C Program to Check Whether a Number is Prime or Not

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

To understand this example, you should have the knowledge of following C programming topics:

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 (n == 1) 
    {
      printf("1 is neither a prime nor a composite number.");
    }
    else 
    {
        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.
Visit this page to learn, how you can display all prime numbers between two intervals entered by the user.

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