Unfortunately, I have lost the original problem specification, so I have 
answered a different question. Could you please help me by repeating the 
original problem specification in full?

  I agree that my solution is not 'obviously correct' since it doesn't 
answer the right question. In one version of the solution, I had a 
prologue that catered for spaces, etc. Not having a textual form of the 
algorithm that provided for the extensions, I chose to omit them.



On tirsdag.05.05.2020 15:20, Richard O'Keefe wrote:
> The irony is that the code I was responding to ISN'T obviously correct.
> Indeed, I found it rather puzzling.
> The problem specification says that the input string may contain digits
> AND SPACES.  The original message includes this:
>
> 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.
>
> Now it isn't clear what "disallowed" means.  I took it to mean "may occur and
> should simply mean the input is rejected as invalid."  Perhaps "may not occur"
> was the intention.  So we shall not quibble about such characters.
>
> But I can't for the life of me figure out how Trygve's code checks for spaces.
> One reason this is an issue is that the behaviour of #digitValue is not
> consistent between systems.
>    Character space digitValue
>      does not exist in the ANSI standard
>      answers -1 in many Smalltalks (which is a pain)
>      answers a positive integer that can't be mistake for a digit in my Smalltalk
>      raises an exception in some Smalltalks.
>
> This is a comment I now have in my Smalltalk library for #digitValue
>        "This is in the Blue Book, but unspecified on non-digits.
>         Squeak, Pharo, Dolphin, VW, VAST, and Apple Smalltalk-80
>         answer -1 for characters that are not digits (or ASCII letters),
>         which is unfortunate but consistent with Inside Smalltalk
>         which specifies this result for non-digits.
>         ST/X and GST raise an exception which is worse.
>         Digitalk ST/V documentation doesn't specify the result.
>         This selector is *much* easier to use safely if it
>         returns a 'large' (>= 36) value for non-digits."
>
> Let's compare three versions, the two I compared last time,
> and the "version A" code I discussed before, which to my mind
> is fairly readable.
>
> "Don't add slowness": 1 (normalised time)
> "Trygve's code":  6.5
> "High level code": 30.6 (or 4.7 times slower than Trygve's)
>
> Here's the "High level code".
>        ^(aString allSatisfy: [:each | each isSpace or: [each isDigit]]) and: [
>          |digitsReverse|
>          digitsReverse := (aString select: [:each | each isDigit]) reverse.
>          digitsReverse size > 1 and: [
>            |evens odds evenSum oddSum|
>            odds  := digitsReverse withIndexSelect: [:y :i | i odd].
>            evens := digitsReverse withIndexSelect: [:x :i | i even].
>            oddSum  := odds  detectSum: [:y | y digitValue].
>            evenSum := evens detectSum: [:x |
>                         #(0 2 4 6 8 1 3 5 7 9) at: x digitValue + 1].
>            (oddSum + evenSum) \\ 10 = 0]]
>
> This is the kind of code I was recommending that Roelof write.
>
> As a rough guide, by counting traversals (including ones inside existing
> methods), I'd expect the "high level" code to be at least 10 times slower
> than the "no added slowness" code.
>
> We are in vehement agreement that there is a time to write high level
> really obvious easily testable and debuggable code, and that's most
> of the time, especially with programming exercises.
>
> I hope that we are also in agreement that factors of 30 (or even 6)
> *can* be a serious problem.  I mean, if I wanted something that slow,
> I'd use Ruby.
>
> I hope we are also agreed that (with the exception of investigations
> like this one) the time to hack on something to make it faster is AFTER
> you have profiled it and determined that you have a problem.
>
> But I respectfully suggest that there is a difference taking slowness OUT
> and simply not going out of your way to add slowness in the first place.
>
> I'd also like to remark that my preference for methods that traverse a
> sequence exactly once has more to do with Smalltalk protocols than
> with efficiency.  If the only method I perform on an object is #do:
> the method will work just as well for readable streams as for
> collections.  If the only method I perform on an object is #reverseDo:
> the method will work just as well for Read[Write]Streams as for
> SequenceReadableCollections, at least in my library.   It's just like
> trying to write #mean so that it works for Durations as well as Numbers.
>
> Oh heck, I suppose I should point out that much of the overheads in
> this case could be eliminated by a Self-style compiler doing dynamic
> inlining + loop fusion.    There's no reason *in principle*, given enough
> people, money, and time, that the differences couldn't be greatly
> reduced in Pharo.
>
> On Tue, 5 May 2020 at 21:50, Trygve Reenskaug <trygver at ifi.uio.no> wrote:
>> Richard,
>>
>> Thank you for looking at the code. It is comforting to learn that the code has been executed for a large number of examples without breaking. The code is not primarily written for execution but for being read and checked by the human end user. It would be nice if we could also check that it gave the right answers, but I don't know how to do that.
>>
>> The first question is: Can a human domain expert read the code and sign their name for its correctness?
>>
>>
>> When this is achieved, a programming expert will transcribe the first code to a professional quality program. This time, the second code should be reviewed by an independent programmer who signs their name for its correct transcription from the first version.
>>
>> --Trygve
>>
>> PS: In his 1991 Turing Award Lecture, Tony Hoare said: "There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies and the other is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult."
>>
>> --Trygve
>>
>> On tirsdag.05.05.2020 04:41, Richard O'Keefe wrote:
>>
>> As a coding experiment, I adapted Trygve  Reenskoug's code to my
>> Smalltalk compiler, put in my code slightly tweaked, and benchmarked
>> them on randomly generated data.
>>
>> Result: a factor of 6.3.
>>
>> In Squeak it was a factor of ten.
>>
>> I had not, in all honesty, expected it to to be so high.
>>
>> On Tue, 5 May 2020 at 02:00, Trygve Reenskaug <trygver at ifi.uio.no> wrote:
>>
>>   A coding experiment.
>> Consider a Scrum development environment. Every programming team has an end user as a member.
>> The team's task is to code a credit card validity check.
>> A first goal is that the user representative shall read the code and agree that it is a correct rendering of their code checker:
>>
>>      luhnTest: trialNumber
>>          | s1 odd s2 even charValue reverse |
>> -----------------------------------------------
>> " Luhn test according to Rosetta"
>> "Reverse the order of the digits in the number."
>>      reverse := trialNumber reversed.
>> "Take the first, third, ... and every other odd digit in the reversed digits and sum them to form the partial sum s1"
>>      s1 := 0.
>>      odd := true.
>>      reverse do:
>>          [:char |
>>              odd
>>                  ifTrue: [
>>                      s1 := s1 + char digitValue.
>>                  ].
>>                  odd := odd not
>>          ].
>> "Taking the second, fourth ... and every other even digit in the reversed digits:
>> Multiply each digit by two and sum the digits if the answer is greater than nine to form partial sums for the even digits"
>>      "The subtracting 9 gives the same answer. "
>> "Sum the partial sums of the even digits to form s2"
>>      s2 := 0.
>>      even := false.
>>      reverse do:
>>          [:char |
>>              even
>>                  ifTrue: [
>>                      charValue := char digitValue * 2.
>>                      charValue > 9 ifTrue: [charValue := charValue - 9].
>>                      s2 := s2 + charValue
>>                  ].
>>                  even := even not
>>          ].
>> "If s1 + s2 ends in zero then the original number is in the form of a valid credit card number as verified by the Luhn test."
>>      ^(s1 + s2) asString last = $0
>> ---------------------------------
>> Once this step is completed, the next step will be to make the code right without altering the algorithm (refactoring). The result should be readable and follow the team's conventions.
>>
>>
>> P.S. code attached.
>>
>>
>> --
>>
>> The essence of object orientation is that objects collaborate  to achieve a goal.
>> Trygve Reenskaug      mailto: trygver at ifi.uio.no
>> Morgedalsvn. 5A       http://folk.uio.no/trygver/
>> N-0378 Oslo             http://fullOO.info
>> Norway                     Tel: (+47) 468 58 625

-- 

/The essence of object orientation is that objects collaborateto achieve 
a goal. /
Trygve Reenskaug mailto: trygver at ifi.uio.no <mailto:%20trygver at ifi.uio.no>
Morgedalsvn. 5A http://folk.uio.no/trygver/
N-0378 Oslo http://fullOO.info
Norway                     Tel: (+47) 468 58 625

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