Re: [Pharo-users] mentor question 4

[Roelof Wobben via Pharo-users]

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Op 30-4-2020 om 15:10 schreef Stéphane Ducasse:

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 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: 

Hmm, I looks like #do does not change the collection and I need to

collection := '4539148803436467'.
1 to: (collection size + 1) by: 2 do: [:item | item asInteger * 2 ].
^ collection

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