function for checking a prime number in c++ code example

Example 1: c++ find prime numbers

#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;
}

Example 2: check prime number c++

#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 {
        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;
}

Example 3: C++ prime number check

// 6k+-1 optimisation
bool is_prime6(int num) {
  int i;
  if (num == 1)
    return false;
  if (num <= 3)
    return true;
  if (num % 2 == 0 || num % 3 == 0)
    return false;
  if (num < 25)
    return true;
  for (i = 5; i * i <= num; i += 6)
    if (num % i == 0 || num % (i + 2) == 0)
      return false;
  return true;
}