C# Program to Determine if a Number is Prime

using System;

class Program
{
    static void Main()
    {
        // Ask the user to input an integer
        Console.Write("Enter an integer: ");
        int number = int.Parse(Console.ReadLine()); // Read the number as an integer

        // Check if the number is less than 2, as prime numbers are greater than 1
        if (number <= 1)
        {
            Console.WriteLine($"{number} is not a prime number.");
        }
        else
        {
            bool isPrime = true; // Flag to indicate if the number is prime

            // Check divisibility from 2 to the square root of the number
            for (int i = 2; i <= Math.Sqrt(number); i++)
            {
                if (number % i == 0) // If number is divisible by i, it is not prime
                {
                    isPrime = false;
                    break; // No need to check further, number is not prime
                }
            }

            // Output whether the number is prime or not
            if (isPrime)
            {
                Console.WriteLine($"{number} is a prime number.");
            }
            else
            {
                Console.WriteLine($"{number} is not a prime number.");
            }
        }
    }
}

 Output

//Example 1: Input = 7
Enter an integer: 7
7 is a prime number.

//Example 2: Input = 10
Enter an integer: 10
10 is not a prime number.

//Example 3: Input = 1
Enter an integer: 1
1 is not a prime number.

//Example 4: Input = 13
Enter an integer: 13
13 is a prime number.