end of this lecture.</div> <div> Command Line Arguments</div> <div> Until now, we have always written the ‘main()’ function as under:</div> <div> main( )</div> <div> {</div> <div> . . . // code statements</div> <div> }</div> <div> But we are now in a position to write something inside the parenthesis of the ‘main()’</div> <div> function. In C language, whenever a program is executed, the user can provide the</div> <div> command-line arguments to it like:</div> <div> C:\Dev-cpp\work>Program-name argument1 argument2 ……argumentN</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 178</div> <div> We have so far been taking input using the ‘cout’ and ‘cin’ in the program. But now</div> <div> we can also pass arguments from the command line just before executing the</div> <div> program. For this purpose, we will need a mechanism. In C, this can be done by using</div> <div> ‘argc’ and ‘argv’ arguments inside the main( ) function as:</div> <div> void main(int argc, char **argv)</div> <div> {</div> <div> . . .</div> <div> }</div> <div> Note that ‘argc’ and ‘argv’ are conventional names of the command line parameters</div> <div> of the ‘main()’ function. However, you can give the desired names to them.</div> <div> argc = Number of command line arguments. Its type is ‘int’.</div> <div> argv = It is a pointer to an array of character strings that contain the arguments, one</div> <div> per string. ‘**argv’ can be read as pointer to pointer to char.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 179</div> <div> This has been further explained in the following program. It counts down from a</div> <div> value specified on the command line and beeps when it reaches 0.</div> <div> /* This program explains the use of command line arguments */</div> <div> #include <iostream.h></div> <div> #include <stdlib.h> //Included for ‘atoi( )’ function</div> <div> main(int argc, char **argv)</div> <div> {</div> <div> int disp, count;</div> <div> if(argc < 2)</div> <div> {</div> <div> cout << "Enter the length of the count\n";</div> <div> cout << "on the command line. Try again.\n";</div> <div> return 1;</div> <div> }</div> <div> if(argc == 3 && !strcmp(*(argv + 2), "display"))</div> <div> {</div> <div> disp = 1;</div> <div> }</div> <div> else</div> <div> {</div> <div> disp = 0;</div> <div> }</div> <div> for(count = atoi(*(argv + 1)); count; --count)</div> <div> {</div> <div> if(disp)</div> <div> {</div> <div> cout << count <<' ';</div> <div> }</div> <div> }</div> <div> cout << '\a'; // ’\a’causes the computer to beep</div> <div> return 0;</div> <div> }</div> <div> You must have noted that if no arguments are specified, an error message will be</div> <div> printed. It is common for a program that uses command-line arguments to issue</div> <div> instructions if an attempt has been made to run it without the availability of proper</div> <div> information. The first argument containing the number is converted into an integer</div> <div> using the standard function ‘atoi( )’. Similarly, if the string ‘display’ is present as the</div> <div> second command-line argument, the count will also be displayed on the screen.</div> <div> In theory, you can have up to 32,767 arguments but most operating systems do not</div> <div> allow more than a few because of the fixed maximum length of command-line. These</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 180</div> <div> arguments are normally used to indicate a file name or an option. Using commandline</div> <div> arguments lends your program a very professional touch and facilitates the</div> <div> program’s use in batch files.</div> <div> Case Study: A Card Shuffling and Dealing Simulation</div> <div> Now we want to move on to a real-world example where we can demonstrate pointer</div> <div> to pointer mechanism.</div> <div> Problem:</div> <div> Write a program to randomly shuffle the deck of cards and to deal it out.</div> <div> Some Facts of Card Games:</div> <div> - There are 4 suits in one deck: Hearts, Spades, Diamonds and Clubs.</div> <div> - Each suit has 13 cards: Ace, Deuce, Three, Four, Five, Six, Seven, Eight, Nine,</div> <div> Ten, Jack, Queen and King.</div> <div> - A deck has 13 * 4 = 52 cards in total.</div> <div> Problem Analysis, Design and Implementation:</div> <div> As obvious from the problem statement, we are dealing with the deck of cards,</div> <div> required to be identified. A card is identified by its suit i.e. it may be one of the</div> <div> Hearts, Spades, Diamonds or Clubs. Also every card has one value in the range</div> <div> starting from Ace to King. So we want to identify them in our program and our</div> <div> requirement is to use English like ‘five of Clubs’. We will declare one array of suit</div> <div> like:</div> <div> const char *suite[4] = {“Hearts”, “Diamonds”, “Clubs”, “Spades” };</div> <div> The second array is of values of cards:</div> <div> const char *face[13] = { “Ace”, “Deuce”, “Three”, “Four”, “Five”, “Six”,</div> <div> “Seven”, “Eight”, “Nine”, “Ten”, “Jack”, “Queen” and “King”};</div> <div> You must have noticed the use of array of pointers and ‘const’ keyword here. Both the</div> <div> arrays are declared in a way to avoid any wastage of space. Also notice the use of</div> <div> ‘const’ keyword. We declared arrays as constants because we want to use these values</div> <div> without modifying them.</div> <div> Now we come to deck which has 52 cards. The deck is the one that is being shuffled</div> <div> and dealt. Definitely, it has some algorithmic requirements.</div> <div> Firstly, what should be size and structure of the deck. It can either be linear array of</div> <div> 52 elements or 4 suites and 13 values (faces) per suit. Logically, it makes sense to</div> <div> have two-dimensional array of 4 suites and 13 faces per suit like:</div> <div> int deck[4][13] = {0};</div> <div> We will now think in terms of Algorithm Analysis.</div> <div> The ‘deck’ is initialized with the 0 value, so that it holds no cards at start or it is</div> <div> empty. We want to distribute 52 cards. Who will load the ‘deck’ first, shuffle the</div> <div> cards and deal them out. How to do it?</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 181</div> <div> As we want to select 52 cards (a deck) randomly, therefore, we can think of a loop to</div> <div> get one card randomly in every iteration. We will randomly choose one out of the 4</div> <div> suites and select one value out of 13 values and store the card with its card number</div> <div> value in the deck. By this way, we will be writing numbers in the two-dimensional</div> <div> array of ‘deck’ randomly. That functionality is part of ‘shuffle ()’ function.</div> <div> void shuffle( int wDeck[][13] )</div> <div> {</div> <div> int row, column, card;</div> <div> for ( card = 1; card <= 52; card++){</div> <div> do{</div> <div> row = rand() % 4;</div> <div> column = rand() % 13;</div> <div> } while( wDeck [ row ][ column ] != 0 );</div> <div> wDeck[ row ][ column ] = card;</div> <div> }</div> <div> }</div> <div> You have noticed the ‘rand()’ function usage to generate random numbers. We are</div> <div> dividing the randomly generated number by 4 and 13 to ensure that we get numbers</div> <div> within our desired range. That is 0 to 3 for suites and 0 to 12 for values or faces. You</div> <div> also see the condition inside the ‘while statement, ‘wDeck[ row ][ column ] != 0 ‘.</div> <div> This is to ensure that we don’t overwrite row and column, which has already been</div> <div> occupied by some card.</div> <div> Now we want to deal the deck. How to deal it?</div> <div> “At first, search for card number 1 inside the deck, wherever it is found inside the</div> <div> ‘deck’ array, note down the row of this element. Use this row to get the name of the</div> <div> suite from the ‘suite’ array. Similarly use the column to take out the value of the card</div> <div> from the ‘face’ array.” See that the deal function is quite simple now.</div> <div> void deal( const int wDeck[][ 13 ], const char *wFace[], const char *wSuit[])</div> <div> {</div> <div> int card, row, column;</div> <div> for ( card = 1; card <= 52; card++ )</div> <div> for( row = 0; row <= 3; row++)</div> <div> for( column = 0; column <= 12; column++)</div> <div> if( wDeck[ row ][ column ] == card )</div> <div> cout << card << ". " <<wFace[ column ]</div> <div> << " of " << wSuit [row ] << '\n';</div> <div> }</div> <div> Here, we are not doing binary search that is more efficient. Instead, we are using</div> <div> simple brute force search. Also see the ‘for loops’ carefully and how we are printing</div> <div> the desired output.</div> <div> Now we will discuss a little bit about the srand() function used while generating</div> <div> random numbers. We know that computers can generate random numbers through the</div> <div> ‘rand()’ function. Is it truly random? Be sure , it is not truly random. If you call</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 182</div> <div> ‘rand()’ function again and again. It will give you numbers in the same sequence. If</div> <div> you want your number to be really random number, it is better to set the sequence to</div> <div> start every time from a new value. We have used ‘srand()’ function for this purpose. It</div> <div> is a seed to the random number generator. Seed initializes the random number</div> <div> generator with a different value every time to generate true random numbers. We call</div> <div> ‘srand()’ function with a different value every time. The argument to ‘srand()’</div> <div> function is taken from the ‘time()’ function which is giving us a new value after every</div> <div> one second. Every time we try to run the program, ‘time()’ returns a different number</div> <div> of seconds, which are passed to ‘srand()’ function as an argument so that the seed to</div> <div> the random number generator is a different number. It means that the random number</div> <div> generator now generates a different sequence of random numbers.</div> <div> Although, you can copy this program and see the output after executing it, but this is</div> <div> not the objective of this exercise. You are required to study the problem and see the</div> <div> constructs very carefully. In this problem, you have examples of nested loops, array of</div> <div> pointers, variable sized strings in an array of pointers and random number usage in the</div> <div> real world problem etc.</div> <div> /* Card shuffling and dealing program */</div> <div> #include <iostream.h></div> <div> #include <stdlib.h></div> <div> #include <time.h></div> <div> void shuffle( int [] [ 13 ]);</div> <div> void deal( const int [][ 13 ], const char *[], const char *[]);</div> <div> int main()</div> <div> {</div> <div> const char *suite[ 4 ] = {"Hearts", "Diamonds", "Clubs", "Spades" };</div> <div> const char *face[ 13 ] = { "Ace", "Deuce", "Three", "Four", "Five", "Six", "Seven",</div> <div> "Eight", "Nine", "Ten", "Jack", "Queen", "King"};</div> <div> int deck[ 4 ][ 13 ] = { 0 };</div> <div> srand( time( 0 ) );</div> <div> shuffle( deck );</div> <div> deal( deck, face, suite );</div> <div> return 0;</div> <div> }</div> <div> void shuffle( int wDeck[][13] )</div> <div> {</div> <div> int row, column, card;</div> <div> for ( card = 1; card <= 52; card++){</div> <div> do{</div> <div> row = rand() % 4;</div> <div> column = rand() % 13;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 183</div> <div> } while( wDeck [ row ][ column ] != 0 );</div> <div> wDeck[ row ][ column ] = card;</div> <div> }</div> <div> }</div> <div> void deal( const int wDeck[][ 13 ], const char *wFace[], const char *wSuit[])</div> <div> {</div> <div> int card, row, column;</div> <div> const char *space;</div> <div> for ( card = 1; card <= 52; card++ )</div> <div> for( row = 0; row <= 3; row++)</div> <div> for( column = 0; column <= 12; column++)</div> <div> if( wDeck[ row ][ column ] == card )</div> <div> cout << card << ". " <<wFace[ column ] << " of " << wSuit</div> <div> [row ] << '\n';</div> <div> }</div> <div> A sample output of the program is:</div> <div> 1. Six of Diamonds</div> <div> 2. Ten of Hearts</div> <div> 3. Nine of Clubs</div> <div> 4. King of Hearts</div> <div> 5. Queen of Clubs</div> <div> 6. Five of Clubs</div> <div> 7. Queen of Hearts</div> <div> 8. Eight of Hearts</div> <div> 9. Ace of Diamonds</div> <div> 10. Ten of Diamonds</div> <div> 11. Seven of Spades</div> <div> 12. Ten of Clubs</div> <div> 13. Seven of Clubs</div> <div> 14. Three of Spades</div> <div> 15. Deuce of Clubs</div> <div> 16. Eight of Diamonds</div> <div> 17. Eight of Clubs</div> <div> 18. Nine of Spades</div> <div> 19. Three of Clubs</div> <div> 20. Jack of Clubs</div> <div> 21. Queen of Spades</div> <div> 22. Jack of Hearts</div> <div> 23. Jack of Spades</div> <div> 24. Jack of Diamonds</div> <div> 25. King of Diamonds</div> <div> 26. Seven of Hearts</div> <div> 27. Five of Spades</div> <div> 28. Seven of Diamonds</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 184</div> <div> 29. Deuce of Hearts</div> <div> 30. Ace of Spades</div> <div> 31. Five of Diamonds</div> <div> 32. Three of Hearts</div> <div> 33. Six of Clubs</div> <div> 34. Four of Hearts</div> <div> 35. Ten of Spades</div> <div> 36. Deuce of Spades</div> <div> 37. Three of Diamonds</div> <div> 38. Eight of Spades</div> <div> 39. Nine of Hearts</div> <div> 40. Ace of Clubs</div> <div> 41. Four of Spades</div> <div> 42. Queen of Diamonds</div> <div> 43. King of Clubs</div> <div> 44. Five of Hearts</div> <div> 45. Ace of Hearts</div> <div> 46. Deuce of Diamonds</div> <div> 47. Four of Diamonds</div> <div> 48. Four of Clubs</div> <div> 49. Six of Hearts</div> <div> 50. Six of Spades</div> <div> 51. King of Spades</div> <div> 52. Nine of Diamonds</div> <div> Exercises</div> <div> 1. Write the program ‘tail’, which prints the last n lines of its input. By default, n is</div> <div> 10,</div> <div> let’s say, but it can be changed by an optional argument, so that</div> <div> tail -n</div> <div> prints the last n lines.</div> <div> Tips</div> <div> 􀂾 Pointers and arrays are closely related in C. The array names can be used as</div> <div> pointers but only as constant pointers.</div> <div> 􀂾 A function receiving a multi-dimensional array as a parameter must minimally</div> <div> define all dimensions except the leftmost one.</div> <div> 􀂾 Each time a pointer is incremented, it points to the memory location of the next</div> <div> element of its base type but in case of two-dimensional array, if you add some</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 185</div> <div> number in a two-dimensional array name, it will jump as many rows as the added</div> <div> number.</div> <div> 􀂾 If the array is to be accessed in random order, then the pointer approach may not</div> <div> be better than array indexing.</div> <div> 􀂾 The use of pointers may reduce the wastage of memory space. As discussed in this</div> <div> lecture if we store a set of character strings of different lengths in a twodimensional</div> <div> array, the memory space is wasted.</div> <div> 􀂾 Pointers may be arrayed (stored in an array) like any other data type.</div> <div> 􀂾 An array of pointers is the same as pointers to pointers.</div> <div> 􀂾 Although, you can give your desired names to the command line parameters</div> <div> inside ‘main()’ function but ‘argc’ and ‘argv’ are conventionally used.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 186</div> <div> Lecture No. 17</div> <div> Reading Material</div> <div> Deitel & Deitel - C++ How to Program Chapter 5</div> <div> 5.29,5.30, 5.31, 5.32,</div> <div> 5.33, 5.34</div> <div> Chapter 16</div> <div> 16.16 –</div> <div> 16.33 ( Pages 869 –</div> <div> 884)</div> <div> Summary</div> <div> • String Handling</div> <div> • String Manipulation Functions</div> <div> • Character Handling Functions</div> <div> • Sample Program</div> <div> • String Conversion Functions</div> <div> • String Functions</div> <div> • Search Functions</div> <div> • Examples</div> <div> • Exercises</div> <div> String Handling</div> <div> We have briefly talked about 'Strings' in some of the previous lectures. In this lecture,</div> <div> you will see how a string may be handled. Before actually discussing the subject, it is</div> <div> pertinent to know how the things were going on before the evolution of the concept of</div> <div> 'strings'.</div> <div> When C language and UNIX operating system were being developed in BELL</div> <div> Laboratories, the scientists wanted to publish the articles. They needed a text editor to</div> <div> publish the articles. What they needed was some easy mechanism by which the</div> <div> articles could be formatted and published. We are talking about the times when PCs</div> <div> and word processors did not exist. It may be very strange thing for you people who</div> <div> can perform the tasks like making the characters bold, large or format a paragraph</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 187</div> <div> with the help of word processors these days. Those scientists had not such a facility</div> <div> available with them. The task of writing article and turning into publishable material</div> <div> was mainly done with the help of typewriters. Then these computer experts decided to</div> <div> develop a program, which could help in the processing of text editing in an easy</div> <div> manner. The resultant efforts led to the development of a program for editing the text.</div> <div> The process to edit text was called text processing. The in- line commands were</div> <div> written as a part of the text and were processed on out put. Later, such programs were</div> <div> evolved in which a command was inserted for the functions like making the character</div> <div> bold. The effect of this command could be preview and then modified if needed.</div> <div> Now coming to the topic of strings again, we will discuss in detail the in-built</div> <div> functions to handle the strings.</div> <div> String Manipulation Functions</div> <div> C language provides many functions to manipulate strings. To understand the</div> <div> functions, let’s consider building block (or unit) of a string i.e., a character. Characters</div> <div> are represented inside the computers in terms of numbers. There is a code number for</div> <div> each character, used by a computer. Mostly the computers use ASCII (American</div> <div> Standard Code for Information Interchange) code for a character to store it. This is</div> <div> used in the computer memory for manipulation. It is used as an output in the form of</div> <div> character. We can write a program to see the ASCII values.</div> <div> We have a data type char to store a character. A character includes every thing, which</div> <div> we can type with a keyboard for example white space, comma, full stop and colon etc</div> <div> all are characters. 0, 1, 2 are also characters. Though, as numbers, they are treated</div> <div> differently, yet they are typed as characters. Another data type is called as int, which</div> <div> stores whole numbers. As we know that characters are stored in side computer as</div> <div> numbers so these can be manipulated in the same form. A character is stored in the</div> <div> memory in one byte i.e. 8 bits. It means that 28 (256) different combinations for</div> <div> different values can be stored. We want to ascertain what number it stores, when we</div> <div> press a key on the board. In other words, we will see what character will be displayed</div> <div> when we have a number in memory.</div> <div> The code of the program, which displays the characters and their corresponding</div> <div> integer, values (ASCII codes) as under.</div> <div> In the program the statement c = i ; has integer value on right hand side (as i is an int)</div> <div> while c has its character representation. We display the value of i and c. It shows us</div> <div> the characters and their integer values.</div> <div> //This program displays the ASCII code table</div> <div> # include <iostream.h></div> <div> main ( )</div> <div> {</div> <div> int i, char c ;</div> <div> for (i = 0 ; i < 256 ; i ++)</div> <div> {</div> <div> c = i ;</div> <div> cout << i << “\t” << c << “\n” ;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 188</div> <div> }</div> <div> }</div> <div> In the output of this program, we will see integer numbers and their character</div> <div> representation. For example, there is a character, say white space (which we use</div> <div> between two words). It is a non-printable character and leaves a space. From the</div> <div> ASCII table, we can see that the values of a-z and A-Z are continuos. We can get the</div> <div> value of an alphabet letter by adding 1 to the value of its previous letter. So what we</div> <div> need to remember as a baseline is the value of ‘a’ and ‘A’.</div> <div> Character Handling Functions</div> <div> C language provides many functions to perform useful tests and manipulations of</div> <div> character data. These functions are found in the header file ctype.h. The programs</div> <div> that have character manipulation or tests on character data must have included this</div> <div> header file to avoid a compiler error. Each function in ctype.h receives a character (an</div> <div> int ) or EOF (end of file; it is a special character) as an argument. ctype.h has many</div> <div> functions, which have self-explanatory names.</div> <div> Of these, int isdigit (int c) takes a simple character as its argument and returns true or</div> <div> false. This function is like a question being asked. The question can be described</div> <div> whether it is a character digit? The answer may be true or false. If the argument is a</div> <div> numeric character (digit), then this function will return true otherwise false. This is a</div> <div> useful function to test the input. To check for an alphabet (i.e. a-z), the function</div> <div> isalpha can be used. isalpha will return true for alphabet a-z for small and capital</div> <div> letters. Other than alphabets, it will return false. The function isalnum (is</div> <div> alphanumeric) returns true if its argument is a digit or letter. It will return false</div> <div> otherwise. All the functions included in ctype.h are shown in the following table with</div> <div> their description.</div> <div> Prototype Description</div> <div> int isdigit( int c ) Returns true if c is a digit and false otherwise.</div> <div> int isalpha( int c ) Returns true if c is a letter and false otherwise.</div> <div> int isalnum( int c ) Returns true if c is a digit or a letter and false otherwise.</div> <div> int isxdigit( int c ) Returns true if c is a hexadecimal digit character and false</div> <div> otherwise.</div> <div> int islower( int c ) Returns true if c is a lowercase letter and false otherwise.</div> <div> int isupper( int c ) Returns true if c is an uppercase letter; false otherwise.</div> <div> int tolower( int c ) If c is an uppercase letter, tolower returns c as a lowercase letter.</div> <div> Otherwise, tolower returns the argument unchanged.</div> <div> int toupper( int c ) If c is a lowercase letter, toupper returns c as an uppercase letter.</div> <div> Otherwise, toupper returns the argument unchanged.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 189</div> <div> int isspace( int c ) Returns true if c is a white-space character—newline ('\n'), space</div> <div> (' '), form feed ('\f'), carriage return ('\r'), horizontal tab ('\t'), or</div> <div> vertical tab ('\v')—and false otherwise</div> <div> int iscntrl( int c ) Returns true if c is a control character and false otherwise.</div> <div> int ispunct( int c ) Returns true if c is a printing character other than a space, a digit,</div> <div> or a letter and false otherwise.</div> <div> int isprint( int c ) Returns true value if c is a printing character including space (' ')</div> <div> and false otherwise.</div> <div> int isgraph( int c ) Returns true if c is a printing character other than space (' ') and</div> <div> false otherwise.</div> <div> The functions tolower and toupper are conversion functions. The tolower function</div> <div> converts its uppercase letter argument into a lowercase letter. If its argument is other</div> <div> than uppercase letter, it returns the argument unchanged. Similarly the toupper</div> <div> function converts its lowercase letter argument into uppercase letter. If its argument is</div> <div> other than lowercase letter, it returns the argument without effecting any change.</div> <div> Sample Program</div> <div> Let’s consider the following example to further demonstrate the use of the functions</div> <div> of ctype.h. Suppose, we write a program which prompts the user to enter a string.</div> <div> Then the string entered is checked to count different types of characters (digit, upper</div> <div> and lowercase letters, white space etc). We keep a counter for each category of</div> <div> character entered. When the user ends the input, the number of characters entered in</div> <div> different types will be displayed. In this example we are using a function getchar(),</div> <div> instead of cin to get the input. This function is defined in header file as stdio.h.</div> <div> While carrying out character manipulation, we use the getchar() function. This</div> <div> function reads a single character from the input buffer or keyboard. This function can</div> <div> get the new line character ‘\n’ (the ENTER key) so we run the loop for input until</div> <div> user presses the ENTER key. As soon as the getchar() gets the ENTER key pressed</div> <div> (i.e. new line character ‘\n’), the loop is terminated. We know that, every C statement</div> <div> returns a value. When we use an assignment statement ( as used in our program c =</div> <div> getchar()), the value assigned to the left hand side variable is the value of the</div> <div> statement too. Thus, the statement (c = getchar()) returns the value that is assigned to</div> <div> char c. Afterwards, this value is compared with the new line character ‘\n’. If it is not</div> <div> equal inside the loop, we apply the tests on c to check whether it is uppercase letter,</div> <div> lowercase letter or a digit etc. In this program, the whole string entered by the user is</div> <div> manipulated character.</div> <div> Following is the code of this program.</div> <div> // Example: analysis of text using <ctype.h> library</div> <div> #include <iostream.h></div> <div> #include <stdio.h></div> <div> #include <ctype.h></div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 190</div> <div> main()</div> <div> {</div> <div> char c;</div> <div> int i = 0, lc = 0, uc = 0, dig = 0, ws = 0, pun = 0, oth = 0;</div> <div> cout << "Please enter a character string and then press ENTER: ";</div> <div> // Analyse text as it is input:</div> <div> while ((c = getchar()) != '\n')</div> <div> {</div> <div> if (islower(c))</div> <div> lc++;</div> <div> else if (isupper(c))</div> <div> uc++;</div> <div> else if (isdigit(c))</div> <div> dig++;</div> <div> else if (isspace(c))</div> <div> ws++;</div> <div> else if (ispunct(c))</div> <div> pun++;</div> <div> else</div> <div> oth++;</div> <div> }</div> <div> // display the counts of different types of characters</div> <div> cout << "You typed:"<< endl;</div> <div> cout<< "lower case letters = "<< lc<< endl;</div> <div> cout << "upper case letters = " << uc <<endl;</div> <div> cout<< "digits = " << dig << endl;</div> <div> cout<< "white space = "<< ws << endl;</div> <div> cout<< "punctuation = "<< pun<< endl;</div> <div> cout<< "others = "<< oth;</div> <div> }</div> <div> A sample output of the program is given below.</div> <div> Please enter a character string and then press ENTER: Sixty Five = 65.00</div> <div> You typed:</div> <div> lower case letters = 7</div> <div> upper case letters = 2</div> <div> digits = 4</div> <div> white space = 3</div> <div> punctuation = 2</div> <div> others = 0</div> <div> String Conversion Functions</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 191</div> <div> The header file stdlib.h includes functions, used for different conversions. When we</div> <div> get input of a different type other than the type of variable in which the value is being</div> <div> stored, it warrants the need to convert that type into another type. These conversion</div> <div> functions take an argument of a type and return it after converting into another type.</div> <div> These functions and their description are given in the table below.</div> <div> Prototype Description</div> <div> double atof( const char *nPtr ) Converts the string nPtr to double.</div> <div> Int atoi( const char *nPtr ) Converts the string nPtr to int.</div> <div> long atol( const char *nPtr ) Converts the string nPtr to long int.</div> <div> double strtod( const char *nPtr, char</div> <div> **endPtr )</div> <div> Converts the string nPtr to double.</div> <div> long strtol( const char *nPtr, char</div> <div> **endPtr, int base )</div> <div> Converts the string nPtr to long.</div> <div> unsigned long strtoul( const char</div> <div> *nPtr, char **endPtr, int base )</div> <div> Converts the string nPtr to unsigned long.</div> <div> Use of these functions:</div> <div> While writing main () in a program, we can put them inside the parentheses of main.</div> <div> ‘int arg c, char ** arg v are written inside the parentheses. The arg c is the count of</div> <div> number of arguments passed to the program including the name of the program itself</div> <div> while arg v is a vector of strings or an array of strings. It is used while giving</div> <div> command line arguments to the program. The arguments in the command line will</div> <div> always be character strings. The number in the command line (for example 12.8 or</div> <div> 45) are stored as strings. While using the numbers in the program, we need these</div> <div> conversion functions.</div> <div> Following is a simple program which demonstrate the use of atoi function. This</div> <div> program prompts the user to enter an integer between 10-100, and checks if a valid</div> <div> integer is entered.</div> <div> //This program demonstrate the use of atoi function</div> <div> # include <iostream.h></div> <div> # include <stdlib.h></div> <div> main( )</div> <div> {</div> <div> int anInteger;</div> <div> char myInt [20]</div> <div> cout << "Enter an integer between 10-100 : ";</div> <div> cin >> myInt;</div> <div> if (atoi(myInt) == 0)</div> <div> cout << "\nError : Not a valid input"; // could be non numeric</div> <div> else</div> <div> {</div> <div> anInteger = atoi(myInt);</div> <div> if (anInteger < 10 || anInteger > 100)</div> <div> cout << "\nError : only integers between 10-100 are allowed!";</div> <div> else</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 192</div> <div> cout << "\n OK, you have entered " << anInteger;</div> <div> }</div> <div> }</div> <div> The output of the program is as follows.</div> <div> Enter an integer between 10-100 : 45.5</div> <div> OK, you have entered 45</div> <div> String Functions</div> <div> We know a program to guess a number, stored in the computer. To find out a name</div> <div> (which is a character array) among many names in the memory, we can perform string</div> <div> comparison on two strings by comparing a character of first string with the</div> <div> corresponding character of the second string. Before doing this, we check the length</div> <div> of both the strings to compare. C library provides functions to compare strings, copy a</div> <div> string and for other string manipulations.</div> <div> The following table shows the string manipulation functions and their description. All</div> <div> these functions are defined in the header file string.h, in the C library.</div> <div> Function prototype Function description</div> <div> char *strcpy( char *s1, const char *s2 ) Copies string s2 into character array s1.</div> <div> The value of s1 is returned.</div> <div> char *strncpy( char *s1, const char *s2,</div> <div> size_t n )</div> <div> Copies at most n characters of string s2</div> <div> into array s1. The value of s1 is</div> <div> returned.</div> <div> char *strcat( char *s1, const char *s2 ) Appends string s2 to array s1. The first</div> <div> character of s2 overwrites the</div> <div> terminating null character of s1. The</div> <div> value of s1 is returned.</div> <div> char *strncat( char *s1, const char *s2,</div> <div> size_t n )</div> <div> Appends at most n characters of string s2</div> <div> to array s1. The first character of s2</div> <div> overwrites the terminating null character</div> <div> of s1. The value of s1 is returned.</div> <div> int strcmp( const char *s1, const char *s2) Compares string s1 to s2. Returns a</div> <div> negative number if s1 < s2, zero if s1 ==</div> <div> s2 or a positive number if s1 > s2</div> <div> int strncmp( const char *s1, const char *s2,</div> <div> size_t n )</div> <div> Compares up to n characters of string s1</div> <div> to s2. Returns a negative number if s1 <</div> <div> s2, zero if s1 == s2 or a positive number</div> <div> if s1 > s2.</div> <div> int strlen ( const char *s) Determines the length of string s. The</div> <div> number of characters preceding the</div> <div> terminating null character is returned.</div> <div> Let’s look at the string copy function which is strcpy. The prototype of this function</div> <div> is</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 193</div> <div> char *strcpy( char *s1, const char *s2 )</div> <div> Here the first argument is a pointer to a character array or string s1 whereas the</div> <div> second argument is a pointer to a string s2. The string s2 is copied to string s1 and a</div> <div> pointer to that resultant string is returned. The string s2 remains the same. We can</div> <div> describe the string s1 as the destination string and s2 as the source string. As the</div> <div> source remains the same during the execution of strcpy and other string functions, the</div> <div> const keyword is used before the name of source string. The const keyword prevents</div> <div> any change in the source string (i.e. s2). If we want to copy a number of characters of</div> <div> a string instead of the entire string, the function strncpy is employed. The function</div> <div> strncpy has arguments a pointer to destination strings (s1), a pointer to source string</div> <div> (s2) . The third argument is int n. Here n is the number of characters which we want</div> <div> to copy from s2 into s1. Here s1 must be large enough to copy the n number of</div> <div> characters.</div> <div> The next function is strcat (string concatenation). This function concatenates (joins)</div> <div> two strings. For example, in a string, we have first name of a student, followed by</div> <div> another string, the last name of the student is found. We can concatenate these two</div> <div> strings to get a string, which holds the first and the last name of the student. For this</div> <div> purpose, we use the strcat function. The prototype of this function is char *strcat(</div> <div> char *s1, const char *s2 ). This function writes the string s2 (source) at the end of</div> <div> the string s1(destination). The characters of s1 are not overwritten. We can</div> <div> concatenate a number of characters of s2 to s1 by using the function strncat. Here we</div> <div> provide the function three arguments, a character pointer to s1, a character pointer to</div> <div> s2 while third argument is the number of characters to be concatenated. The prototype</div> <div> of this function is written as</div> <div> char *strncat( char *s1, const char *s2, size_t n )</div> <div> Examples</div> <div> Let’s consider some simple examples to demonstrate the use of strcpy, strncpy,</div> <div> strcat and strncat functions. To begin with, we can fully understand the use of the</div> <div> function strcpy and strncpy.</div> <div> Example 1</div> <div> //Program to display the operation of the strcpy() and strncpy()</div> <div> # include<iostream.h></div> <div> # include<string.h></div> <div> void main()</div> <div> {</div> <div> char string1[15]="String1";</div> <div> char string2[15]="String2";</div> <div> cout<<"Before the copy :"<<endl;</div> <div> cout<<"String 1:\t"<<string1<<endl;</div> <div> cout<<"String 2:\t"<<string2<<endl;</div> <div> //copy the whole string</div> <div> strcpy(string2,string1); //copy string1 into string2</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 194</div> <div> cout<<"After the copy :"<<endl;</div> <div> cout<<"String 1:\t"<<string1<<endl;</div> <div> cout<<"String 2:\t"<<string2<<endl;</div> <div> //copy three characters of the string1 into string3</div> <div> strncpy(string3, string1, 3);</div> <div> cout << “strncpy (string3, string1, 3) = “ << string3 ;</div> <div> }</div> <div> Following is the output of the program.</div> <div> Before the copy :</div> <div> String 1: String1</div> <div> String 2: String2</div> <div> After the copy :</div> <div> String 1: String1</div> <div> String 2: String1</div> <div> Strncpy (string3, string1, 3) = Str</div> <div> Example 2 (strcat and strncat)</div> <div> The following example demonstrates the use of function strcat and strncat.</div> <div> //Program to display the operation of the strcat() and strncat()</div> <div> #include <iostream.h></div> <div> #include <string.h></div> <div> int main()</div> <div> {</div> <div> char s1[ 20 ] = "Welcome to ";</div> <div> char s2[] = "Virtual University ";</div> <div> char s3[ 40 ] = "";</div> <div> cout<< "s1 = " << s1 << endl << "s2 = " << s2 << endl << "s3 = " << s3 << endl;</div> <div> cout<< "strcat( s1, s2 ) = “<< strcat( s1, s2 );</div> <div> cout << "strncat( s3, s1, 6 ) = “ << strncat( s3, s1, 6 );</div> <div> }</div> <div> The output of the program is given below.</div> <div> s1 = Welcome to</div> <div> s2 = Virtual University</div> <div> s3 =</div> <div> strcat( s1, s2 ) = Welcome to Virtual University</div> <div> strncat( s3, s1, 7 ) = Welcome</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 195</div> <div> Now we come across the function strcmp. This function compares two strings, and</div> <div> returns an integer value depending upon the result of the comparison. The prototype</div> <div> of this function is int strcmp( const char *s1, const char *s2)</div> <div> This function returns a number less than zero (a negative number), if s1 is less than</div> <div> s2. It returns zero if s1 and s2 are identical and returns a positive number (greater than</div> <div> zero) if s1 is greater than s2. The space character in a string and lower and upper case</div> <div> letters are also considered while comparing two strings. So the strings “Hello”,</div> <div> “hello” and “He llo” are three different strings these are not identical.</div> <div> Similarly there is a function strncmp, which can be used to compare a number of</div> <div> characters of two strings. The prototype of this function is</div> <div> int strncmp( const char *s1, const char *s2, size_t n )</div> <div> Here s1 and s2 are two strings and n is the number upto which the characters of s1</div> <div> and s2 are compared. Its return type is also int. It returns a negative number if first n</div> <div> characters of s1 are less than first n characters of s2. It returns zero if n characters of</div> <div> s1 and n characters of s2 are identical. However, it returns a positive number if n</div> <div> characters of s1 are greater than n characters of s2.</div> <div> Now we will talk about the function, ‘strlen’ (string length) which is used to</div> <div> determine the length of a character string. This function returns the length of the</div> <div> string passed to it. The prototype of this function is given below.</div> <div> int strlen ( const char *s)</div> <div> This function determines the length of string s. the number of characters preceding the</div> <div> terminating null character is returned.</div> <div> Search Functions</div> <div> C provides another set of functions relating to strings, called search functions. With</div> <div> the help of these functions, we can do different types of search in a string. For</div> <div> example, we can find at what position a specific character exists. We can search a</div> <div> character starting from any position in the string. We can find the preceding or</div> <div> proceeding string from a specific position. We can find a string inside another string.</div> <div> These functions are given in the following table.</div> <div> Function prototype Function description</div> <div> char *strchr( const char *s, int c ); Locates the first occurrence of character c in string</div> <div> s. If c is found, a pointer to c in s is returned.</div> <div> Otherwise, a NULL pointer is returned.</div> <div> size_t strcspn( const char *s1,</div> <div> const char *s2 );</div> <div> Determines and returns the length of the initial</div> <div> segment of string s1 consisting of characters not</div> <div> contained in string s2.</div> <div> size_t strspn( const char *s1, const</div> <div> char *s2 );</div> <div> Determines and returns the length of the initial</div> <div> segment of string s1 consisting only of characters</div> <div> contained in string s2.</div> <div> char *strpbrk( const char *s1,</div> <div> const char *s2 );</div> <div> Locates the first occurrence in string s1 of any</div> <div> character in string s2. If a character from string s2 is</div> <div> found, a pointer to the character in string s1 is</div> <div> returned. Otherwise, a NULL pointer is returned.</div> <div> char *strrchr( const char *s, int c</div> <div> );</div> <div> Locates the last occurrence of c in string s. If c is</div> <div> found, a pointer to c in string s is returned.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 196</div> <div> Otherwise, a NULL pointer is returned.</div> <div> char *strstr( const char *s1, const</div> <div> char *s2 );</div> <div> Locates the first occurrence in string s1 of string s2.</div> <div> If the string is found, a pointer to the string in s1 is</div> <div> returned. Otherwise, a NULL pointer is returned.</div> <div> char *strtok( char *s1, const char</div> <div> *s2 );</div> <div> A sequence of calls to strtok breaks string s1 into</div> <div> “tokens”—logical pieces such as words in a line of</div> <div> text—separated by characters contained in string s2.</div> <div> The first call contains s1 as the first argument, and</div> <div> subsequent calls to continue tokenizing the same</div> <div> string contain NULL as the first argument. A</div> <div> pointer to the current token is returned by each call.</div> <div> If there are no more tokens when the function is</div> <div> called, NULL is returned.</div> <div> Example 3</div> <div> Here is an example, which shows the use of different string manipulation functions.</div> <div> The code of the program is given below.</div> <div> //A program which shows string manipulation using <string.h> library</div> <div> #include <iostream.h></div> <div> #include <string.h></div> <div> #include <stdlib.h></div> <div> main()</div> <div> {</div> <div> char s1[] = "Welcome to " ;</div> <div> char s2[] = "Virtual University" ;</div> <div> char s3[] = "Welcome to Karachi" ;</div> <div> char city[] = "Karachi";</div> <div> char province[] = "Sind";</div> <div> char s[80];</div> <div> char *pc;</div> <div> int n;</div> <div> cout << "s1 = " << s1 << endl << "s2 = " << s2 << endl ;</div> <div> cout << "s3 = " << s3 << endl ;</div> <div> // function for string length</div> <div> cout << "The length of s1 = " << strlen(s1) << endl ;</div> <div> cout << "The length of s2 = " << strlen(s2) << endl ;</div> <div> cout << "The length of s3 = " << strlen(s3) << endl ;</div> <div> strcpy(s, "Hyderabad"); // string copy</div> <div> cout<< "The nearest city to "<< city << " is " << s << endl ;</div> <div> strcat(s, " and "); // string concatenation</div> <div> strcat(s,city);</div> <div> strcat(s, " are in ");</div> <div> strcat(s, province);</div> <div> strcat(s, ".\n");</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 197</div> <div> cout << s;</div> <div> if (!(strcmp (s1,s2))) // ! is used as zero is returned if s1 & s2 are equal</div> <div> cout << "s1 and s2 are identical" << endl ;</div> <div> else</div> <div> cout << "s1 and s2 are not identical" << endl ;</div> <div> if (!(strncmp (s1,s3,7))) // ! is used as zero is returned for equality</div> <div> cout << "First 7 characters of s1 and s3 are identical" << endl ;</div> <div> else</div> <div> cout << "First 7 characters of s1 and s3 are not identical" << endl ;</div> <div> }</div> <div> Following is the output of the program.</div> <div> S1 = Welcome to</div> <div> S2 = Virtual University</div> <div> S3 = Welcome to Karachi</div> <div> The length of s1 = 11</div> <div> The length of s2 = 18</div> <div> The length of s3 = 18</div> <div> The nearest city to Karachi is Hyderabad</div> <div> Hyderabad and Karachi are in Sind.</div> <div> S1 and s2 are not identical</div> <div> First 7 characters of s1 and s3 are identical</div> <div> Exercises</div> <div> 1: Write a program that displays the ASCII code set in tabular form on the screen.</div> <div> 2: Write your own functions for different manipulations of strings.</div> <div> 3: Write a program, which uses different search functions.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 198</div> <div> Lecture No. 18</div> <div> Reading Material</div> <div> Deitel & Deitel - C++ How to Program Chapter 14</div> <div> 14.3, 14.4, 14.5, 14.6</div> <div> Summary</div> <div> Files</div> <div> Text File Handling</div> <div> Example 1</div> <div> Output File Handling</div> <div> Example 2</div> <div> Tips</div> <div> Files</div> <div> Today’s topic is about files and file handling. We have been talking about bit, bytes,</div> <div> character, numbers etc. Then we discuss about strings, which are actually character</div> <div> arrays. When we combine all these things, it becomes a program. We type a letter or a</div> <div> document in word processor. Similarly, we can have a novel as a combinations of</div> <div> characters, words, sentences, bigger collection of words and sentences. These are no</div> <div> longer bits and bytes. We call these sentences, paragraphs as files. There are many</div> <div> types of files in computer. Primarily, there are two types of files i.e. text files and</div> <div> executable program files. Text files consist of readable English characters. These</div> <div> include our simple text files, or word processor file etc. On the other hand, the</div> <div> executable program files run the program. In the dos (command prompt window),</div> <div> when we issue the command ‘dir’, a list of files is displayed. Similarly, Windows</div> <div> explorer is used in the windows, click on some folder to see the list of the files in that</div> <div> folder in the right panel. These are the names of the files, which we see. The file</div> <div> properties show the length of the file, date of creation etc. One category of data files is</div> <div> plain text files. We can create plain text files using the windows note pad, type the</div> <div> text and save it. It is an ordinary text, meaning that there is no formatting of text</div> <div> involved and we can view this text using the ‘type’ command of the dos (type</div> <div> filename). Similarly, our source programs are also plain text files. There is no</div> <div> formatted text in cpp files. There is another kind of text files, which are not plain</div> <div> ones. These are the word processor files, containing text that is formatted like, bold,</div> <div> italic, underline, colored text and tables. This formatting information is also stored in</div> <div> the file along with the text. Therefore such files are not plain text files. Same thing</div> <div> applies to spreadsheets having formatting, formulae, cell characteristic etc. Though</div> <div> these files contain some binary information along with the text, yet these are not</div> <div> program files. We created these files using some other program like Microsoft Word,</div> <div> excel etc. Such files also fall in the category of text files. The other type is the</div> <div> program file that executes on the computer. Normally, executable files contain only</div> <div> non-printable binary information. There are different ways of handling these files.</div> <div> Today we will see what is the utility of files in our programs. We know that all the</div> <div> information in the computer memory is volatile. It means when we turn off the</div> <div> computer that information will be lost. The data, written in a program, is actually the</div> <div> part of the program and is saved on the disk. Whenever we execute the program that</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 199</div> <div> data will be available. Suppose we have to develop a payroll system for a factory. For</div> <div> this purpose, we will at first need to gather the data like name of the employees, their</div> <div> salaries etc. Enter all this information before getting their attendance. After collecting</div> <div> all the information, you can calculate their salary and print a report of the salary. Now</div> <div> the question arises whether we will have to enter the name and salary of employees</div> <div> every month. The better way is to store this information once and re-use it every</div> <div> month. We can save this information in a file and can calculate the salary after getting</div> <div> the current month’s attendance of employees. We have to do all the calculations</div> <div> again in case of not saving the report on the disk. It will be nicer if we have saved the</div> <div> output file on the disk. We can take the print out whenever we need. We are</div> <div> discussing this just to give you the justification of using files. The data in the memory</div> <div> is volatile. Similarly, the data, which we key in the program during the execution of a</div> <div> program, is also volatile. To save the data on permanent basis, we need files so that</div> <div> we keep these on the disk and can use whenever needed. Now there is need to learn</div> <div> how to create a file on the disk, read from the file, and write into the file and how to</div> <div> manipulate the data in it. This is the file handling.</div> <div> Text file Handling</div> <div> Let's look what are the basic steps we need for file handling. Suppose we have a file</div> <div> on the disk and want to open it. Then read from or write into the file before finally</div> <div> closing it. The basic steps of file handling are:</div> <div> Open the file</div> <div> Read and write</div> <div> Close the file</div> <div> We have been using cin and cout a lot in our programs. We know that these are the</div> <div> doors by which data can enter and come out. cin is used to enter the data and cout is</div> <div> used to display the data on the screen. Technically, these are known as streams in</div> <div> C++. We will discuss in detail about streams in later lectures. Today we will see some</div> <div> more streams about file handling. This is how 'C++ language' handles files. For this</div> <div> purpose, the header file to be used is <fstream.h> (i.e. file stream). Whenever using</div> <div> files in our program, we will include this header file as #include <fstream.h>. These</div> <div> streams are used the way we have been employing cin and cout but we can do more</div> <div> with these streams. While handling files, one can have three options. Firstly, we will</div> <div> only read the file i.e. read only file. It means the file is used as input for the program.</div> <div> We need to have a stream for input file i.e. ifstream (input file stream). Similarly, if</div> <div> we want to write in some file, ofstream (output file stream) can be used. Sometimes</div> <div> we may need to read and write in the same file. One way is to read from a file,</div> <div> manipulate it and write it in another file, delete the original file and renaming the new</div> <div> file with the deleted file name. We can read, write and manipulate the same file using</div> <div> fstream.h.</div> <div> Let us take a look how can we use these files in our programs. First, we have to</div> <div> include the fstream.h in our programs. Then we need to declare file streams. cin and</div> <div> cout are predefined streams therefore we did not declare these. We can declare file</div> <div> stream as:</div> <div> ifstream inFile; // object for reading from a file</div> <div> ofstream outFile; // object for writing to a file</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 200</div> <div> The variables inFile and outFile are used as handle to refer files. These are like</div> <div> internal variables which will be used to handle the files that are on the disk. If we</div> <div> want to read a file, we will use inFile as declared above to read a file. Any meaningful</div> <div> and self-explanatory name can be used. To deal with a payroll system,</div> <div> payrollDataFile can be used as a file stream variable i.e. ifstream payrollDataFile;.</div> <div> Consider the following statement:</div> <div> ifstream myFile;</div> <div> Here myFile is an internal variable used to handle the file. So far, we did not attach a</div> <div> file with this handle. Before going for attachment, we will have to open a file.</div> <div> Logically, there is function named ‘open’ to open a file. While associating a file with</div> <div> the variable myFile, the syntax will be as under:</div> <div> myFile.open(filename);</div> <div> You have noted that this is a new way of function calling. We are using dot (.)</div> <div> between the myFile and open function. myFile is an object of ifstream and open() is a</div> <div> function of ifstream. The argument for the open function filename is the name of the</div> <div> file on the disk. The data type of argument filename is character string, used to give</div> <div> the file name in double quotation marks. The file name can be simple file name like</div> <div> “payroll.txt”. It can be fully qualified path name like “C:\myprogs\payroll.txt”. In the</div> <div> modern operating systems like Windows, disks are denoted as C: or D: etc. We have</div> <div> different folders in it like ‘myprogs’ and can have files in this folder. The fully</div> <div> qualified path means that we have to give the path beginning from C:\.</div> <div> To under stand it further, suppose that we are working in the folder ‘myprogs’ and our</div> <div> source and executable files are also in this folder. Here, we don’t need to give a</div> <div> complete path and can write it as “payroll.txt”. If the file to be opened is in the current</div> <div> directory (i.e. the program and text file are in the same folder), you can open it by</div> <div> simply giving the name. If you are not familiar with the windows file system, get</div> <div> some information from windows help system. It is a hierarchical system. The disk,</div> <div> which is at the top, contains folder and files. Folders can contain subfolders and files.</div> <div> It is a multi-level hierarchical system. In UNIX, the top level is “root”, which contains</div> <div> files and directories. So it’s like a bottom-up tree. Root is at the top while the</div> <div> branches are spreading downward. Here ‘root’ is considered as root of a tree and files</div> <div> or subfolders are branches.</div> <div> To open a file, we use open function while giving it the name of the file as fully</div> <div> qualified path name or simple name. Then we also tell it what we want to do with that</div> <div> file i.e. we want to read that file or write into that file or want to modify that file. We</div> <div> have declared myFile as ifstream (input file stream) variable so whenever we tried to</div> <div> open a file with ifstream variable it can only be opened for input. Once the file is</div> <div> open, we can read it. The access mechanism is same, as we have been using with</div> <div> streams. So to read a word from the file we can write as:</div> <div> myFile >> c;</div> <div> So the first word of the file will be read in c, where c is a character array. It is similar</div> <div> as we used with cin. There are certain limitations to this. It can read just one word at</div> <div> one time. It means, on encountering a space, it will stop reading further. Therefore,</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 201</div> <div> we have to use it repeatedly to read the complete file. We can also read multiple</div> <div> words at a time as:</div> <div> myFile >> c1 >> c2 >> c3;</div> <div> The first word will be read in c1, 2nd in c2 and 3rd in c3. Before reading the file, we</div> <div> should know some information regarding the structure of the file. If we have a file of</div> <div> an employee, we should know that the first word is employee’s name, 2nd word is</div> <div> salary etc, so that we can read the first word in a string and 2nd word in an int variable.</div> <div> Once we have read the file, it must be closed. It is the responsibility of the</div> <div> programmer to close the file. We can close the file as:</div> <div> myFile.close();</div> <div> The function close() does not require any argument, as we are going to close the file</div> <div> associated with myFile. Once we close the file, no file is associated with myfile now.</div> <div> Lets take a look on error checking mechanism while handling files. Error checking is</div> <div> very important. Suppose we have to open a text file myfile.txt from the current</div> <div> directory, we will write as:</div> <div> ifstream myFile;</div> <div> myFile.open(“myfile.txt”);</div> <div> If this file does not exist on the disk, the variable myFile will not be associated with</div> <div> any file. There may be many reasons due to which the myFile will not be able to get</div> <div> the handle of the file. Therefore, before going ahead, we have to make sure that the</div> <div> file opening process is successful. We can write as:</div> <div> if (!myFile)</div> <div> {</div> <div> cout << “There is some error opening file” << endl;</div> <div> cout << “ File cannot be opened” << end;</div> <div> exit(1);</div> <div> }</div> <div> else</div> <div> cout << “ File opened successfully “ << end;</div> <div> Example 1</div> <div> Let’s write a simple program, which will read from a file ‘myfile.txt’ and print it on</div> <div> the screen. “myfile.txt” contains employee’s name, salary and department of</div> <div> employees. Following is the complete program along with “myfile.txt” file.</div> <div> Sample “myfile.txt”.</div> <div> Name Salary Department</div> <div> Aamir 12000 Sales</div> <div> Amara 15000 HR</div> <div> Adnan 13000 IT</div> <div> Afzal 11500 Marketing</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 202</div> <div> Code of the program.</div> <div> /*</div> <div> * This program reads from a txt file “myfile.txt” which contains the</div> <div> * employee information</div> <div> */</div> <div> #include <iostream.h></div> <div> #include <fstream.h></div> <div> main()</div> <div> {</div> <div> char name[50]; // used to read name of employee from file</div> <div> char sal[10]; // used to read salary of employee from file</div> <div> char dept[30]; // used to read dept of employee from file</div> <div> ifstream inFile; // Handle for the input file</div> <div> char inputFileName[] = "myfile.txt"; // file name, this file is in the current directory</div> <div> inFile.open(inputFileName); // Opening the file</div> <div> // checking that file is successfully opened or not</div> <div> if (!inFile)</div> <div> {</div> <div> cout << "Can't open input file named " << inputFileName << endl;</div> <div> exit(1);</div> <div> }</div> <div> // Reading the complete file word by word and printing on screen</div> <div> while (!inFile.eof())</div> <div> {</div> <div> inFile >> name >> sal >> dept;</div> <div> cout << name << "\t" << sal << " \t" << dept << endl;</div> <div> }</div> <div> inFile.close();</div> <div> }</div> <div> Output of the program.</div> <div> Name Salary Department</div> <div> Aamir 12000 Sales</div> <div> Amara 15000 HR</div> <div> Adnan 13000 IT</div> <div> Afzal 11500 Marketing</div> <div> In the above program, we have declared three variables for reading the data from the</div> <div> input file (i.e. name, sal, dept). The text file “myfile.txt” and the program file should</div> <div> be in the same directory as there is no fully qualified path used with the file name in</div> <div> the open() function. After opening the file, we will check that file is successfully</div> <div> opened or not. If there is some error while opening the file, we will display the error</div> <div> on screen and exit from the program. The statement exit(1) is used to exit from the</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 203</div> <div> program at any time and the control is given back to the operating system. Later, we</div> <div> will read all the data from the file and put it into the variables. The condition in ‘while</div> <div> loop’ is “!inFile.eof()” means until the end of file reached. The function eof() returns</div> <div> true when we reached at the end of file.</div> <div> Output File Handling</div> <div> Let’s talk about the output file handling. You can do several things with output files</div> <div> like, creation of a new file on the disk and writing data in it. Secondly, we may like to</div> <div> open an existing file and overwrite it in such a manner that all the old information is</div> <div> lost from it and new information is stored. Thirdly, we may want to open an existing</div> <div> file and append it in the end. Fourthly, an existing file can be opened and modified in</div> <div> a way that it can be written anywhere in the file. Therefore, when we open a file for</div> <div> output we have several options and we might use any one of these methods. All these</div> <div> things are related to the file-opening mode. The actual syntax of open function is:</div> <div> open (filename, mode)</div> <div> The first argument is the name of the file while the second will be the mode in which</div> <div> file is to be opened. Mode is basically an integer variable but its values are predefined.</div> <div> When we open a file for input, its mode is input file that is defined and</div> <div> available through the header files, we have included. So the correct syntax of file</div> <div> opening for input is:</div> <div> myFile.open(“myfile.txt” , ios::in);</div> <div> The 2nd argument ios::in associates myFile stream object with the “myfile.txt” for</div> <div> input. Similarly, for output files, there are different modes available. To open a file for</div> <div> output mode, ios::out is used. Here is the complete list of modes:</div> <div> Mode Meaning</div> <div> in Open a file or stream for extraction (input)</div> <div> out Open a file or stream for insertion (output)</div> <div> app Append rather than truncate an existing file. Each insertion</div> <div> (output) will be written to the end of the file</div> <div> trunc Discards the file’s contents if it exists. (similar to default</div> <div> behavior)</div> <div> ate Opens the file without truncating, but allows data to be</div> <div> written anywhere in the file</div> <div> binary Treat the file as binary rather than text. A binary file has</div> <div> data stored in internal formats, rather than readable text</div> <div> format</div> <div> If a file is opened with ios::out mode, a new file is created. However, if the file</div> <div> already exists, its contents will be deleted and get empty unless you write something</div> <div> into it. If we want to append into the file, the mode will be ios::app. When we write</div> <div> into the file, it will be added in the end of the file. If we want to write anywhere in the</div> <div> file, the mode is ios::ate. We can position at some particular point and can write there.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 204</div> <div> It is like append mode. But in ‘ate mode’ we can write anywhere in the file. With the</div> <div> trunc mode, the file is truncated, it is similar to out mode.</div> <div> Exercise:</div> <div> Write a program, which creates a new file, and write “Welcome to VU” in it.</div> <div> The code of the program is:</div> <div> /*</div> <div> * This program writes into a txt file “myfileOut.txt” which contains the</div> <div> * “Welcome to VU”</div> <div> */</div> <div> #include <iostream.h></div> <div> #include <fstream.h></div> <div> main()</div> <div> {</div> <div> ofstream outFile; // Handle for the input file</div> <div> char outputFileName[] = "myFileOut.txt"; // The file is created in the current directory</div> <div> char ouputText[100] = "Welcome to VU"; // used to write into the file</div> <div> outFile.open(outputFileName, ios::out); // Opening the file</div> <div> // checking that file is successfully opened or not</div> <div> if (!outFile)</div> <div> {</div> <div> cout << "Can't open input file named " << outputFileName << endl;</div> <div> exit(1);</div> <div> }</div> <div> // Writing into the file</div> <div> outFile << ouputText;</div> <div> outFile.close();</div> <div> }</div> <div> The file “myFileOut.txt”:</div> <div> Welcome to VU</div> <div> Exercise:</div> <div> Write a program, which reads an input file of employee’s i.e. “employeein.txt”, add</div> <div> the salary of each employee by 2000, and write the result in a new file</div> <div> “employeeout.txt”.</div> <div> The sample input file “employeein.txt”</div> <div> Aamir 12000</div> <div> Amara 15000</div> <div> Adnan 13000</div> <div> Afzal 11500</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 205</div> <div> The output file “employeeout.txt” should be as:</div> <div> Name Salary</div> <div> Aamir 14000</div> <div> Amara 17000</div> <div> Adnan 15000</div> <div> Afzal 13500</div> <div> We have been using ‘>>’ sign for reading data from the file. There are some other</div> <div> ways to read from the file. The get() function is used to get a character from the file,</div> <div> so that we can use get() to read a character and put it in a char variable. The last</div> <div> character in the file is EOF, defined in header files. When we are reading file using</div> <div> get() function the loop will be as:</div> <div> char c;</div> <div> while ( (c = inFile.get()) != EOF)</div> <div> {</div> <div> // do all the processing</div> <div> outFile.put(c);</div> <div> }</div> <div> There is one limitation with the ‘>>’ and that is it does not read the new line character</div> <div> and in the output file we have to insert the new line character explicitly, whereas get()</div> <div> function reads each character as it was typed. So if we have to make a copy of a file,</div> <div> the function get() should be used. Can we have a function to put a character in the</div> <div> output file? Yes, the function to write a single character in the out put file is put(), so</div> <div> with the output file stream handle, we can use this function to write a character in the</div> <div> output file.</div> <div> Exercise:</div> <div> Write the above programs using the get() function and verify the difference of ‘>>’</div> <div> and ’get()’ using different input files.</div> <div> Whenever we declare a variable we initialize it like if we declare an integer as int i.</div> <div> We initialize it as i = 0. Similarly we can declare and initialize an input or output file</div> <div> stream variable as:</div> <div> ifstream inFile(“myFileIn.txt”);</div> <div> ofstream outFile(“myfileOut.txt”, ios::out);</div> <div> This is a short hand for initialization. This is same as we open it with open() function.</div> <div> Normally we open a file explicitly with the open() function and close it explicitly with</div> <div> close() function. Another advantage of using explicitly opening a file using the open()</div> <div> function is, we can use the same variable to associate with other files after closing the</div> <div> first file.</div> <div> We can also read a line from the file. The benefit of reading a line is efficiency, but</div> <div> clarity should not be sacrificed over efficiency. We read from the disk and write to the</div> <div> disk. The disk is an electro mechanical device and is the slowest component in the</div> <div> computer. Other parts like processors, memory etc are very fast nowadays i.e. up o</div> <div> 2Ghz. When we talk about hard disk, we say its average access time is 7 mili sec. It</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 206</div> <div> means when we request hard disk to get data it will take 7 mili sec (7/1000 of a sec) to</div> <div> get the data where as processor is running on GHz speed which is thousand million</div> <div> cycles per sec. Processor and memory are much much faster than the hard disk.</div> <div> Therefore reading a single character from the file is too slow. Although nowadays, the</div> <div> buffering and other techniques are used to make the disk access faster. It will be quite</div> <div> efficient if we read the data in bigger chunks i.e. 64k or 256k bytes and also write in</div> <div> bigger chunks. Today’s operating system applies the buffering and similar techniques.</div> <div> Instead of reading and writing character-by-character or word-by-word, reading and</div> <div> writing line by line is efficient. A function is available for this purpose i.e. getLine()</div> <div> for input file stream and putLine() for output file stream. The syntax of getLine() is as</div> <div> follows:</div> <div> char name[100];</div> <div> int maxChar = 100;</div> <div> int stopChar = ‘o’;</div> <div> inFile.getLine(name, maxChar, stopChar);</div> <div> The first argument is a character array, the array should be large enough to hold the</div> <div> complete line. The second argument is the maximum number of characters to be read.</div> <div> The third one is the character if we want to stop somewhere. Suppose we have an</div> <div> input file containing the line ‘Hello World’, then the statements:</div> <div> char str[20];</div> <div> inFile.getLine(str, 20, ‘W’);</div> <div> cout << “The line read from the input file till W is ” << str;</div> <div> The getLine() function will read ‘Hello ’. Normally we do not use the third argument.</div> <div> The default value for the third argument is new line character so getLine() will read</div> <div> the complete line up to the new line character. The new line character will not be</div> <div> read. The line read will be stored in the array, used in the first argument. It is our</div> <div> responsibility that the array should be large enough to hold the entire line and then we</div> <div> can manipulate this data. Using the getLine() repeatedly to read the file is much more</div> <div> efficient rather than using the get() function. As the getLine() function does not read</div> <div> the new line character, we have to put it explicitly. If we have large file to be read</div> <div> then difference in speed with both the programs i.e. using get() and getLine() can be</div> <div> noted.</div> <div> Exercise:</div> <div> Write a program which reads a file using the getLine() function and display it on the</div> <div> screen.</div> <div> Sample input file:</div> <div> This is a test program</div> <div> In this program we learn how to use getLine() function</div> <div> This function is faster than using the get() function</div> <div> The complete code of the program:</div> <div> /*</div> <div> * This program reads from a txt file line by line</div> <div> *</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 207</div> <div> */</div> <div> #include <iostream.h></div> <div> #include <fstream.h></div> <div> main()</div> <div> {</div> <div> ifstream inFile; // Handle for the input file</div> <div> char inputFileName[] = "test.txt"; // file name, this file is in the current directory</div> <div> const int MAX_CHAR_TO_READ = 100; // maximum character to read in one line</div> <div> char completeLineText[MAX_CHAR_TO_READ]; // to be used in getLine function</div> <div> inFile.open(inputFileName); // Opening the file</div> <div> // checking that file is successfuly opened or not</div> <div> if (!inFile)</div> <div> {</div> <div> cout << "Can't open input file named " << inputFileName << endl;</div> <div> exit(1);</div> <div> }</div> <div> // Reading the complete file line by line and printing on screen</div> <div> while (!inFile.eof())</div> <div> {</div> <div> inFile.getline(completeLineText, MAX_CHAR_TO_READ);</div> <div> cout << completeLineText << endl;</div> <div> }</div> <div> inFile.close();</div> <div> }</div> <div> The output of the program is:</div> <div> This is a test program</div> <div> In this program we learn how to use getLine() function</div> <div> This function is faster than using the get() function</div> <div> Example 2</div> <div> Problem statement:</div> <div> A given input file contains Name of the employee and salary of current month. There</div> <div> is a single space between the name and the salary. Name and salary can not contain</div> <div> spaces. Calculate the total salaries of the employees. Create an output file and write</div> <div> the total salary in that file.</div> <div> Solution:</div> <div> We can read a line from the input file using the getLine() function. Now we need to</div> <div> break this line into pieces and get the name and salary in different variables. Here we</div> <div> can use the string token function i.e. strtok(). The string token function (strtok()) takes</div> <div> a string and a delimiter i.e. the character that separates tokens from each other. As</div> <div> there is a space between the name and the salary, we can use the space character as</div> <div> delimiter. So the first call to the string token function will return the name of the</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 208</div> <div> employee, the second call will return the salary of the employee. The syntax to get the</div> <div> next token from the strtok() function is: strtok(NULL, ‘ ‘), it means return the next</div> <div> token from the same string. The second token contains the salary of the employee and</div> <div> is in a char string. We need to add the salaries of all the employees. So we need to</div> <div> convert the salary from character to integer. For this purpose we can use atoi()</div> <div> function.</div> <div> Sample input file:</div> <div> Aamir 12000</div> <div> Amara 15000</div> <div> Adnan 13000</div> <div> Afzal 11500</div> <div> Complete code of the program:</div> <div> /*</div> <div> * This program reads name and salary from a txt file</div> <div> * Calculate the salaries and write the total in an output file</div> <div> */</div> <div> #include <iostream.h></div> <div> #include <fstream.h></div> <div> #include <cstring></div> <div> #include <cstdlib></div> <div> main()</div> <div> {</div> <div> ifstream inFile; // Handle for the input file</div> <div> char inputFileName[] = "salin.txt"; // file name, this file is in the current directory</div> <div> ofstream outFile; // Handle for the output file</div> <div> char outputFileName[] = "salout.txt"; // file name, this file is in the current directory</div> <div> const int MAX_CHAR_TO_READ = 100; // maximum character to read in one line</div> <div> char completeLineText[MAX_CHAR_TO_READ]; // used in getLine function</div> <div> char *tokenPtr; // Used to get the token of a string</div> <div> int salary, totalSalary;</div> <div> salary = 0;</div> <div> totalSalary = 0;</div> <div> inFile.open(inputFileName); // Opening the input file</div> <div> outFile.open(outputFileName); // Opening the output file</div> <div> // Checking that file is successfully opened or not</div> <div> if (!inFile)</div> <div> {</div> <div> cout << "Can't open input file named " << inputFileName << endl;</div> <div> exit(1);</div> <div> }</div> <div> if (!outFile)</div> <div> {</div> <div> cout << "Can't open output file named " << outputFileName << endl;</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 209</div> <div> exit(1);</div> <div> }</div> <div> // Reading the complete file line by line and calculating the total salary</div> <div> while (!inFile.eof())</div> <div> {</div> <div> inFile.getline(completeLineText, MAX_CHAR_TO_READ);</div> <div> tokenPtr = strtok(completeLineText, " "); // First token is name</div> <div> tokenPtr = strtok(NULL, " "); // 2nd token is salary</div> <div> salary = atoi(tokenPtr);</div> <div> totalSalary += salary;</div> <div> }</div> <div> // Writing the total into the output file</div> <div> outFile << "The total salary = " << totalSalary;</div> <div> // closing the files</div> <div> inFile.close();</div> <div> outFile.close();</div> <div> }</div> <div> The contents of output file:</div> <div> The total salary = 51500</div> <div> Exercise:</div> <div> Modify the above program such that the input and output files are given as the</div> <div> command line arguments. Add another information in the input file i.e. the age of the</div> <div> employee. Calculate the average age of the employees and write it in the out put file.</div> <div> Write a program, which reads an input file. The structure of the input file is First</div> <div> Name, Middle Initial, Last Name. Create an output file with the structure First Name,</div> <div> Login Name, Password. First name is same as in the input file. The login name is</div> <div> middle initial and last name together. The password is the first four digits of the first</div> <div> name. First name, middle initial and last name does not contain space.</div> <div> The sample input file is:</div> <div> Syed N Ali</div> <div> Muhammad A Butt</div> <div> Faisal A Malik</div> <div> Muhammad A Jamil</div> <div> If the above file is used as input, the output should be as follows:</div> <div> Syed Nali Syed</div> <div> Muhammad Abutt Muha</div> <div> Faisal Amalik Fais</div> <div> Muhammad Ajamil Muha</div> <div> Tips</div> <div> Always close the file with the close function.</div> <div> CS201 – Introduction to Programming</div> <div> © Copyright Virtual University of Pakistan</div> <div> 210</div> </div> <div> <br /></div> <div> <b><span style="font-size: large;"><a href="http://www.genrica.com/vustuff/BIT701/BIT701_handouts_1_45.pdf" target="_blank">Introduction to Programming BIT701/Introduction to Programming CS201 book HANDOUTS in pdf</a></span></b></div>

Introduction to Programming BIT701/Introduction to Programming CS201 book HANDOUTS in pdf

Introduction to Programming BIT701/Introduction to Programming CS201 book HANDOUTS in pdf Handouts | Lectures | Contents | Books Lectu...

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