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 positive integer which is only divisible by 1 and itself is known as prime number.

For example: 13 is a prime number because it is only divisible by 1 and 13 but, 15 is not prime number because it is divisible by 1, 3, 5 and 15.


Example: Check Prime Number

#include <iostream>
using namespace std;

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

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

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

  return 0;
}

Output

Enter a positive integer: 29
This is a prime number.

This program takes a positive integer from user and stores it in variable n.

Then, for loop is executed which checks whether the number entered by user is perfectly divisible by i or not.

The foor loop initiates with an initial value of i equals to 2 and increasing the value of i in each iteration.

If the number entered by user is perfectly divisible by i then, isPrime is set to false and the number will not be a prime number.

But, if the number is not perfectly divisible by i until test condition i <= n/2 is true means, it is only divisible by 1 and that number itself.

So, the given number is a prime number.