C++ Program to check Prime Number

A prime is a natural number greater than 1 that has no positive divisors other than 1 and itself. For example, 5 is prime but 10 is not prime. In this article, we will learn how to write a C++ program to check whether a number is prime or not.

Algorithm to Check Prime Numbers in C++

• Run a loop from 2 to n/2 using a variable i.
• Inside the loop, check whether n is divisible by i using the modulo operator ( n % i == 0 ).
• If n is not divisible by i, it means n is a prime number.
• If n is divisible by i, it means n has divisors other than 1 and itself, and thus, it is not a prime number.

Prime Number Program in C++

Below is the C++ program to check if a number is a prime number or not.

C++

// A school method based C++ program
// to check if a number is prime
#include <bits/stdc++.h>
using namespace std;

bool isPrime(int n)
{
    // Corner case
    if (n <= 1)
        return false;

    // Check from 2 to n-1
    for (int i = 2; i <= n / 2; i++)
        if (n % i == 0)
            return false;

    return true;
}

// Driver code
int main()
{
    isPrime(11) ? cout << " true\n" : cout << " false\n";
    isPrime(15) ? cout << " true\n" : cout << " false\n";
    return 0;
}

Output

 true
 false

Complexity Analysis

• Time Complexity: O(n) 
• Auxiliary Space: O(1)