C# Program to Check Prime Number

Posted in C# Tutorial

Updated on Jan 11, 2024

By Mari Selvan

👁️ 683 - Views

⏳ 4 mins

💬 1 Comment

Photo Credit to CodeToFun

🙋 Introduction

In the landscape of programming, dealing with prime numbers is a fundamental and interesting task. Prime numbers, those divisible only by 1 and themselves, play a crucial role in various mathematical and computational applications.

In this tutorial, we will delve into a simple yet effective C# program to check whether a given number is a prime number.

📄 Example

Let's explore the C# code that performs the prime number check.

Program.cs

Copied

using System;

class Program {
  // Function to check if a number is prime
  static bool IsPrime(int number) {
    // 0 and 1 are not prime numbers
    if (number <= 1) {
      return false;
    }

    // Check for factors from 2 to the square root of the number
    for (int i = 2; i * i <= number; ++i) {
      if (number % i == 0) {
        return false; // Found a factor, not a prime number
      }
    }

    return true; // No factors found, it's a prime number
  }

  // Driver program
  static void Main() {
    // Replace this value with your desired number
    int testNumber = 17;

    // Call the function to check if the number is prime
    if (IsPrime(testNumber)) {
      Console.WriteLine($"{testNumber} is a prime number.");
    } else {
      Console.WriteLine($"{testNumber} is not a prime number.");
    }
  }
}

💻 Testing the Program

To test the program with a different number, replace the value of testNumber in the Main method.

Output

17 is a prime number.

Compile and run the program to see whether the number is a prime number.

🧠 How the Program Works

The program defines a function IsPrime that takes an integer as input and returns a boolean indicating whether the number is prime.
Inside the function, it checks if the number is less than or equal to 1; if so, it's not prime.
It then iterates through potential factors from 2 to the square root of the number.
If it finds any factor, the number is not prime; otherwise, it is prime.
The Main method tests the program with a specific number (in this case, 17).

📏 Between the Given Range

Let's dive into the C# code that checks and lists prime numbers in the specified range.

Program.cs

Copied

using System;

class Program {
  // Function to check if a number is prime
  static bool IsPrime(int number) {
    if (number <= 1) {
      return false;
    }

    for (int i = 2; i <= Math.Sqrt(number); ++i) {
      if (number % i == 0) {
        return false;
      }
    }

    return true;
  }

  // Driver program
  static void Main() {
    Console.Write("Prime Numbers in the Range 1 to 20: \n");

    // Check and list prime numbers in the range 1 to 20
    for (int i = 1; i <= 20; ++i) {
      if (IsPrime(i)) {
        Console.Write(i + " ");
      }
    }

    Console.WriteLine();
  }
}

💻 Testing the Program

The program is set to check and list prime numbers in the range from 1 to 20. No additional input is required.

Output

Prime Numbers in the Range 1 to 20:
2 3 5 7 11 13 17 19

Compile and run the program to see the prime numbers in the specified range.

🧠 How the Program Works

The program defines a function IsPrime that checks whether a given number is prime.
Inside the function, it iterates through numbers from 2 to the square root of the given number to check for divisors.
The Main method then iterates through the range from 1 to 20, calling the IsPrime function to determine and print the prime numbers.

🧐 Understanding the Concept of Prime Numbers

Before diving into the code, let's briefly understand prime numbers.

A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. For example, 2, 3, 5, 7, 11, and 13 are prime numbers.

🎢 Optimizing the Program

While the provided program is effective, you can explore optimizations such as using the square root of the number as the loop limit for improved efficiency.

Feel free to incorporate and modify this code as needed for your specific use case. Happy coding!