Check for Prime number

A prime number is greater than one and can be divided by one or itself. In other words, prime numbers cannot be divided by anything other than themselves or 1. Prime numbers include 2, 3, 5, 7, 11, 13, 17, 19, 23, etc.

Program code:

#include <iostream>
using namespace std;

int main() {

  int num, i, flag = 0;
  printf("Enter a positive integer: ");
  cin>>num;

  // 0 and 1 are not prime numbers
  // change flag to 1 for non-prime number
  if (num == 0 || num == 1)
    flag = 1;

  for (i = 2; i <= num / 2; ++i) {

    // if n is divisible by i, then n is not prime
    // change flag to 1 for non-prime number
    if (num % i == 0) {
      flag = 1;
      break;
    }
  }

  // flag is 0 for prime numbers
  if (flag == 0)
    cout<<num<<" is a prime number.";
  else
    cout<<num<<" is NOT a prime number.";

  return 0;
}

Output:

Enter a positive integer: 56
56 is NOT a prime number.

Enter a positive integer: 73
73 is a prime number.

Note: also read about Diamond Pattern Using Numbers

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