Program to Check Whether a Number is Prime or Not - (C Programs)

Introduction

In this article, we will discuss how to write a C program to check whether a given integer number is prime or not. Prime numbers are those numbers that are divisible only by 1 and itself. For example, 2, 3, 5, 7, 11, 13, 17, 19, 23, and so on are prime numbers.

Example

Let's take an example to understand how this program works:

Input: 17

Output: 17 is a prime number

Explanation

To check whether a number is prime or not, we need to divide that number by every integer number starting from 2 to the number itself. If the number is divisible by any integer, other than 1 and itself, it is not a prime number. If the number is not divisible by any integer, other than 1 and itself, it is a prime number.

Program

#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 non-prime number
        if(n%i == 0)
        {
            flag = 1;
            break;
        }
    }

    if (n == 1)
    {
        printf("1 is not a prime number.");
    }
    else
    {
        if (flag == 0)
        {
            printf("%d is a prime number.", n);
        }
        else
        {
            printf("%d is not a prime number.", n);
        }
    }

    return 0;
}

Output

If we run the program with the input value as 17, it will output:

Enter a positive integer: 17
17 is a prime number.

Use

This program can be used to check whether a given number is prime or not. It is useful in various mathematical calculations and algorithms.

Summary

In this article, we have discussed how to write a C program to check whether a given number is prime or not. We have also provided an example and explained the logic of the program. This program can be used in various mathematical calculations and algorithms.