C++, C Program to Check whether a Number is Prime or Not

Write a C, C++ program to check whether a number is prime or not. In this program, we take a number as input from a user and your program prints whether an input number is prime or not.

What is a Prime Number?


A prime number is a number which is greater than 1 , and divisible by 1 and itself. 

For example -  7 is a prime number it is divisible by 1 and 7 only.

NOTE -  2 is the only even prime number. 


There are various methods to check whether a number is prime or not. You can check this tutorial for explanation. 

Program to check whether a number is prime or not

Algorithm to  check whether number is prime or not

We run a loop from 2 to num/2 and check whether the number is divisible. If the number is not divisible by any number then it's a prime number.


Program to Print all prime numbers between 1 to 100.

Print even numbers from 1 to 100.

Sorting algorithms and their time complexity.

C++ Program to Check whether a Number is Prime or Not

#include<iostream.h>
using namespace std;

int main(){

    int num, i, flag=0;
    

    cout << "Enter a number\n";
    cin  >> num;

    /* Check whether number is divisible 
     by number other than 1 and itself 
    */
    for(i = 2; i <= num/2; i++){

        if(num % i == 0){
            
            val = 1;
            break;

        }
    }

   if(flag == 0)
        cout << "An input number is a prime number";
   else
      cout<< " An input number is not a prime number";

   return 0;
}

C Program to Check whether a Number is Prime or Not

#include <stdio.h>

int main(void) {
 
 int num,i,val=0;
    
    printf ("Enter a number\n");
    scanf ("%d" , &num);

    /* Check whether number is divisible 
       by number other than 1 and itself 
    */
    for(i = 2; i <= num/2; i++){

        if(num % i == 0){
            
            val=1;
            break;

        }
    }

   if(val==0)
        printf (" An input number is a prime number");
   else
      printf (" An input number is not a prime number");
      
 return 0;
}

Output :

Enter a number :  5

An input number is a prime number


Enter a number : 6

An input number is not a prime number