Determine Prime Number in C++

Introduction

In this lab, you will learn how to write a C++ program to check whether a given number is a prime number or not. A prime number is a number that is only divisible by 1 and itself. It is an important concept in number theory and has many important applications in cryptography.

Skills Graph

Create a new C++ source file

First, create a new C++ source file named main.cpp in ~/project/ directory.

cd ~/project
touch main.cpp

Write the C++ program to check prime number

Copy the following code into the main.cpp file:

#include <iostream>

using namespace std;

int main() {
    int n, i;
    bool isPrime = true;

    // Get the number from the user
    cout << "Enter a positive integer: ";
    cin >> n;

    // Check whether the number is prime
    for(i=2; i<=n/2; i++) {
        if(n%i == 0) {
            isPrime = false;
            break;
        }
    }

    if(isPrime) {
        cout << n << " is a prime number." << endl;
    } else {
        cout << n << " is not a prime number." << endl;
    }

    return 0;
}

This program takes an integer n from the user and checks whether it is prime or not. If it's prime, print n is a prime number.; otherwise, print n is not a prime number.

Compile and Run the Program

Open the terminal in Ubuntu system and navigate to the ~/project/ directory:

cd ~/project

Compile the main.cpp code by running the following command:

g++ main.cpp -o main

Run the compiled executable file by executing the following command:

./main

Test the Program

Now, enter different numbers to check whether they are prime or not:

Enter a positive integer: 17
17 is a prime number.

Enter a positive integer: 57
57 is not a prime number.

Summary

In this lab, you learned how to write a C++ program to check whether a given number is a prime number or not. You now have a solid understanding of this concept, which is important in number theory and cryptography.