Credit Card Validator

In this code challenge we must validate a credit card using the card's checksum supplied by the card vendor.

Problem

Takes in a credit card number from a common credit card vendor (Visa, MasterCard, American Express, Discoverer) and validates it to make sure that it is a valid number (look into how credit cards use a checksum).

Solution

How checksum works

Credit card vendors want an easy way to validate cards. This is to ensure, when the card is charged, that the card number is correct and therefore the payment can go through. If you've ever paid for something online and entered your card number wrong, you would know the website instantly knows and won't let you continue. Behind the scenes the website is running a checksum algorithm on the card number to ensure it is correct.

The algorithm behind checksum

Example card number: 4716 1065 5423 0864

1. Take the first 15 digits and double every other digit: 8 7 2 6 2 0 12 5 10 4 4 3 0 8 12
2. Any numbers that are longer than a single digit we add the two number together (e.g. 12 = 1+2 = 3): 8 7 2 6 2 0 3 5 1 4 4 3 0 8 3
3. Now we add all the numbers together: 56
4. Take the 16th digit and add it onto our sum: 56 + 4 = 60
5. Finally, if our number is a multiple of 10 then it is valid, otherwise it is not.

I recommend you do this on pen and paper first ad it'll help you understand what we're doing. Try using this online fake credit card number generator and doing the algorithm yourself.

Read more:

• What is Checksum on a credit card?

pseudocode

Here's some pseudocode to showcase how an implementation of the algorithm could look like.
First, we get the credit_card_number from user input, and the last_digit from the last number of credit_card_number. We also remove the last digit from the credit_card_number so it doesn't mess with any of our calculations for the time being.

credit_card_number = INPUT
last_digit = LAST NUMBER OF credit_card_number
REMOVE LAST NUMBER OF credit_card_number

Next, we loop over the credit_card_number and run a series of checks.

1. Check if i is a multiple of 2. This allows every other number in credit_card_number is multiplied by 2.
2. Check the lengths of the number and add the two digits if the number is longer than a single digit (e.g. >9).
3. We add the current number to our sum variable which is adding up all of our numbers as we loop over credit_card_number.

i = 0
sum = 0
LOOP UNLESS i = LENGTH OF credit_card_number
    IF i IS MULTIPLE OF 2
        credit_card_number[i] = credit_card_number[i] * 2
    IF LENGTH OF credit_card_number[i] IS MORE THAN 1
        x = credit_card_number[i][0]
        y = credit_card_number[i][1]
        credit_card_number[i] = x + y
    sum = sum + credit_card_number[i]
END

Finally, we add the last_digit (the 16th digit of the credit_card_number to the sum). If the sum is a multiple of 10 it is valid, otherwise it is invalid.

sum = sum + last_digit
IF sum IS MULTIPLE OF 10
    OUTPUT "Credit card number is valid!"
ELSE
    OUTPUT "Credit card numebr is invalid!"