using System;

​

public class Program

{

    static private double fact(double a)

    {

        ulong factorial = 1;

        ulong i = 1;

        do

        {

            factorial = factorial * i;

            i++;

        }

        while (i <= a);

        return factorial;

    }

​

    static private double DectoPer(double a)

    {

        return a * 100;

    }

​

    static private double PertoDec(double a)

    {

        return a / 100;

    }

​

    static private double nPr(double n, double r)

    {

        double nfact = fact(n);

        double nrfact = fact(n - r);

        return nfact / nrfact;

    }

​

    static private double nCr(double n, double r)

    {

        double nfact = fact(n);

        double nrfact = fact(n - r);

        return (nfact) / (nrfact) * (nfact);

    }

​

    public static void Main()

    {

        string restart = "Hi";

        while (restart != "q")

        {

            Console.Clear();

            Console.WriteLine("Please choose what you would like to calulate:");

            Console.WriteLine("Press 1 to calculate Factorial");

            Console.WriteLine("Press 2 to convert a Percent to a Decimal");

            Console.WriteLine("Press 3 to convert a Decimal to a Percent");

            Console.WriteLine("Press 4 to calculate Permutation");

            Console.WriteLine("Press 5 to calculate Combination");

            string choice = Console.ReadLine();

            if (choice == "1")

            {

                Console.WriteLine("What number would you like the factorial of?");

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

                Console.WriteLine(ans1 + "! = " + fact(ans1));

            }

            else if (choice == "2")

            {

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

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

                Console.WriteLine(ans2 + " = " + PertoDec(ans2));

            }

            else if (choice == "3")

            {

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

                double ans3 = double.Parse(Console.ReadLine());

                Console.WriteLine(ans3 + " = " + DectoPer(ans3) + "%");

            }

            else if (choice == "4")

            {

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

                double ans4 = double.Parse(Console.ReadLine());

                Console.WriteLine("How many ways would you like to arrange this?");

                double ans5 = double.Parse(Console.ReadLine());

                Console.WriteLine(ans4 + "P" + ans5 + " = " + nPr(ans4, ans5));

            }

            else if (choice == "5")

            {

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

                double ans6 = double.Parse(Console.ReadLine());

                Console.WriteLine("How many ways would you like to arrange this?");

                double ans7 = double.Parse(Console.ReadLine());

                Console.WriteLine(ans6 + "P" + ans7 + " = " + nCr(ans6, ans7));

            }

​

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

            restart = Console.ReadLine();

        }

    }

}