Encyclopedia of Computer Science and Technology

66 CTotal = Total + 1;Total += 1;Total ++;Unlike Pascal’s two separate kinds of procedures (func,or function, which returns a value, and proc, or procedure,which does not), C has only functions. Arguments arepassed to functions by value, but can be passed by referenceby using a pointer. (See procedures and functions.)Sample ProgramThe following is a brief example program:#include float Average (void);main () {printf (“The average is: %f”, Average() );}float Average (void) {int NumbersRead = 0;int Number;int Total = 0;while (scanf(“%d\n”, &Number) == 1){Total = Total + Number;NumbersRead = NumbersRead + 1;}return (Total / NumbersRead);}}Statements at the beginning of the program that beginwith # are preprocessor directives. These make changes tothe source code before it is compiled. The #include directiveadds the specified source file to the program. Unlike manyother languages, the C language itself does not includemany basic functions, such as input/output (I/O) statements.Instead, these are provided in standard libraries.(The purpose of this arrangement is to keep the languageitself simple and portable while keeping the implementationof functions likely to vary on different platforms separate.)The stdio.h file here is a “header file” that definesthe I/O functions, such as printf() (which prints formatteddata) and scanf() (which reads data into the program andformats it).The next part of the program declares any functions thatwill be defined and used in the program (in this case, thereis only one function, Average). The function declarationbegins with the type of data that will be returned by thefunction to the calling statement (a floating point value inthis case). After the function name comes declarations forany parameters that are to be passed to the function by thecaller. Since the Average function will get its data from userinput rather than the calling statement, the value (void) isused as the parameter.Following the declaration of Average comes the main()function. Every C program must have a main function. Mainis the function that runs when the program begins to execute.Typically, main will call a number of other functionsto perform the necessary tasks. Here main calls Averagewithin the printf statement, which will print the average asreturned by that function. (Calling functions within otherstatements is an example of C’s concise syntax.)Finally, the Average function is defined. It uses a loopto read in the data numbers, which are totaled and thendivided to get the average, which is sent back to the callingstatement by the return statement.A programmer could create this program on a UNIXsystem by typing the code into a source file (test.c in thiscase) using a text editor such as vi. A C compiler (gcc inthis case) is then given the source code. The source code iscompiled, and linked, creating the executable program filea.out. Typing that name at the command prompt runs theprogram, which asks for and averages the numbers.% gcc test.c% a.out579.The average is: 7.000000Success and ChangeIn the three decades after its first appearance, C became oneof the most successful programming languages in history.In addition to becoming the language of choice for mostUNIX programming, as microcomputers became capable ofrunning high-level languages, C became the language ofchoice for developing MS-DOS, Windows, and Macintoshprograms. The application programming interface (API) forWindows, for example, consists of hundreds of C functions,structures, and definitions (see application programminginterface and Microsoft Windows).However, C has not been without its critics amongcomputer scientists. Besides containing idioms that canencourage cryptic coding, the original version of C (asdefined in Kernighan and Ritchie’s The C ProgrammingLanguage) did not check function parameters to makesure they matched the data types expected in the functiondefinitions. This problem led to a large number ofhard-to-catch bugs. However, the development of ANSIstandard C with its stricter requirements, as well as typechecking built into compilers has considerably amelioratedthis problem. At about the same time, C++ becameavailable as an object-oriented extension and partial rectificationof C. While C++ and Java have considerablysupplanted C for developing new programs, C programmershave a relatively easy learning path to the newerlanguages and the extensive legacy of C code will remainuseful for years to come.Further ReadingKernighan, B. W., and D. M. Ritchie. The C Programming Language,2nd ed. Upper Saddle River, N.J.: Prentice-Hall, 1988.Prata, Stephen. C Primer Plus. 5th ed. Indianapolis: SAMS, 2004.Ritchie, D. M. “The Development of the C Language,” in History ofProgramming Languages II, ed. T. J. Bergin and R. G. Gibson,678–698. Reading, Mass.: Addison-Wesley, 1995.