Program to Check if a Number is Prime in C++

Checking Prime Number

A prime number is a natural number greater than 1 that has no divisors other than 1 and itself.

We will explore different methods to check if a number is prime using C++ programming.

Method 1: Using a for Loop

We iterate from 2 to the square root of the number and check for divisibility.

#include <iostream>
using namespace std;

int main() {
    int num, i;
    bool isPrime = true;
    cout << "Enter a number: ";
    cin >> num;
    
    if (num < 2) {
        isPrime = false;
    } else {
        for (i = 2; i * i <= num; i++) {
            if (num % i == 0) {
                isPrime = false;
                break;
            }
        }
    }
    
    if (isPrime)
        cout << num << " is a prime number";
    else
        cout << num << " is not a prime number";
    
    return 0;
}
            

Output:

Enter a number: 7
7 is a prime number

Method 2: Using a Function

We create a function to check for primality and return the result.

#include <iostream>
using namespace std;

bool isPrime(int num) {
    if (num < 2) return false;
    for (int i = 2; i * i <= num; i++) {
        if (num % i == 0)
            return false;
    }
    return true;
}

int main() {
    int num;
    cout << "Enter a number: ";
    cin >> num;
    
    if (isPrime(num))
        cout << num << " is a prime number";
    else
        cout << num << " is not a prime number";
    
    return 0;
}
            

Output:

Enter a number: 10
10 is not a prime number