C Program to Check if a given Number is Prime or not

C Program to Check if a given Number is Prime or not - Source code

#include <stdio.h>
 
int main()
{
    int n, i, flag;
 
    printf("Enter a number \n");
    scanf("%d", &n);
 
    if (n <= 1)
    {
        printf("Please enter a positive number.", n);
        return 0;
    }
    flag = 0;
    for (i = 2; i <= n / 2; i++)
    {
        if ((n % i) == 0)
        {
            flag = 1;
            break;
        }
    }
    if (flag == 0)
    {
         printf("%d is a prime number \n", n);
    }
 
     else
     {
        printf("%d is not a prime number \n", n);
     }
 
     return 0;
}

Program Output

Case 1:

Enter a number
451
451 is not a prime number

Case 2:

Enter a number
79
79 is a prime number

Case 3:

Enter a number
-12
Please enter a positive number.

Program Explanation

1. The program accept a positive integer using the scanf function. If the number entered by the user is negative, it prompt the user to enter a positive number.

2. To check for the prime number, modulus operator is used which returns the remainder after dividing the entered number with each number up to n/2 (starting from 2).

3. If the remainder comes out zero in any case, that means the number is not prime and the variable flag is set to 1 and for loop is existed. Otherwise the value of flag remains 0.

4. Depending on the value of variable flag, message is printed on the screen using if-else statement.