On Thu, 30 Apr 2020 at 14:33, Roelof Wobben via Pharo-users < pharo-users@lists.pharo.org> wrote:
> Hello, > > I hope I can discuss my approch to this problem : > > Given a number determine whether or not it is valid per the Luhn formula. > > The Luhn algorithm <https://en.wikipedia.org/wiki/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! > > > my idea was to do these steps > > 1) reverse the input. > 2) use this to double every second digit and calculate the sum of all > the numbers : > > checkNumber := (collection reverse selectwith index: [:item :index | > (index % 2 == 0) . IfTrue: [item *2]] ) sumNumbers > I just want to check that before you are ask such questions, did you actually run this code? Because it would be fairly easy to see how close you are by evaluating it without #sumNumbers. i.e Please report what you see when evaluating this.... ``` collection := '8569 2478 0383 3437'. (collection reverse selectWithIndex: [:item :index | (index % 2 == 0) ifTrue: [item *2]] ) inspect ``` 3) check if its a valid number by doing this : > > > ^ (checkNumber % 10 == 0) > That would work, but there is a more intention revealing method... #isDivisibleBy: cheers -ben
