C: User-defined Functions

A function is a group of statements (i.e. block of code) that perform a task. Every C program has at least one function called the main where program execution begins. Additional functions are created for numerous reasons:

1. Organize, or modularize, the program
2. Facilitate code re-use, i.e. write-once, run many
3. Improved program/code readability
4. Breaking a large project into smaller ones and assigning parts to teams
5. Easier debugging

There are a few important things to know about C functions:

• Function name - every function is given a name
• Function declaration - informs the compiler about the function's name, return type, and parameters
• Function definition - the function body, i.e. statements to be executed/run
• Function call - the code that calls (runs) the function
• Return type - function may return a value. The return type is the data type of the value the function returns. If a function does not return a value then the return type is void
• Parameter - functions may have parameters (i.e. optional). They are are any data passed to it
• Function body - synonymous with function definition
• Recursion - when a function calls itself

Example: C that does not return anything

	#include <stdio.h>
	void greeting()
	{
		printf("Welcome.\n");
		printf("This is a C program.\n");
		printf("I hope you enjoy.");
	}

	int main()
	{
		 greeting();  /* function call */
		 return 0;
	}
	

Output:

  Welcome.
  This is a C program.
  I hope you enjoy.

Example: Simple Function in C - returns an integer

	#include <stdio.h> 
	int sq(int num);

	int main() {
		/* calling foo from main */
		printf("2 squared = %d", sq(2));
	}

	int sq(int num) {
		return num * num;
	}
	

Output:

  2 squared = 4

Example: Simple Function in C - returns an integer

	#include <stdio.h>
	 
	/* function declaration */
	int min(int num1, int num2);
	 
	int main () {

	   /* local variable declations */
	   int a = 23;
	   int b = 15;
	   int answer;
	 
	   answer = min(a, b);		/* function call */
	 
	   printf( "Minimum value is : %d\n", answer );
	 
	   return 0;
	}
	 
	int min(int num1, int num2) {
	   int answer;
	 
	   if (num1 < num2)
		  answer = num1;
	   else
		  answer = num2;
	 
	   return answer; 
	}
	

Output:

  15

Example: Simple Function in C - returns nothing (void)

	#include <stdio.h> 
	void greeting(int count);   /* function prototype */

	main()
	{ 
		greeting(4);  
	} 

	void greeting(int count) 
	{ 
		int x; 
		for(x = 0; x < count; x++) 
			printf("Welcome.\n"); 
	} 
	

Output:

  Welcome.
  Welcome.
  Welcome.
  Welcome.

Example: Recursion Example - Calculate Factorial

	#include <stdio.h>
	int recurs(int);
	main() 
	{ 
		int x, fact ; 
		printf("Enter number to calculate factorial: "); 
		scanf ("%d", &x); 
		fact = recurs(x); 
		printf ("Factorial value: %d", fact); 
	} 
	int recurs (int x) 
	{ 
		int fact; 
		if (x == 1) 
			return (1); 
		else 
			fact = x * recurs (x - 1) ; 
		return (fact) ; 
	}
	

Output: (4 is user input, 24 is output)

  Enter number to calculate factorial: 4
  Factorial value: 24