projecteuler_problem_3.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LargestPrimeFactor
{
    class Program
    {
        static void Main(string[] args)
        {
            //http://projecteuler.net/problem=3

            var before = DateTime.Now;
            double UserInput = 600851475143;// 600,  851,  475, 143 --> four places

            int initialValue = 10000;

            IEnumerable<int> PrimeNumbersList = GetListOfPrimeNumbers(initialValue);

            Console.WriteLine("Input Number is: " + UserInput);            
            Console.WriteLine( );
            double LargestPrime = UserInput;
            foreach (int item in PrimeNumbersList)
            {
                //System.IO.File.AppendAllText("a.txt" ,Environment.NewLine +  item.ToString() );                 
                if (LargestPrime % item == 0)
                {
                    Console.WriteLine(item + ", "  );
                    var res = LargestPrime / item;
                    if (LargestPrime <= item)
                    {
                        Console.WriteLine(item);
                        break;
                    }

                }
                    
            }

            
             var after = DateTime.Now;
             Console.WriteLine();
            Console.WriteLine(after - before);
            

        }

        private static IEnumerable<int> GetListOfPrimeNumbers(int num)
        {
            IEnumerable<int> req = Enumerable.Range(0, (int)
                Math.Floor(2.52 * Math.Sqrt(num) / Math.Log(num))).Aggregate(
                Enumerable.Range(2, (int)num - 1).ToList(),

                (result, index) =>
                {
                    var bp = result[index];
                    var sqr = bp * bp;
                    result.RemoveAll(i => i >= sqr && i % bp == 0);
                    return result;
                }
            );
            return req;
        } 



    }
}