It looks like a cool problem from where did you take it? > 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. > >
I like people that are always thinking in string as if a collection of number would not make it :) > 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 > you can also 1 to: xx by: 2 do: > 3) check if its a valid number by doing this : > > > ^ (checkNumber % 10 == 0) > > > > is this a good game plan or has it flaws or can I do it better ? > > Regards, > > > Roelof > > > > > -------------------------------------------- Stéphane Ducasse http://stephane.ducasse.free.fr / http://www.pharo.org 03 59 35 87 52 Assistant: Julie Jonas FAX 03 59 57 78 50 TEL 03 59 35 86 16 S. Ducasse - Inria 40, avenue Halley, Parc Scientifique de la Haute Borne, Bât.A, Park Plaza Villeneuve d'Ascq 59650 France
