C Program to Check Whether a Number is Prime or Not Using User Defined Function

Question: write a program in C language to read a number from a user and check whether it is a prime number or not using a user defined function.

A Prime Number is a positive integer greater than 1 which is divisible by 1 and itself. In other words, Prime number is a whole number greater than whose factors are 1 and itself. Examples of first few prime numbers are 2, 3, 5, 7, 11, 13..

Note : Number 2 is first and only even prime number.

This program checks whether a number given by a user is prime or not in c language using a user defined function.

Program

``````
#include<stdio.h>
#include<conio.h>

#include

/* Function prototype to check prime number */
int check_prime(int n);

/* Main function */
int main()
{
int number, i, flag=0;

// Inputs
printf("Enter integer number: ");
scanf("%d", &number);

// Function call and taking decision
if(check_prime(number)==1)
{
printf("%d is PRIME.", number);
}
else
{
printf("%d is NOT PRIME.", number);
}

return(0);
}

int check_prime(int n)
{
int i, flag=0;
for(i=2;i<=n/2; i++)
{
if(n%i==0)
{
return 0;
}
}
return 1;
}
``````

Output of above program :

```Run 1:
-----------
Enter integer number: 13 ↲
13 is PRIME.

Run 2:
-----------
Enter integer number: 16 ↲
16 is NOT PRIME.

Run 3:
-----------
Enter integer number: 1 ↲
1 is NOT PRIME.

Run 4:
-----------
Enter integer number: 2 ↲
2 is PRIME.

Note: ↲ indicates enter is pressed.
```