//Programmer: June Oh

//Date: Start: Nov 19-21

//Project: U7 A3 greatest common factor

//Description: calculate the greatest common factor among three given numbers 

using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Greatest Common Factor (GCF)");//display the title

        loop1://loop1 so we can come back

        Console.WriteLine("Please enter three numbers you want to find the Greatest Common Factor (GCF)");

        int no1;//set an integer variable for number 1

        if(!int.TryParse(Console.ReadLine(), out no1)||no1<=0)//loop back if the answer is not in a double value

           {

               Console.WriteLine("Please enter a valid positive number.");

               goto loop1;

           }

        int no2;//set an integer variable for number 2

        if(!int.TryParse(Console.ReadLine(), out no2)||no2<=0)//loop back if the answer is not in a double value

           {

               Console.WriteLine("Please enter a valid positive number.");

               goto loop1;

           }

        int no3;//set an integer variable for number 3

        if(!int.TryParse(Console.ReadLine(), out no3)||no3<=0)//loop back if the answer is not in a double value

           {

               Console.WriteLine("Please enter a valid positive number.");

               goto loop1;

           }

        int a=0;//int a,b,c,d

        int b=0;

        int c=0;

        int d=0;

        if (no1>=no2)//if no1 is no2,

        {

            while(no1%no2!=0)//continue the loop until no1 is divisible by no2

            {

                a=no1%no2;//a is a remainder

                no1=no2;//bigger number becomes smaller number

                no2=a;//smaller number becomes the remainder.

            }//keep reducing numbers 

            b=no2;//store the value, GCF in b

        }

        else if (no1<no2)

        {

            while(no2%no1!=0)//continue the loop until no2 divisible by no1

            {

                a=no2%no1;//a is a remainder

                no2=no1;//no2 is a second number

                no1=a;//no1 is equal to a

            }

            b=no1;//store the value, GCF in b

        }   

        if (b>=no3)// this is for GCF of the second and the third number.

        {

            while(b%no3!=0)//continue the loop until b is disible by no3

            {

                c=b%no3;//c is a remainder

                b=no3;//b is a second number

                no3=c;//number 3 is now c

            }

            d=no3;//store the value, GCF in d

            Console.WriteLine("The Greatest common factor is "+no3);//display the result

        }

        else if (b<no3)// if b is less than no3

        {

            while(no3%b!=0)//continue the loop until no3 divisible by b

            {

                c=no3%b;//c is a remainder

                no3=b;//bigger number becomes smaller number

                b=c;//smaller number becomes the remainder

            }

            d=b;//store the value, GCF in d

            Console.WriteLine("The Greatest common factor is "+b);//display the result

        }

        Console.WriteLine("would you like to try again?");//ask users for the replay and store the answer

        string ans1=Console.ReadLine();

        if (ans1.Substring(0,1).ToUpper() == "Y")//if they are willing to replay,

        {

            goto loop1;//goto first loop1 we set

        }

        else if(ans1.Substring(0,1).ToUpper() == "N")//if they are not,

        {

            Console.WriteLine("byebye :)");//byebye

        }

        else 

        {

            Console.WriteLine("please enter a valid answer");//fool proof

        }

    }

}

​