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

What Is Prime Number

A prime number is a positive integer that is exactly divisible by the 1 and the number itself.

1, 3, 5, 7, 11, 17, 19 are the examples of prime numbers from 1 to 20

Example Prime Number

For example, 3 is a prime number because it is only divisible by 1 and 3, also 7 is a prime number because it is only divisible by 1 and the number itself 7.

So, 27 is not a prime number because it is divisible by 1, 3, 9, and 27.

MCQ: 2 is the only even number which is a prime number.

Related: C++ Program Factorial of a Number

C++ Program Prime Number 1

#include <iostream>
using namespace std;

int main() {
    int i, n;
    bool isPrime = true;

    cout << "Enter a positive integer: ";
    cin >> n;

    // 0 and 1 are not prime numbers
    if (n == 0 || n == 1) {
        isPrime = false;
    }
    else {

   // 2 is a prime number
        for (i = 2; i <= n / 2; ++i) {
            if (n % i == 0) {
                isPrime = false;
                break;
            }
        }
    }
    if (isPrime)
        cout << n << " is a prime number";
    else
        cout << n << " is not a prime number";

    return 0;
}
Code language: PHP (php)

Also Read: C++ Program Factorial using Recursion

Prime Number C++ Program 2

//C++ program to check for prime number
#include<iostream>
using namespace std;

int main()
{
   int num, i, j = 0;

   cout << "Enter number: ";
   cin >> num;

   //check for prime number
   for (i = 1; i <= num; i++)
   {
      if ((num % i) == 0)
      {
         j++;
      }
   }

   if (j == 2)
      cout << "\n" << num << " is a prime number.\n";
   else
      cout << num << " is not a prime number.\n";

   return 0;
}Code language: PHP (php)