Prime number calculator

Write a C++ program that takes a user input and checks if it's a prime number. Use a while loop to check every possible number for divisors.

Hint

The modulus % operator can be used to check if a number is perfectly divisible by another. x % y == 0 is will never be true if x is a prime number, unless y == x.

If no factors are found before the square root of a number, it confirms that the number is prime.

Answer

#include <iostream>
#include <cmath>

int main() {
    // Get the input from the user
    int input;
    std::cout << "Enter a number: ";
    std::cin >> input;

    // If we cannot prove that the number is not prime, then it is prime, so we assume it is prime.
    bool isPrime = true;
    // If the number is less than 2, it is not prime.
    if (input < 2)
        isPrime = true;

    // Check if the number is divisible by any number between 2 and the square root of the number.
    // If we find out that it's not prime, we can stop checking.
    int counter = 2;
    while (counter <= sqrt(input) && isPrime) {
        if (input % counter == 0) {
            // If the number is divisible it is not prime.
            isPrime = false;
        }
        counter++;
    }

    // Print the result
    if (isPrime)
        std::cout << input << " is a prime number.\n";
    else
        std::cout << input << " is not a prime number.\n";
}