diff --git a/exercises/luhn/description.md b/exercises/luhn/description.md index c84ac520ef..3b48003cd9 100644 --- a/exercises/luhn/description.md +++ b/exercises/luhn/description.md @@ -7,62 +7,40 @@ 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. -As an example of a valid string, here is a fictitious Canadian Social Insurance -Number. +## Example 1: valid credit card number ``` -046 454 286 +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_ 4_4 _8_ +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: ``` -086 858 276 +8569 2478 0383 3437 ``` -Then sum all of the digits +Then sum all of the digits: ``` -0+8+6+8+5+8+2+7+6 = 50 +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! -An example of an invalid Canadian SIN where we've changed the final digit - -``` -046 454 287 -``` - -Double the second digits, starting from the right - -``` -086 858 277 -``` - -Sum the digits - -``` -0+8+6+8+5+8+2+7+7 = 51 -``` - -51 is not evenly divisible by 10, so this number is not valid. - ----- - -An example of an invalid credit card account +## Example 2: invalid credit card number ``` 8273 1232 7352 0569 @@ -77,7 +55,7 @@ Double the second digits, starting from the right Sum the digits ``` -7+2+5+3+2+2+5+2+5+3+1+2+0+5+3+9 = 57 +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.