Introduction
Fundamentals
C Flow Control
C Functions
C Arrays
User Defined Functions in C Language
User-defined functions have contained the block of statements which are written by the user to perform a task. In other words we can say that a function created by user is known as user defined function. In this tutorial, we will cover different - different type of user defined functions in C Language.
Example 1: Function with no arguments and no return value
#include<stdio.h>
void smallNum();// function declaration
int main()
{
smallNum(); // function call
return 0;
}
void smallNum(){ // function definition
int i, j;
printf("Enter 2 numbers to compare: \n");
scanf("%d%d", &i, &j);
if(i < j)
{
printf("The smaller number is: %d", i);
}
else
{
printf("The smaller number is: %d", j);
}
}Output :
Enter 2 numbers to compare: 6 9 The smaller number is: 6
Above is an example of smallNum() function, which takes 2 numbers as input from user, and display which is the smaller number.
The return type of the function is void. Hence, no value is returned from the function.
Example 2: No Arguments Passed But Returns a Value
#include<stdio.h>
int smallNum(); // function declaration
int main()
{
int result;
result = smallNum(); // function call
printf("The smaller number is: %d", result);
return 0;
}
int smallNum() // function definition
{
int i, j, smallNum;
printf("Enter 2 numbers that you want to compare: \n");
scanf("%d%d", &i, &j);
if(i < j)
{
smallNum = i;
}
else
{
smallNum = j;
}
// returning the result
return smallNum;
}Output :
Enter 2 numbers that you want to compare: 9 23 The smaller number is: 9
Example 3: Argument Passed But No Return Value
#include <stdio.h>
void checkPrimeAndDisplay(int n);
int main() {
int n;
printf("Enter a positive integer: ");
scanf("%d",&n);
// n is passed to the function
checkPrimeAndDisplay(n);
return 0;
}
// return type is void meaning doesn't return any value
void checkPrimeAndDisplay(int n) {
int i, check = 0;
// 0 and 1 are not prime numbers
if (n == 0 || n == 1)
check = 1;
for(i = 2; i <= n/2; ++i) {
if(n%i == 0){
check = 1;
break;
}
}
if(check == 1)
printf("%d is not a prime number.",n);
else
printf("%d is a prime number.", n);
}Output :
Enter a positive integer: 67 67 is a prime number.
The integer value entered by the user is passed to the checkPrimeAndDisplay() function.
Here, the checkPrimeAndDisplay() function checks whether the argument passed is a prime number or not and displays the appropriate message.
Example 4: Argument Passed and Returns a Value
#include <stdio.h>
int checkPrimeNumber(int n);
int main() {
int n, flag;
printf("Enter a positive integer: ");
scanf("%d",&n);
// n is passed to the checkPrimeNumber() function
// the returned value is assigned to the flag variable
flag = checkPrimeNumber(n);
if(flag == 1)
printf("%d is not a prime number",n);
else
printf("%d is a prime number",n);
return 0;
}
// int is returned from the function
int checkPrimeNumber(int n) {
// 0 and 1 are not prime numbers
if (n == 0 || n == 1)
return 1;
int i;
for(i=2; i <= n/2; ++i) {
if(n%i == 0)
return 1;
}
return 0;
}Output :
Enter a positive integer: 56 56 is not a prime number
