Now that you should have learned about variables, loops, and conditional statements it is time to learn about functions. You should have an idea of their uses as we have already used them and defined one in the guise of main. Getchar is another example of a function. In general, functions are blocks of code that perform a number of pre-defined commands to accomplish something productive. You can either use the built-in library functions or you can create your own functions.
Functions that a programmer writes will generally require a prototype. Just like a blueprint, the prototype gives basic structural information: it tells the compiler what the function will return, what the function will be called, as well as what arguments the function can be passed. When I say that the function returns a value, I mean that the function can be used in the same manner as a variable would be. For example, a variable can be set equal to a function that returns a value between zero and four.
#include <stdlib.h> /* Include rand() */
int a = rand(); /* rand is a standard function that all compilers have */
When the programmer actually defines the function, it will begin with the prototype, minus the semi-colon. Then there should always be a block (surrounded by curly braces) with the code that the function is to execute, just as you would write it for the main function. Any of the arguments passed to the function can be used as if they were declared in the block. Finally, end it all with a cherry and a closing brace. Okay, maybe not a cherry.
Let's look at an example program:
int mult ( int x, int y );
printf( "Please input two numbers to be multiplied: " );
scanf( "%d", &x );
scanf( "%d", &y );
printf( "The product of your two numbers is %d\n", mult( x, y ) );
int mult (int x, int y)
return x * y;
The mult function is actually defined below main. Because its prototype is above main, the compiler still recognizes it as being declared, and so the compiler will not give an error about mult being undeclared. As long as the prototype is present, a function can be used even if there is no definition. However, the code cannot be run without a definition even though it will compile.
Prototypes are declarations of the function, but they are only necessary to alert the compiler about the existence of a function if we don't want to go ahead and fully define the function. If mult were defined before it is used, we could do away with the prototype--the definition basically acts as a prototype as well.