# Luhn Welcome to Luhn on Exercism's Bash Track. If you need help running the tests or submitting your code, check out `HELP.md`. ## Instructions Given a number determine whether or not it is valid per the Luhn formula. The [Luhn algorithm][luhn] is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers. The task is to check if a given string is valid. ## Validating a Number Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed. ### Example 1: valid credit card number ```text 4539 3195 0343 6467 ``` The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling ```text 4_3_ 3_9_ 0_4_ 6_6_ ``` If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling: ```text 8569 6195 0383 3437 ``` Then sum all of the digits: ```text 8+5+6+9+6+1+9+5+0+3+8+3+3+4+3+7 = 80 ``` If the sum is evenly divisible by 10, then the number is valid. This number is valid! ### Example 2: invalid credit card number ```text 8273 1232 7352 0569 ``` Double the second digits, starting from the right ```text 7253 2262 5312 0539 ``` Sum the digits ```text 7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57 ``` 57 is not evenly divisible by 10, so this number is not valid. [luhn]: https://en.wikipedia.org/wiki/Luhn_algorithm ## Source ### Created by - @sjwarner-bp ### Contributed to by - @bkhl - @budmc29 - @glennj - @guygastineau - @IsaacG - @kotp - @ZapAnton ### Based on The Luhn Algorithm on Wikipedia - https://en.wikipedia.org/wiki/Luhn_algorithm