using System;

​

public class Program

{

    //This is 1

    static private ulong fact(ulong a)

    {

        ulong factorial = 1;

        ulong i = 1;

        do

        {

            factorial = factorial * i;

            i++;

        }

        while (i <= a);

        return factorial;

    }

​

    //This is 2

    static private double DectoPer(double a)

    {

        return a * 100;

    }

​

    //This is 3

    static private double PertoDec(double a)

    {

        return a / 100;

    }

​

    // This is 4

    static private ulong nPr(ulong n, ulong r)

    {

        ulong nfact = fact(n);

        ulong nrfact = fact(n - r);

        return nfact / nrfact;

    }

​

    public static void Main()

    {

        string restart = "This variable will let my quit the program.";

        while (restart != "q")

        {

            start:

                Console.WriteLine("Please choose which option you would like to calculate");

            Console.WriteLine("Press 1 for factorial, 2 to convert from decimal to percent, 3 to convert from percent to decimal, 4 to calulate the permutation of a number, 5 to calculate the combination.");

            string choice = Console.ReadLine();

            if (choice == "1")

            {

                int i, fact = 1, number;

                Console.WriteLine("Please enter what number you want the factorial of.");

                number = int.Parse(Console.ReadLine());

                for (i = 1; i <= number; i++)

                {

                    fact = fact * i;

                }

​

                Console.Write("Factorial of " + number + " is: " + fact);

            }

            else if (choice == "2")

            {

                Console.WriteLine("What decimal would you like to convert to percent?");

                double ans = double.Parse(Console.ReadLine());

                Console.WriteLine(ans +  " =  " +DectoPer(ans));

            }

        else if (choice == "3")

        {

            {

                Console.WriteLine("What percent would you like to convert to decimal?");

                double ans = double.Parse(Console.ReadLine());

                Console.WriteLine(ans +  " =  " +PertoDec(ans));

            }

        }

        else if (choice == "4")

        {

            Console.WriteLine("How many total items do you have?");

            ulong ans1 = ulong.Parse(Console.ReadLine());

            Console.WriteLine("How many items do you want to arrange?");

            ulong ans2 = ulong.Parse(Console.ReadLine());

            Console.WriteLine(ans1 + "p" + ans2 + "=" + nPr(ans1, ans2));

        }

            else

            {

                Console.WriteLine("Please press 1, 2, 3, 4 or 5.");

                goto start;

            }

​

            Console.WriteLine(". Press q to quit the program or any other key to continue.");

            restart = Console.ReadLine();

        }

    }

}