Luhn

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.

Run the tests with:

bats luhn_test.sh

After the first test(s) pass, continue by commenting out or removing the [[ $BATS_RUN_SKIPPED == true ]] || skip annotations prepending other tests.

To run all tests, including the ones with skip annotations, run:

BATS_RUN_SKIPPED=true bats luhn_test.sh

Source

The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

External utilities

Bash is a language to write "scripts" -- programs that can call external tools, such as sed, awk, date and even programs written in other programming languages, like Python. This track does not restrict the usage of these utilities, and as long as your solution is portable between systems and does not require installation of third party applications, feel free to use them to solve the exercise.

For an extra challenge, if you would like to have a better understanding of the language, try to re-implement the solution in pure Bash, without using any external tools. Note that there are some types of problems that bash cannot solve, such as performing floating point arithmetic and manipulating dates: for those, you must call out to an external tool.

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

2.9 KiB Raw Blame History