C Functions

Why Use Functions?

Imagine you need to print a welcome message in five different places. Without functions you would copy the same printf() lines five times. With a function you write it once and call it by name whenever you need it. This idea is often called 'don't repeat yourself'.

• Reuse code without copying and pasting it
• Break a big problem into small, named steps
• Make programs easier to read and to test
• Fix a bug in one place instead of many

Creating and Calling a Function

To create a function you write a return type, a name, a pair of parentheses, and a body inside curly braces. To use it you simply write its name followed by parentheses. The example below creates a function called greet() and calls it from main().

#include <stdio.h>

void greet() {
  printf("Welcome to C programming!\n");
}

int main() {
  greet();
  greet();
  return 0;
}

Declaring Functions Before main()

C reads your file from top to bottom, so a function must be known before it is called. In the example above we defined greet() above main(). If you prefer to keep main() first, you can instead write a declaration (also called a prototype) at the top and put the full definition below.

#include <stdio.h>

void greet();

int main() {
  greet();
  return 0;
}

void greet() {
  printf("Hello from a function!\n");
}

Returning a Value

A function can also send a value back to the caller. You set the return type (for example int) and use the return keyword to hand back a result. The caller can then store or print that result.

#include <stdio.h>

int square(int n) {
  return n * n;
}

int main() {
  int result = square(5);
  printf("5 squared is %d\n", result);
  return 0;
}