//Programmer: Jiamiao Zeng

//Date: Start - Dec. 1, 2024

//Project: Credit Card Checker

/*Description: A program that checks a set of credit card number input to see if the

card number is valid based on a check digit*/

​

using System;

using System.Collections.Generic; //imported to use list

using System.Linq; //imported to use .sum method

​

public class Program

{

    public static void Main()

    {

        List<int> cardNum = new List<int>(); //list to put user input in

        List<int> multiplied = new List<int>(); //list to put multipled numbers in for the calculation later

        Console.WriteLine("Credit Card Checker" + "\n" + "Please input card number:");

        takeNum:

        string input = Console.ReadLine(); //reads user input number as string

        foreach (char number in input) //separates the string input into several chars

        {

            if (char.IsNumber(number)) //checks if the char is a number

            {

                cardNum.Add(number - '0'); //converts char to int and add to cardNum list

            } //the loop ignores all input that is not a number, such as white space and letters

        }

        if(cardNum.Count() > 16){ //check if the length of the input is less than 17

            Console.WriteLine("Please input a number with 16 or less digits.");

            cardNum.Clear(); //if more than 17, clear the list and restart user input

            goto takeNum;

        }

​

        cardNum.Reverse(); //flip the index of the elements in the list

        for (int position = 0; position < cardNum.Count; position++) //looping through the list

        {

            if (position % 2 != 0) //since list index starts with 0, the second/fourth/sixth element would have an odd index

            { //checks all odd positions 

                string num = Convert.ToString(cardNum[position] * 2); //doubles the value of alternate digits of the list beginning with the second digit and converts to string

                foreach (char digit in num) //separate the num string into chars so that calculations are done with individual digits later

                {

                    multiplied.Add(Convert.ToInt32(digit - '0')); //add doubled digits to multiplied list after converting them to int

                }

                cardNum[position] = 0; //for the doubled numbers, change it to 0 in first list so that it does not affect the sum of said list

            }

        }

​

        int sum = cardNum.Sum() + multiplied.Sum(); //calculate sum of int in both lists                                                                                                                                                                                                                                                                                                                              

        if (sum % 10 == 0) //checks if the sum ends in 0 (is divisible by 10)

        {

            Console.WriteLine("The card is valid.");

        }

        else

        {

            Console.WriteLine("The card is invalid.");

        }

    }

}