Example: Check Prime Number by User-defined Function in C++

// Check Prime Number by User-defined Function
#include <iostream>
#include <cmath> // Required for sqrt() function

// Function to check if a number is prime
bool isPrime(int num) {
    // Step 1: Handle edge cases
    if (num <= 1) {
        return false; // Numbers less than or equal to 1 are not prime
    }
    if (num == 2) {
        return true;  // 2 is the only even prime number
    }
    if (num % 2 == 0) {
        return false; // All other even numbers are not prime
    }

    // Step 2: Check for divisibility from 3 up to sqrt(num)
    // We only need to check odd divisors
    for (int i = 3; i <= sqrt(num); i += 2) {
        if (num % i == 0) {
            return false; // Found a divisor, so it's not prime
        }
    }

    // Step 3: If no divisors were found, the number is prime
    return true;
}

int main() {
    // Step 1: Declare a variable to store user input
    int number;

    // Step 2: Prompt the user to enter a number
    std::cout << "Enter a positive integer: ";
    std::cin >> number;

    // Step 3: Call the isPrime function and display the result
    if (isPrime(number)) {
        std::cout << number << " is a prime number." << std::endl;
    } else {
        std::cout << number << " is not a prime number." << std::endl;
    }

    return 0;
}