Check Whether a Number is Prime or Not in C Program

Example to read how to check whether an integer (entered by the user) is a prime number or not using for loop and if...else statement.

A prime number is a positive integer which is divisible only by 1 and itself. For example: 2, 3, 5, 7, 11, 13

Example: Program to Check Prime Number

#include <stdio.h>
int main()
{
    int n, i, flag = 0;

    printf("Enter a positive integer: ");
    scanf("%d",&n);

    for(i=2; i<=n/2; ++i)
    {
        // condition for nonprime number
        if(n%i==0)
        {
            flag=1;
            break;
        }
    }

    if (flag==0)
        printf("%d is a prime number.",n);
    else
        printf("%d is not a prime number.",n);
    
    return 0;
}

Output

Enter a positive integer: 29
29 is a prime number. 

If the for loop terminates when the test expression of loop i <= n/2 is false, the entered number is a prime number. The value of flag is equal to 0 in this case.

If the loop terminates because of break statement inside the if statement, the entered number is a nonprime number. The value of flag is 1 in this case.