Lecture notes Computer Programming C

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Horton_735-4C01.fm Page 1 Tuesday, September 19, 2006 10:59 AM CH A P TER 1 ■ ■ ■ Programming in C C is a powerful and compact computer language that allows you to write programs that specify exactly what you want your computer to do. You’re in charge: you create a program, which is just a set of instructions, and your computer will follow them. Programming in C isn’t difficult, as you’re about to find out. I’m going to teach you all the fundamentals of C programming in an enjoyable and easy-to-understand way, and by the end of this chapter you’ll have written your first few C programs. It’s as easy as that In this chapter you’ll learn the following: � How to create C programs � How C programs are organized � How to write your own program to display text on the screen Creating C Programs There are four fundamental stages, or processes, in the creation of any C program: �Editing �Compiling �Linking �Executing You’ll soon know all these processes like the back of your hand (you’ll be doing them so easily and so often), but first let’s consider what each process is and how it contributes to the creation of a C program. Editing This is the process of creating and modifying C source code—the name given to the program instruc- tions you write. Some C compilers come with a specific editor that can provide a lot of assistance in managing your programs. In fact, an editor often provides a complete environment for writing, managing, developing, and testing your programs. This is sometimes called an integrated development environment, or IDE. 1Horton_735-4C01.fm Page 2 Tuesday, September 19, 2006 10:59 AM 2 CHAPTER 1 ■ PROGRAMMING IN C You can also use other editors to create your source files, but they must store the code as plain text without any extra formatting data embedded in it. In general, if you have a compiler system with an editor included, it will provide a lot of features that make it easier to write and organize your source programs. There will usually be automatic facilities for laying out the program text appropriately, and color highlighting for important language elements, which not only makes your code more readable but also provides a clear indicator when you make errors when keying in such words. If you’re working in UNIX or Linux, the most common text editor is the vi editor. Alternately you might prefer to use the emacs editor. On a PC you could use one of the many freeware and shareware programming editors. These will often provide a lot of help in ensuring your code is correct with syntax highlighting and autoin- denting of your code. Don’t use a word processor such as Microsoft Word, as these aren’t suitable for producing program code because of the extra formatting information they store along with the text. Of course, you also have the option of purchasing one of the professionally created programming development environments that support C, such as those from Borland or Microsoft, in which case you will have very extensive editing capabilities. Before parting with your cash though, it’s a good idea to check that the level of C that is supported is approximate to the current C standard. With some of the products out there that are primarily aimed at C++ developers, C has been left behind somewhat. A further possibility is to get the emacs editor for Windows. emacs is the editor of choice for some programming professionals. Compiling The compiler converts your source code into machine language and detects and reports errors in the compilation process. The input to this stage is the file you produce during your editing, which is usually referred to as a source file. The compiler can detect a wide range of errors that are due to invalid or unrecognized program code, as well as structural errors where, for example, part of a program can never be executed. The output from the compiler is known as object code and is stored in files called object files, which usually have names with the extension .obj in the Microsoft Windows environment, or .o in the Linux/UNIX environment. The compiler can detect several different kinds of errors during the trans- lation process, and most of these will prevent the object file from being created. The result of a successful compilation is a file with the same name as that used for the source file, but with the .o or .obj extension. If you’re working in UNIX, at the command line, the standard command to compile your C programs will be cc (or the GNU’s Not UNIX (GNU) compiler, which is gcc). You can use it like this: cc -c myprog.c where myprog.c is the program you want to compile. Note that if you omit the -c flag, your program will automatically be linked as well. The result of a successful compilation will be an object file. Most C compilers will have a standard compile option, whether it’s from the command line (such as cc myprog.c) or a menu option from within an IDE (where you’ll find a Compile menu option).Horton_735-4C01.fm Page 3 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 3 Linking The linker combines the various modules generated by the compiler from source code files, adds required code modules from program libraries supplied as part of C, and welds everything into an executable whole. The linker can also detect and report errors, for example, if part of your program is missing or a nonexistent library component is referenced. In practice, if your program is of any significant size, it will consist of several separate source code files, which can then be linked. A large program may be difficult to write in one working session, and it may be impossible to work with as a single file. By breaking it up into a number of smaller source files that each provide a coherent part of what the whole program does, you can make the development of the program a whole lot easier. The source files can be compiled separately, which makes eliminating simple typographical errors a bit easier. Furthermore, the whole program can usually be developed incrementally. The set of source files that make up the program will usually be integrated under a project name, which is used to refer to the whole program. Program libraries support and extend the C language by providing routines to carry out opera- tions that aren’t part of the language. For example, libraries contain routines that support operations such as performing input and output, calculating a square root, comparing two character strings, or obtaining date and time information. A failure during the linking phase means that once again you have to go back and edit your source code. Success on the other hand will produce an executable file. In a Microsoft Windows environment, this executable file will have an .exe extension; in UNIX, there will be no such exten- sion, but the file will be of an executable type. Many IDEs also have a Build option, which will compile and link your program in one step. This option will usually be found, within an IDE, in the Compile menu; alternatively, it may have a menu of its own. Executing The execution stage is where you run your program, having completed all the previous processes successfully. Unfortunately, this stage can also generate a wide variety of error conditions that can include producing the wrong output or just sitting there and doing nothing, perhaps crashing your computer for good measure. In all cases, it’s back to the editing process to check your source code. Now for the good news: this is the stage where, at last, you get to see your computer doing exactly what you told it to do In UNIX and Linux you can just enter the name of the file that has been compiled and linked to execute the program. In most IDEs, you’ll find an appropriate menu command that allows you to run or execute your compiled program. This Run or Execute option may have a menu of its own, or you may find it under the Compile menu option. In Windows, you can run the .exe file for your program as you would any other executable. The processes of editing, compiling, linking, and executing are essentially the same for devel- oping programs in any environment and with any compiled language. Figure 1-1 summarizes how you would typically pass through processes as you create your own C programs.Horton_735-4C01.fm Page 4 Tuesday, September 19, 2006 10:59 AM 4 CHAPTER 1 ■ PROGRAMMING IN C Figure 1-1. Creating and executing a program Creating Your First Program Let’s step through the processes of creating a simple C program, from entering the program itself to executing it. Don’t worry if what you type doesn’t mean anything to you at this stage—I’ll explain everything as we go along.Horton_735-4C01.fm Page 5 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 5 TRY IT OUT: AN EXAMPLE C PROGRAM Run your editor, and type in the following program exactly as it’s written. Be careful to use the punctuation exactly as you see here. The brackets used on the fourth and last lines are braces—the curly ones , not the square ones or the round ones ()—it really does matter. Also, make sure you put the slashes the right way (/), as later you’ll be using the backslash (\) as well. Don’t forget the semicolon (;). / Program 1.1 Your Very First C Program - Displaying Hello World / include stdio.h int main(void) printf("Hello world"); return 0; When you’ve entered the preceding source code, save the program as hello.c. You can use whatever name you like instead of hello, but the extension must be .c. This extension is the common convention when you write C programs and identifies the contents of the file as C source code. Most compilers will expect the source file to have the extension .c, and if it doesn’t, the compiler may refuse to process it. Next you’ll compile your program as described in the “Compiling” section previously in this chapter and link all the pieces necessary to create an executable program as discussed in the previous “Linking” section. This is typi- cally carried out in a single operation, and once the source code has been compiled successfully, the linker will add code from the standard libraries that your program needs and create the single executable file for your program. Finally, you can execute your program. Remember that you can do this in several ways. There is the usual method of double-clicking the .exe file from Windows Explorer if you’re using Windows, but you will be better off opening a command-line window because the window showing the output will disappear when execution is complete. On all platforms, you can run your program from the command line. Just start a command-line session, change the current directory to the one that contains the executable file for your program, and then enter the program name to run it. If everything worked without producing any error messages, you’ve done it This is your first program, and you should see the following message on the screen: Hello world Editing Your First Program You could try altering the same program to display something else on the screen. For example, you might want to try editing the program to read like this: / Program 1.2 Your Second C Program / includestdio.h int main(void) printf("If at first you don\'t succeed, try, try, try again"); return 0; Horton_735-4C01.fm Page 6 Tuesday, September 19, 2006 10:59 AM 6 CHAPTER 1 ■ PROGRAMMING IN C The \' sequence in the middle of the text to be displayed is called an escape sequence. Here it’s a special way of including a single quote in the text because single quotes are usually used to indicate where a character constant begins and ends. You’ll learn more about escape sequences in the “Control Characters” section later in this chapter. You can try recompiling the program, relinking it, and running it again once you’ve altered the source. With a following wind and a bit of luck you have now edited your first program. You’ve written a program using the editor, edited it, and compiled, linked, and executed it. Dealing with Errors To err is human, so there’s no need to be embarrassed about making mistakes. Fortunately computers don’t generally make mistakes themselves and they’re actually very good at indicating where we’ve slipped up. Sooner or later your compiler is going to present you with a list (sometimes a list that’s longer than you want) of the mistakes that are in your source code. You’ll usually get an indication of the statements that are in error. When this happens, you must return to the editing stage, find out what’s wrong with the incorrect code, and fix it. Keep in mind that one error can result in error messages for subsequent statements that may actually be correct. This usually happens with statements that refer to something that is supposed to be defined by a statement containing an error. Of course, if a statement that defines something has an error, then what was supposed to be defined won’t be. Let’s step through what happens when your source code is incorrect by creating an error in your program. Edit your second program example, removing the semicolon (;) at the end of the line with printf() in it, as shown here: / Program 1.2 Your Second C Program / includestdio.h int main(void) printf("If at first you don\'t succeed, try, try, try again") return 0; If you now try to compile this program, you’ll see an error message that will vary slightly depending on which compiler you’re using. A typical error message is as follows: Syntax error : missing ';' before '' HELLO.C - 1 error(s), 0 warning(s) Here, the compiler is able to determine precisely what the error is, and where. There really should be a semicolon at the end of that printf() line. As you start writing your own programs, you’ll probably get a lot of errors during compilation that are caused by simple punctuation mistakes. It’s so easy to forget a comma or a bracket, or to just press the wrong key. Don’t worry about this; a lot of experi- enced programmers make exactly the same mistakes—even after years of practice. As I said earlier, just one mistake can sometimes result in a whole stream of abuse from your compiler, as it throws you a multitude of different things that it doesn’t like. Don’t get put off by the number of errors reported. After you consider the messages carefully, the basic approach is to go back and edit your source code to fix what you can, ignoring the errors that you can’t understand. Then have another go at compiling the source file. With luck, you’ll get fewer errors the next time around. To correct your example program, just go back to your editor and reenter the semicolon. Recompile, check for any other errors, and your program is fit to be run again.Horton_735-4C01.fm Page 7 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 7 Dissecting a Simple Program Now that you’ve written and compiled your first program, let’s go through another that’s very similar and see what the individual lines of code do. Have a look at this program: / Program 1.3 Another Simple C Program - Displaying a Quotation / include stdio.h int main(void) printf("Beware the Ides of March"); return 0; This is virtually identical to your first program. Even so, you could do with the practice, so use your editor to enter this example and see what happens when you compile and run it. If you type it in accurately, compile it, and run it, you should get the following output: Beware the Ides of March Comments Look at the first line of code in the preceding example: / Program 1.3 Another Simple C Program - Displaying a Quotation / This isn’t actually part of the program code, in that it isn’t telling the computer to do anything. It’s simply a comment, and it’s there to remind you, or someone else reading your code, what the program does. Anything between / and / is treated as a comment. As soon as your compiler finds / in your source file, it will simply ignore anything that follows (even if the text looks like program code) until it finds the matching / that marks the end of the comment. This may be on the same line, or it can be several lines further on. You should try to get into the habit of documenting your programs, using comments as you go along. Your programs will, of course, work without comments, but when you write longer programs you may not remember what they do or how they work. Put in enough comments to ensure that a month from now you (and any other programmer) can understand the aim of the program and how it works. As I said, comments don’t have to be in a line of their own. A comment is everything between / and /, wherever / and / are in your code. Let’s add some more comments to the program: / Program 1.3 Another Simple C Program - Displaying a Quotation / include stdio.h / This is a preprocessor directive / int main(void) / This identifies the function main() / / This marks the beginning of main() / printf("Beware the Ides of March"); / This line displays a quotation / return 0; / This returns control to the operating system / / This marks the end of main() / You can see that using comments can be a very useful way of explaining what’s going on in the program. You can place comments wherever you want in your program, and you can use them to explain the general objectives of the code as well as the specifics of how the code works. A single comment can spread over several lines; everything from the / to the / will be treated as a comment Horton_735-4C01.fm Page 8 Tuesday, September 19, 2006 10:59 AM 8 CHAPTER 1 ■ PROGRAMMING IN C and ignored by the compiler. Here’s how you could use a single comment to identify the author of the code and to assert your copyright: / Written by Ivor Horton Copyright 2006 / This is one comment spread over four lines. I have used asterisks to mark the beginning of each line of text here but they are not obligatory, just part of the comment as I wrote it. You can use anything you like to improve the readability of a comment, but don’t forget that / will be interpreted as the end of the comment. Preprocessing Directives Look at the following line of code: include stdio.h / This is a preprocessor directive / This is not strictly part of the executable program, but it is essential in this case—in fact, the program won’t work without it. The symbol indicates this is a preprocessing directive, which is an instruction to your compiler to do something before compiling the source code. The compiler handles these directives during an initial preprocessing phase before the compilation process starts. There are quite a few preprocessing directives, and they’re usually placed at the beginning of the program source file. In this case, the compiler is instructed to “include” in your program the contents of the file with the name stdio.h. This file is called a header file, because it’s usually included at the head of a program. In this case the header file defines information about some of the functions that are provided by the standard C library but, in general, header files specify information that the compiler uses to integrate any predefined functions or other global objects with a program, so you’ll be creating your own header files for use with your programs. In this case, because you’re using the printf() function from the standard library, you have to include the stdio.h header file. This is because stdio.h contains the information that the compiler needs to understand what printf() means, as well as other functions that deal with input and output. As such, its name, stdio, is short for standard input/output. All header files in C have file names with the extension .h. You’ll use other C header files later in the book. ■Note Although the header file names are not case sensitive, it’s common practice to write them in include directives in lowercase letters. Every C compiler that conforms to the international standard (ISO/IEC 9899) for the language will have a set of standard header files supplied with it. These header files primarily contain declara- tions relating to standard library functions that are available with C. Although all C compilers that conform with the standard will support the same set of standard library functions and will have the same set of standard header files available, there may be extra library functions provided with a particular compiler that may not be available with other compilers, and these will typically provide functionality that is specific to the type of computer on which the compiler runs.Horton_735-4C01.fm Page 9 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 9 Defining the main() Function The next five statements define the function main(): int main(void) / This identifies the function main() / / This marks the beginning of main() / printf("Beware the Ides of March"); / This line displays a quotation / return 0; / This returns control to the operating system / / This marks the end of main() / A function is just a named block of code between braces that carries out some specific set of operations. Every C program consists of one or more functions, and every C program must contain a function called main()—the reason being that a program will always start execution from the beginning of this function. So imagine that you’ve created, compiled, and linked a file called progname.exe. When you execute this program, the operating system calls the function main() for the program. The first line of the definition for the function main() is as follows: int main(void) / This identifies the function main() / This defines the start of the function main(). Notice that there is no semicolon at the end of the line. The first line identifying this as the function main() has the word int at the beginning. What appears here defines the type of value to be returned by the function, and the word int signifies that the function main() returns an integer value. The integer value that is returned when the execution of main() ends represents a code that is returned to the operating system that indicates the program state. You end execution of the main() function and specify the value to be returned in the statement: return 0; / This returns control to the operating system / This is a return statement that ends execution of the main() function and returns that value 0 to the operating system. You return a zero value from main() to indicate that the program terminated normally; a nonzero value would indicate an abnormal return, which means, in other words, things were not as they should be when the program ended. The parentheses that immediately follow the name of the function, main, enclose a definition of what information is to be transferred to main() when it starts executing. In this example, however, you can see that there’s the word void between the parentheses, and this signifies that no data can be transferred to main(). Later, you’ll see how data is transferred to main() and to other functions in a program. The function main() can call other functions, which in turn may call further functions, and so on. For every function that’s called, you have the opportunity to pass some information to it within the parentheses that follow its name. A function will stop execution when a return statement in the body of the function is reached, and control will then transfer to the calling function (or the operating system in the case of the function main()). Keywords In C, a keyword is a word with special significance, so you shouldn’t use keywords for any other purposes in your program. For this reason, keywords are also referred to as reserved words. In the preceding example, int is a keyword and void and return are also keywords. C has several keywords, and you’ll become familiar with more of them as you learn more of the language. You’ll find a complete list of C keywords in Appendix C.Horton_735-4C01.fm Page 10 Tuesday, September 19, 2006 10:59 AM 10 CHAPTER 1 ■ PROGRAMMING IN C The Body of a Function The general structure of the function main() is illustrated in Figure 1-2. Figure 1-2. Structure of the function main() The function body is the bit between the opening and closing braces that follow the line where the function name appears. The function body contains all the statements that define what the function does. The example’s main() function has a very simple function body consisting of just two statements: / This marks the beginning of main() / printf("Beware the Ides of March"); / This line displays a quotation / return 0; / This returns control to the operating system / / This marks the end of main() / Every function must have a body, although the body can be empty and just consist of the opening and closing braces without any statements between them. In this case, the function will do nothing. You may wonder where the use is for a function that does nothing. Actually, this can be very useful when you’re developing a program that will have many functions. You can declare the set of (empty) functions that you think you’ll need to write to solve the problem at hand, which should give you an idea of the programming that needs to be done, and then gradually create the program code for each function. This technique helps you to build your program in a logical and incremental manner. ■Note You can see that I’ve aligned the braces one below the other in Program 1.3. I’ve done this to make it clear where the block of statements that the braces enclose starts and finishes. Statements between braces are usually indented by a fixed amount—usually two or more spaces so that the braces stand out. This is good programming style, as the statements within a block can be readily identified. Outputting Information The body of the main() function in the example includes a statement that calls the printf() function:Horton_735-4C01.fm Page 11 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 11 printf("Beware the Ides of March"); / This line displays a quotation / As I’ve said, printf() is a standard library function, and it outputs information to the display screen based on what appears between the parentheses that immediately follow the function name. In this case, the call to the function displays a simple piece of Shakespearean advice that appears between the double quotes; a string of characters between double quotes like this is called a string literal. Notice that this line does end with a semicolon. Arguments Items enclosed between the parentheses following a function name, as with the printf() function in the previous statement, are called arguments, which specify data that is to be passed to the func- tion. When there is more than one argument to a function, they must be separated by commas. In the previous example the argument to the function is the text string between double quotes. If you don’t like the quotation that is specified here, you could display something else by simply including your own choice of words enclosed within double quotes inside the parentheses. For instance, you might prefer a line from Macbeth: printf("Out, damned Spot Out I say"); Try using this in the example. When you’ve modified the source code, you need to compile and link the program again before executing it. ■Note As with all executable statements in C (as opposed to defining or directive statements) the printf() line must have a semicolon at the end. As you’ve seen, a very common error, particularly when you first start programming in C, is to forget the semicolon. Control Characters You could alter the program to display two sentences on separate lines. Try typing in the following code: / Program 1.4 Another Simple C Program - Displaying a Quotation / include stdio.h int main(void) printf("\nMy formula for success?\nRise early, work late, strike oil."); return 0; The output from this program looks like this: My formula for success? Rise early, work late, strike oil. Look at the printf() statement. At the beginning of the text and after the first sentence, you’ve inserted the characters \n. The combination \n actually represents one character: a newline character. The backslash (\) is of special significance in a text string. As we saw before, it indicates the start of an escape sequence. Escape sequences are used to insert characters in a string that would other- wise be impossible to specify, such as tab and newline, or in some circumstances would confuse the Horton_735-4C01.fm Page 12 Tuesday, September 19, 2006 10:59 AM 12 CHAPTER 1 ■ PROGRAMMING IN C compiler, such as placing a double quote, which you would normally use to delimit a string, within a string. The character following the backslash indicates what character the escape sequence repre- sents. In this case, it’s n for newline, but there are plenty of other possibilities. Obviously, if a backslash is of special significance, you need a way to specify a backslash in a text string. To do this, you simply use two backslashes: \\. Similarly, if you actually want to display a double quote character, you can use \". Type in the following program: / Program 1.5 Another Simple C Program - Displaying Great Quotations / include stdio.h int main(void) printf("\n\"It is a wise father that knows his own child.\" Shakespeare"); return 0; The output displays the following text: "It is a wise father that knows his own child." Shakespeare You can use the \a escape sequence in an output string to sound a beep to signal something interesting or important. Enter and run the following program: / Program 1.6 A Simple C Program – Important / include stdio.h int main(void) printf("\nBe careful\a"); return 0; The output of this program is sound and vision. Listen closely and you should hear the beep through the speaker in your computer. Be careful The \a sequence represents the “bell” character. Table 1-1 shows a summary of the escape sequences that you can use. Table 1-1. Escape Sequences Escape Sequence Description \n Represents a newline character \r Represents a carriage return \b Represents a backspace \f Represents a form-feed character \t Represents a horizontal tabHorton_735-4C01.fm Page 13 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 13 Table 1-1. Escape Sequences Escape Sequence Description \v Represents a vertical tab \a Inserts a bell (alert) character \? Inserts a question mark (?) \" Inserts a double quote (") \' Inserts a single quote (') \\ Inserts a backslash (\) Try displaying different lines of text on the screen and alter the spacing within that text. You can put words on different lines using \n, and you can use \t to space the text. You’ll get a lot more practice with these as you progress through the book. Developing Programs in C The process of developing programs in C may not be evident if you’ve never written a program before. However, it’s very similar to many other situations in life in which at the beginning it just isn’t clear how you’re going to achieve your objective. Normally you start with a rough idea of what you want to achieve, but you need to translate this into a more precise specification of what you want. Once you’ve reached this more precise specification, you can work out the series of steps that will lead to your final objective. So having an idea that you want to build a house just isn’t enough. You need to know what kind of house you want, how large it’s going to be, what kinds of materials you have to build it with, and where you want to build it. This kind of detailed planning is also neces- sary when you want to write a program. Let’s go through the basic steps that you need to follow when you’re writing a program. The house analogy is a useful one, so we’ll work with it for a while. Understanding the Problem The first step is to get a clear idea of what you want to do. It would be lunacy to start building your house before you had established what facilities it should provide: how many bedrooms, how many bathrooms, how big it’s going to be, and so on. All these things affect the cost of the house in terms of materials and the work involved in building it. Generally it comes down to a compromise that best meets your needs within the constraints of the money, the workforce, and the time that’s available for you to complete the project. It’s the same with developing a program of any size. Even for a relatively straightforward problem, you need to know what kind of input to expect, how the input is to be processed, and what kind of output is required—and how it’s going to look. The input could be entered with the keyboard, but it might also involve data from a disk file or information obtained over a telephone line or a network. The output could simply be displayed on the screen, or it could be printed; perhaps it might involve updating a data file on disk. For more complex programs, you’ll need to look at many more aspects of what the program is going to do. A clear definition of the problem that your program is going to solve is an absolutely essential part of understanding the resources and effort that are going to be needed for the creation of a finished product. Considering these details also forces you to establish whether the project is actually feasible.Horton_735-4C01.fm Page 14 Tuesday, September 19, 2006 10:59 AM 14 CHAPTER 1 ■ PROGRAMMING IN C Detailed Design To get the house built, you’ll need detailed plans. These plans enable the construction workers to do their job and the plans describe in detail how the house will go together—the dimensions, the mate- rials to use, and so on. You’ll also need a plan of what is to be done and when. For example, you’ll want the foundation dug before the walls are built, so the plan must involve segmenting the work into manageable units to be performed in a logical sequence. It’s the same with a program. You’ll need to specify what the program does by dividing it into a set of well-defined and manageable chunks that are reasonably self-contained. You’ll also need to detail the way in which these chunks connect, as well as what information each chunk will need when it executes. This will enable you to develop the logic of each chunk relatively independently from the rest of the program. If you treat a large program as one huge process that you try to code as a single chunk, chances are that you’ll never get it to work. Implementation Given the detailed design of a house, the work can start. Each group of construction workers will need to complete its part of the project at the right time. Each stage will need to be inspected to check that it’s been done properly before the next stage begins. Omitting these checks could easily result in the whole house collapsing. Of course, if a program is large, you’ll write the source code one unit at a time. As one part is completed, you can write the code for the next. Each part will be based on the detailed design spec- ifications, and you’ll verify that each piece works, as much as you can, before proceeding. In this way, you’ll gradually progress to a fully working program that does everything you originally intended. Testing The house is complete, but there are a lot of things that need to be tested: the drainage, the water and electricity supplies, the heating, and so on. Any one of these areas can have problems that the contrac- tors need to go back and fix. This is sometimes an iterative process, in which problems with one aspect of the house can be the cause of things going wrong somewhere else. The mechanism with a program is similar. Each of your program modules—the pieces that make up your program—will need to be tested individually. When they don’t work properly, you need to debug them. Debugging is the process of finding and correcting errors in your program. This term is said to have originated in the days when finding the errors in a program involved tracing where the information went and how it was processed by using the circuit diagram for the computer. The story goes that it was discovered that a computer program error was caused by an insect shorting part of the circuit in the computer. The problem was caused by a bug. Subsequently, the term bug was used to refer to any error in a program. With a simple program, you can often find an error simply by inspecting the code. In general, though, the process of debugging usually involves adding extra program code to produce output that will enable you to check what the sequence of events is and what intermediate values are produced in a program. With a large program, you’ll also need to test the program modules in combination because, although the individual modules may work, there’s no guarantee that they’ll work together The jargon for this phase of program development is integration testing. Functions and Modular Programming The word function has appeared a few times so far in this chapter with reference to main(), printf(), function body, and so on. Let’s explore in a little more depth what functions are and why they’re important. Horton_735-4C01.fm Page 15 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 15 Most programming languages, including C, provide a way of breaking up a program into segments, each of which can be written more or less independently of the others. In C these segments are called functions. The program code in the body of one function is insulated from that of other func- tions. A function will have a specific interface to the outside world in terms of how information is transferred to it and how results generated by the function are transmitted back from it. This inter- face is specified in the first line of the function, where the function name appears. Figure 1-3 shows a simple example of a program to analyze baseball scores that is composed of four functions. Figure 1-3. Modular programming Each of the four functions does a specific, well-defined job. Overall control of the sequence of operations in the program is managed by one module, main(). There is a function to read and check the input data, and another function to do the analysis. Once the data has been read and analyzed, a fourth function has the task of outputting the team and player rankings. Segmenting a program into manageable chunks is a very important aspect to programming, so let’s go over the reasons for doing this: � It allows each function to be written and tested separately. This greatly simplifies the process of getting the total program to work. � Several separate functions are easier to handle and understand than one huge function. � Libraries are just sets of functions that people tend to use all the time. Because they’ve been prewritten and pretested, you know they’ll work, so you can use them without worrying about their code details. This will accelerate your program development by allowing you to concen- trate on your own code, and it’s a fundamental part of the philosophy of C. The richness of the libraries greatly amplifies the power of the language. Horton_735-4C01.fm Page 16 Tuesday, September 19, 2006 10:59 AM 16 CHAPTER 1 ■ PROGRAMMING IN C � You can accumulate your own libraries of functions that are applicable to the sort of programs that you’re interested in. If you find yourself writing a particular function frequently, you can write a generalized version of it to suit your needs and build this into your own library. Then, whenever you need to use that particular function, you can simply use your library version. � In the development of large programs, which can vary from a few thousand to millions of lines of code, development can be undertaken by teams of programmers, with each team working with a defined subgroup of the functions that make up the whole program. You’ll learn about C functions in greater detail in Chapter 8. Because the structure of a C program is inherently functional, you’ve already been introduced to one of the standard library functions in one of this chapter’s earliest examples: the function printf(). TRY IT OUT: EXERCISING WHAT YOU KNOW Let’s now look at an example that puts into practice what you’ve learned so far. First, have a look at the following code and see whether you can understand what it does without running it. Then type it in and compile, link, and run it, and see what happens. / Program 1.7 A longer program / include stdio.h / Include the header file for input and output / int main(void) printf("Hi there\n\n\nThis program is a bit"); printf(" longer than the others."); printf("\nBut really it's only more text.\n\n\n\a\a"); printf("Hey, wait a minute What was that???\n\n"); printf("\t1.\tA bird?\n"); printf("\t2.\tA plane?\n"); printf("\t3.\tA control character?\n"); printf("\n\t\t\b\bAnd how will this look when it prints out?\n\n"); return 0; The output will be as follows: Hi there This program is a bit longer than the others. But really it’s only more text. Hey, wait a minute What was that??? A bird? A plane? A control character? And how will this look when it prints out?Horton_735-4C01.fm Page 17 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 17 How It Works The program looks a little bit complicated, but this is only because the text strings between parentheses include a lot of escape sequences. Each text string is bounded by a pair of double quotation marks. However, the program is just a succession of calls to the printf() function, and it demonstrates that output to the screen is controlled by what you pass to the printf() function. Let’s look at this program in detail. You include the stdio.h file from the standard library through the preprocessing directive: include stdio.h / Include the header file for input and output / You can see that this is a preprocessing directive because it begins with . The stdio.h file provides the defi- nitions you need to be able to use the printf() function. You then define the start of the function main() and specify that it returns an integer value with this line: int main(void) The opening brace on the next line indicates that the body of the function follows: The next statement calls the standard library function printf() to output Hi there to your display screen, followed by two blank lines and the phrase This program is a bit. printf("Hi there\n\n\nThis program is a bit"); The two blank lines are produced by the three \n escape sequences. Each of these starts a new line when the characters are written to the display. The first ends the line containing Hi there, and the next two produce the two empty lines. The text This program is a bit appears on the fourth line of output. You can see that this one line of code produces a total of four lines of output on the screen. The next line of output produced by the next printf() starts at the character position immediately following the last character in the previous output. The next statement outputs the text longer than the others with a space as the first character of the text: printf(" longer than the others."); This output will simply continue where the last line left off, following the t in bit. This means that you really do need the space at the beginning of the text, otherwise the computer will display This program is a bitlonger than the others, which isn’t what you want. The next statement starts its output on a new line immediately following the previous line, because of the \n at the beginning of the text string between double quotation marks: printf("\nBut really it's only more text.\n\n\n\a\a"); It then displays the text and adds two empty lines (because of the three \n escape sequences) and beeps twice. The next output to the screen will start at the beginning of the line that follows the second empty line produced here. The next output is produced by the following statement: printf("Hey, wait a minute What was that???\n\n"); This outputs the text and then leaves one empty line. The next output will be on the line following the empty line. Each of the next three statements inserts a tab, displays a number, inserts another tab followed by some text, and ends with a new line. This is useful for making your output easier to read.Horton_735-4C01.fm Page 18 Tuesday, September 19, 2006 10:59 AM 18 CHAPTER 1 ■ PROGRAMMING IN C printf("\t1.\tA bird?\n"); printf("\t2.\tA plane?\n"); printf("\t3.\tA control character?\n"); This produces three numbered lines of output. The next statement initially outputs a new line character, so that there will be an empty line following the previous output. Two tabs are then sent to the display followed by two backspaces, which moves you back two spaces from the last tab position. Finally the text is displayed, and two newline characters are sent to the display. printf("\n\t\t\b\bAnd how will this look when it prints out?\n\n"); The last statement in the body of the function is the following: return 0; This ends execution of main() and returns 0 to the operating system. The closing brace marks the end of the function body: Common Mistakes Mistakes are a fact of life. When you write a computer program in C, the compiler must convert your source code to machine code, and so there must be some very strict rules governing how you use the language. Leave out a comma where one is expected, or add a semicolon where you shouldn’t, and the compiler won’t be able to translate your program into machine code. You’ll be surprised just how easy it is to introduce typographical errors into your program, even after years of practice. If you’re lucky, these errors will be picked up when you compile or link your program. If you’re really unlucky, they can result in your program apparently working fine but producing some intermittent erratic behavior. You can end up spending a lot of time tracking these errors down. Of course, it’s not only typographical errors that cause problems. You’ll often find that your detailed implementation is just not right. Where you’re dealing with complicated decisions in your program, it’s easy to get the logic wrong. Your program may be quite accurate from a language point of view, and it may compile and run without a problem, but it won’t produce the right answers. These kinds of errors can be the most difficult to find. Points to Remember It would be a good idea to review what you’ve gleaned from your first program. You can do this by looking at the overview of the important points in Figure 1-4.Horton_735-4C01.fm Page 19 Tuesday, September 19, 2006 10:59 AM CHAPTER 1 ■ PROGRAMMING IN C 19 Figure 1-4. Elements of a simple program Summary You’ve reached the end of the first chapter, and you’ve already written a few programs in C. You’ve covered quite a lot of ground, but at a fairly gentle pace. The aim of this chapter was to introduce a few basic ideas rather than teach you a lot about the C programming language. You should be confi- dent about editing, compiling, and running your programs. You probably have only a vague idea about how to construct a C program at this point. It will become much clearer when you’ve learned a bit more about C and have written some programs with more meat to them. In the next chapter you’ll move on to more complicated things than just producing text output using the printf() function. You’ll manipulate information and get some rather more interesting results. And by the way, the printf() function does a whole lot more than just display text strings— as you’ll see soon.Horton_735-4C01.fm Page 20 Tuesday, September 19, 2006 10:59 AM 20 CHAPTER 1 ■ PROGRAMMING IN C Exercises The following exercises enable you to try out what you’ve learned in this chapter. If you get stuck, look back over the chapter for help. If you’re still stuck, you can download the solutions from the Source Code/Download section of the Apress web site (http://www.apress.com), but that really should be a last resort. Exercise 1-1. Write a program that will output your name and address using a separate printf() statement for each line of output. Exercise 1-2. Modify your solution for the previous exercise so that it produces all the output using only one printf() statement. Exercise 1-3. Write a program to output the following text exactly as it appears here: "It's freezing in here," he said coldly.

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