Prime Number Checker in C++

Introduction

A prime number is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers.
In other words, a prime number is only divisible by 1 and itself. The objective of this program is to check if a given
number is prime or not using C++ programming.

Objective

The primary goal of this program is to determine whether a given integer is prime or not. The program will:

• Take an integer as input from the user.
• Check if the number is divisible by any number other than 1 and itself.
• Return a result indicating whether the number is prime or not.

Code

        #include 
        using namespace std;

        // Function to check if a number is prime
        bool isPrime(int number) {
            // A number less than 2 is not prime
            if (number <= 1) {
                return false;
            }

            // Check divisibility up to the square root of the number
            for (int i = 2; i * i <= number; i++) {
                if (number % i == 0) {
                    return false; // Divisible by i, so it's not prime
                }
            }

            return true; // Number is prime if no divisors were found
        }

        int main() {
            int num;

            // Ask the user to input a number
            cout << "Enter a number: "; cin >> num;

            // Check if the number is prime
            if (isPrime(num)) {
                cout << num << " is a prime number." << endl;
            } else {
                cout << num << " is not a prime number." << endl;
            }

            return 0;
        }

Explanation of the Program

This C++ program checks whether a given number is prime using the following steps:

1. The program defines a function isPrime(int number) that takes an integer as input and checks whether the number is prime.
2. If the number is less than or equal to 1, it immediately returns false because numbers less than or equal to 1 are not prime.
3. The program uses a for loop to check divisibility from 2 up to the square root of the number. This is an optimization that reduces the number of checks needed.
4. If the number is divisible by any number in this range, the function returns false, indicating the number is not prime.
5. If no divisors are found, the function returns true, indicating the number is prime.
6. In the main function, the program prompts the user to enter a number, calls the isPrime function, and prints the result accordingly.

How to Run the Program

To run the C++ program, follow these steps:

1. Open a text editor (e.g., Notepad++ or Visual Studio Code) and paste the above code into a new file.
2. Save the file with a .cpp extension (e.g., prime_checker.cpp).
3. Open a terminal or command prompt and navigate to the directory where the file is saved.
4. Compile the program using a C++ compiler. For example, if you’re using g++, type the following command:

   g++ -o prime_checker prime_checker.cpp

5. Run the compiled program with this command:

   ./prime_checker

6. Enter an integer when prompted, and the program will tell you whether it is prime or not.