Question

Credit Card Number Check. The last digit of a credit card number is the check digit, which protects against transcription errors such as an error in a single digit or switching two digits. The following method is used to verify actual credit card numbers but, for simplicity, we will describe it for numbers with 8 digits instead of 16:

• Starting from the rightmost digit, form the sum of every other digit. For example, if the credit card number is 4358 9795, then you form the sum 5 + 7 + 8 + 3 = 23.

• Double each of the digits that were not included in the preceding step. Add all digits of the resulting numbers. For example, with the number given above, doubling the digits, starting with the next-to-last one, yields 18 18 10 8. Adding all digits in these values yields 1 + 8 + 1 + 8 + 1 + 0 + 8 = 27.

• Add the sums of the two preceding steps. If the last digit of the result is 0, the number is valid. In our case, 23 + 27 = 50, so the number is valid. Write a program in Java that implements this algorithm. The user should supply an 8-digit number, and you should print out whether the number is valid or not. If it is not valid, you should print the value of the check digit that would make it valid.

Answer 1

Hi there,

I have solved the question according to the steps described above.

The Code:

import java.util.*;

public class creditCard {
   public static void main(String[] args) {
       int num,length=0;
       Scanner sc= new Scanner(System.in);   
       //loop till we get a valid length credit card
       do
       {
           System.out.print("Enter a 8 digit number : ");
           num = sc.nextInt();
           int temp=num;
           length=0;
           while(temp!=0)
           {
               length++;
               temp/=10;
           }  
           if(length!=8)
               System.out.println("Length is not 8. Please try again!!");
       }while(length!=8);
       //create a array to store the digits      
       int[] arr = new int[8];
       int temp=num;
       for(int i=7;i>=0;i--)
       {
           arr[i]=temp%10;
           temp/=10;
       }
       //calculate the value to check validity
       int sum=arr[7]+arr[5]+arr[3]+arr[1]+calc(arr[6])+calc(arr[4])+calc(arr[2])+calc(arr[0]);
       //check for validity
       if(sum%10==0)
           System.out.println("Valid credit card!!");
       else
       {
           int diff;
           if(arr[7] + (10-sum%10) > 9)
               diff=arr[7]-sum%10;
           else
               diff=arr[7]+(10-sum%10);  
           System.out.println("Invalid credit card!! The correct check digit is: "+ diff);  
       }
   }
   //function to calculate the sum of digits of a number
   private static int calc(int x){
       x*=2;
       int sum=0;
       while(x!=0)
       {
           sum+=x%10;
           x/=10;
       }
       return sum;  
   }
}            

The Code(Screenshot) :

The Output(Screenshot) :

Valid output:

Invalid output:

I hope you liked my answer. If you have any doubt, please do mention it in the comment section.

Happy Coding :)