Given a number determine whether or not it is valid per the Luhn formula.
The Luhn algorithm 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
4539 1488 0343 6467
The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling
4_3_ 1_8_ 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:
8569 2478 0383 3437
Then sum all of the digits:
8+5+6+9+2+4+7+8+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
8273 1232 7352 0569
Double the second digits, starting from the right
7253 2262 5312 0539
Sum the digits
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.
Follow the setup instructions for Crystal here:
More help installing can be found here:
Making the Test Suite Pass
Execute the tests with:
$ crystal spec
In each test suite all but the first test have been skipped.
Once you get a test passing, you can unskip the next one by changing
The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm
Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
- about 1 month ago
- March 3, 2021
Tue, 20 Apr 2021 04:17:21 GMT