## Credit Card Checker

To find out if a credit card number is valid or not, the Luhn algorithm is used.

Generally speaking, an algorithm is a series of steps that solve a problem — the Luhn algorithm is a series of mathematical calculations used to validate certain identification numbers, e.g. credit card numbers.

The calculations in the Luhn algorithm can be broken down as the following steps:

- Starting from the farthest digit to the right, AKA the check digit, iterate to the left.
- As you iterate to the left, every other digit is doubled (the check digit is not doubled). If the number is greater than 9 after doubling, subtract 9 from its value.
- Sum up all the digits in the credit card number.
- If the sum modulo 10 is 0 (if the sum divided by 10 has a remainder of 0) then the number is valid, otherwise, it’s invalid.