Write a C# program to check if a number is prime or not

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. To check if a number is prime in C#, we count how many divisors it has by iterating through all numbers from 1 to the number itself.

The basic approach uses a counter that increments each time we find a divisor. If the counter equals 2 at the end (divisible only by 1 and itself), the number is prime.

Algorithm

The algorithm for checking prime numbers follows these steps −

• Initialize a counter to 0

• Loop from 1 to the number (n)

• If the number is divisible by the current iteration value, increment the counter

• If the counter equals 2, the number is prime

Using Basic Method

This example checks if a number is prime by counting all its divisors −

using System;

class Program {
    public static void Main() {
        int n = 17, a = 0;

        for (int i = 1; i <= n; i++) {
            if (n % i == 0) {
                a++;
            }
        }

        if (a == 2) {
            Console.WriteLine("{0}: Prime Number", n);
        } else {
            Console.WriteLine("{0}: Not a Prime Number", n);
        }
    }
}

The output of the above code is −

17: Prime Number

Using Optimized Method

A more efficient approach only checks divisors up to the square root of the number, since factors come in pairs −

using System;

class Program {
    public static bool IsPrime(int number) {
        if (number <= 1) return false;
        if (number == 2) return true;
        if (number % 2 == 0) return false;

        for (int i = 3; i <= Math.Sqrt(number); i += 2) {
            if (number % i == 0) {
                return false;
            }
        }
        return true;
    }

    public static void Main() {
        int[] numbers = {2, 17, 25, 29, 100};

        foreach (int num in numbers) {
            if (IsPrime(num)) {
                Console.WriteLine("{0}: Prime Number", num);
            } else {
                Console.WriteLine("{0}: Not a Prime Number", num);
            }
        }
    }
}

The output of the above code is −

2: Prime Number
17: Prime Number
25: Not a Prime Number
29: Prime Number
100: Not a Prime Number

Using Method with Range Checking

This example demonstrates checking multiple numbers in a range −

using System;

class Program {
    public static void CheckPrime(int n) {
        if (n <= 1) {
            Console.WriteLine("{0}: Not a Prime Number (must be > 1)", n);
            return;
        }

        int divisorCount = 0;
        for (int i = 1; i <= n; i++) {
            if (n % i == 0) {
                divisorCount++;
            }
        }

        if (divisorCount == 2) {
            Console.WriteLine("{0}: Prime Number", n);
        } else {
            Console.WriteLine("{0}: Not a Prime Number", n);
        }
    }

    public static void Main() {
        Console.WriteLine("Checking numbers from 1 to 20:");
        for (int i = 1; i <= 20; i++) {
            CheckPrime(i);
        }
    }
}

The output of the above code is −

Checking numbers from 1 to 20:
1: Not a Prime Number (must be > 1)
2: Prime Number
3: Prime Number
4: Not a Prime Number
5: Prime Number
6: Not a Prime Number
7: Prime Number
8: Not a Prime Number
9: Not a Prime Number
10: Not a Prime Number
11: Prime Number
12: Not a Prime Number
13: Prime Number
14: Not a Prime Number
15: Not a Prime Number
16: Not a Prime Number
17: Prime Number
18: Not a Prime Number
19: Prime Number
20: Not a Prime Number

Comparison of Methods

Conclusion

Checking prime numbers in C# can be done using different approaches. The basic method counts all divisors, while the optimized method checks only up to the square root for better performance. Choose the method based on your specific requirements for accuracy and efficiency.