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Op 30-4-2020 om 16:06 schreef Richard O'Keefe:
This sounds very much like the Luhn test task at RosettaCode.
https://rosettacode.org/wiki/Luhn_test_of_credit_card_numbers
except that there it is described as working on the digits of an
integer.
(1) There are two approaches to traversing a sequence in reverse.
(A) Reverse the sequence, then traverse the copy forward.
aString reverse do: [:each | ...]
(B) Just traverse the sequence in reverse
aString reverseDo: [:each | ...]
My taste is for the second.
(2) There are two approaches to deleting spaces.
(A) Make a copy of the string without spaces.
x := aString reject: [:each | each = Character space].
x do: ...
(B) Ignore spaces as you go:
(i) aString do: [:each | each = Character space ifFalse: [...]]
(ii) aString select: [:each | each ~= Character space] thenDo:
[:each | ...]
Combining (1A) and (2A) you get very obvious code:
(aString reject: [:each | each = Character space]) reverse do:
[:digit } ...]
Combining (1B) and (2Bi) you get more efficient code:
aString reverseDo: [:digit |
digit = Character space ifFalse: [ ...]]
By the way, let's start by checking that the character in the string *are*
digits or spaces:
(aString allSatisfy: [:each | each isDigit or: [each = Character s[ace]])
ifFalse: [^false],
(3) There are two approaches to doubling the even digits.
(A) Make a new string that starts as a copy and change every second
digit from the right.
(B) Simply *act* as if this has been done; keep track of whether the
current digit position is even or odd and multiply by 1 or 2 as
appropriate.
nextIsOdd := true.
aString reverseDo: [:digit |
digit = Character space ifFalse: [
nextIsOdd
ifTrue: [oddSum := ...]
ifFalse: [evenSum := ...].
nextIsOdd := nextIsOdd not]].
I *like* code that traverses a data structure exactly once and
allocates no intermediate garbage, so I'd be making (B) choices.
For me , I use this to practice solving problems and doing the "right"
steps.
So I love it , that so many people share there way of solving it.
I can learn a lot from it
Expecially when they explain there thinking process so detailed.
I like this code also a lot.
Am I correct for testing if it is a valid string by doing this ^
(oddSum + evenSum) dividedBy: 10
Roelof
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