C++ Program to Check Whether a Number is Prime or Not

In this program, we will learn how to check whether a number is prime or not using C++ Programming language.

We will discuss two ways to do it.

1. Traversing till half the given number
2. Traversing till square root of the given number

Let's discuss them one by one.

Check whether a number is prime or not

In this program, we ask the user to enter a number and store it in variable num.

Then, for loop is executed which checks whether the number entered by the user is perfectly divisible by i or not.

The for loop initiates with an initial value of i equals to 2 and increasing the value of i in each iteration.

If the number entered by the user is perfectly divisible by i then, isPrime is set to false and the number will not be a prime number.

But, if the number is not perfectly divisible by i  until test condition i <= n/2 is true means, it is only divisible by 1 and that number itself.

Traversing till half the given number

Code:-

#include <iostream>  

using namespace std;  

int main(){  
    int num;
    bool isPrime = true; 

    cout << "Enter the Number: ";  
    cin >> num; 

    if(num <= 1){
        isPrime = false;
    }

    for(int i = 2; i <= num/2; i++){  
        if(num % i == 0){  
            isPrime = false;  
            break;  
        }  
    }  

    if (isPrime)  
        cout << num << " is Prime." << endl;  
    else
        cout<< num << " is not Prime." << endl;  

    return 0;  
}

Output:-

Traversing till square root of the given number

In this, we just have used the sqrt() function of the math library to get the square root of the number. Now, for loop will traverse till the square root of the given number (num variable).

Code:-

#include <iostream>  
#include <math.h>

using namespace std;  

int main(){  
    int num;
    bool isPrime = true; 

    cout << "Enter the Number: ";  
    cin >> num; 

    if(num <= 1){
        isPrime = false;
    }

    for(int i = 2; i <= sqrt(num); i++){  
        if(num % i == 0){  
            isPrime = false;  
            break;  
        }  
    }  

    if (isPrime)  
        cout << num << " is Prime." << endl;  
    else
        cout<< num << " is not Prime." << endl;  

    return 0;  
}

Output:-