using System;

using System.Collections.Generic;

using System.Diagnostics;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Filter Prime Numbers");

        Stopwatch sw = new Stopwatch();

        sw.Reset();

        sw.Start();

        PrintPrimeNumbers(20);

        sw.Stop();

        Console.WriteLine("Total Time taken {0} ms",sw.ElapsedTicks.ToString());

        sw.Reset();

        sw.Start();

        foreach(int i in ComputePrime())

        {

            Console.WriteLine("{0} is a Prime Number", i);

        }

        sw.Stop();

        Console.WriteLine("Total Time taken {0} ms",sw.ElapsedTicks.ToString());

    }

    public static void PrintPrimeNumbers(int number)

    {

        for(int i=2; i<number; i++)

        {

            bool isPrime = true;

            for(int j=2; j<=i/2; j++)

            {

                if(i%j == 0)

                {

                    isPrime = false;

                    break;

                }

            }

            if(isPrime)

                Console.WriteLine("{0} is a Prime Number", i);

        }

    }

    static IEnumerable<int> ComputePrime()

    {

        for(int i=2; i<20; i++)

        {

            bool isPrime = true;

            for(int j=2; j<=i/2; j++)

            {

                if(i%j == 0)

                {

                    isPrime = false;

                    break;

                }

            }

            if(isPrime)

                yield return i;

        }

    }

}