September 2020 - Pharo-users

Re: [Pharo-users] Can it do this way ?

by Richard O'Keefe

There is simply no point in "taking the first nine numbers out". And there shouldn't BE a test for the string being empty, anywhere. '' '-' '---' and so on should all be handled the same way. Oh well, what stops you doing digits := aString select: [:each | each ~= $-]. digits size = 10 ifFalse: [^false]. lastDigit := digits la ost. digits := digits copyFrom: 1 to: 9. ( (lastDigit = $X or: [lastDigit isDigit]) and: [ digits allSatisfy: #isDigit] ) ifFalse: [^false]. Now my code does not do this, but it is just 16 lines of code with nothing that it would make sense to extract. On Wed, 2 Sep 2020 at 22:24, Roelof Wobben <r.wobben(a)home.nl> wrote: > Yep, I know that isValidIsbn is the method that must output if a isbn is > valid or not. > > What I want to do is take the first 9 characters out so I can convert > them to a array of numbers where I can do the calculation on. > And take out the last char so I can seperate test if that is a valid > char. So between the 0 and 9 or a X > > I do not think I would have do all the checks in that only method > because it would be a very big method then. > > but if I understand you well also the test if a string is empty should > be called from the isValidIsbn method or even checked there. > > Roelof > > >

Sept. 2, 2020

Re: [Pharo-users] Scaled decimals comparisons

by Richard O'Keefe

The behaviour of ScaledDecimal in Squeak and Pharo is - different from what the ANSI Smalltalk standard says - different from what other Smalltalks do (not that they agree) - completely different from decimal arithmetic in COBOL, PL/I, or SQL - seriously confusing. What you EXPECT is an exact rational number that is equal to an integer divided by a power of 10, so that the printed representation faithfully represents the value with no error. What you GET in Pharo is a rational number that PRINTS with a given number of decimal places but that is all. Thus we expect that x := 10/3 asScaledDecimal: 2. x asString ==> '3.33s2' y := x+x+x. y asString ==> '9.99s2' But no, y asString => '10.00s2'. Here's what the comment in the VW class FixedPoint says: "There are two possible ways to express FixedPoint numbers. One is as a scaled Integer, but the problem here is that you can lose precision during intermediate calculations. For example, a property that seems useful is that the calculation (1.000 / 7 * 7) should give you back the number 1.000. For this reason, we have adopted an alternative representation, which is a slight variation on Fraction. The number is expressed as the ratio of a numerator to a denominator, with the addition of a precision that is used for printing and for rounding. The number is not usually rounded off to its scale, but if an intermediate result must be rounded to its scale before being used, the messages #roundedToScale and #truncatedToScale may be used." The last sentence in the first paragraph is one I cannot agree with. If I want a calculation where (1/7 * 7) gives me back 1, then I use exact Fractions. If I am using ScaledDecimal, it is because I *want* fixed point decimal numbers, with the properties appropriate to fixed point decimal numbers. Wanting something that I could use to talk to databases like MariaDB I found that I had to write my own FixedPoint class, only to find that VW called ScaledDecimal FixedPoint. *Sigh*. Unless and until the current ScaledDecimal is ripped out and buried in an unmarked grave at a crossroad with a stake through its heart, you may wish to add VW-compatibility methods roundedToScale |t| t := 10 raisedToInteger: scale. ^(numerator * t / denominator) rounded / t asScaledDecimal: scale truncatedToScale |t| t := 10 raisedToInteger: scale. ^(numerator * t / denominator) truncated / t asScaledDecimal: scale and then use aScaledDecimal roundedToScale = anotherOne roundedToScale Note that the ANSI standard, which was agreed to by a whole bunch of Smalltalk experts, says "A <scaledDecimal> converted to a <Fraction> will be a fraction having the same numeric value but having an integer numerator and a denominator which is ten raised to the power of the <scaledDecimal>âs scale factor." Try "(10.00s2 / 3) asFraction" in your Smalltalk, and if the result has a denominator of 3, SOMEONE stuffed up. On Tue, 1 Sep 2020 at 16:08, Esteban Maringolo <emaringolo(a)gmail.com> wrote: > Hi, > > I was doing some basic calculations based on a formula, and I wanted > to convert the result to a scaled decimal in order to avoid having > these "loose" decimals in 10th position or something similar. > > So I did the following: > 82 - (2 * 35.9) - (0 / 2) * (113/121) asScaledDecimal: 1 -> 9.5s1 > > But When I do this in a test: > (82 - (2 * 35.9) - (0 / 2) * (113/121) asScaledDecimal: 1) = 9.5s1 > > It fails because the comparison returns false. > > I guess this is the proper behavior, but I'd expected that the > conversion from a Float to a scaled decimal would have eliminated the > extra precision from the formula. > > I can do a roundTo: 0.1 before converting, but I'd like to know what > would be the proper way of dealing with this. > > Thanks! > > Esteban A. Maringolo > >

Sept. 2, 2020

Re: [Pharo-users] Can it do this way ?

by Roelof Wobben

Yep, I know that isValidIsbn is the method that must output if a isbn is valid or not. What I want to do is take the first 9 characters out so I can convert them to a array of numbers where I can do the calculation on. And take out the last char so I can seperate test if that is a valid char. So between the 0 and 9 or a X I do not think I would have do all the checks in that only method because it would be a very big method then. but if I understand you wellÂ also the test if a string is empty should be called from the isValidIsbn method or even checked there. Roelof

Sept. 2, 2020

Re: [Pharo-users] Can it do this way ?

by Richard O'Keefe

Briefly, you cannot and (for a different reason) you should not. The IsbnVerifierTest class defines the API; the method must be isValidIsbn: aString and it MUST be on the instance side. The reason why you shouldn't is that an empty string should NOT be a special case. This is a problem where the design style described by Dijkstra in "A Discipline of Programming" and further fleshed out by Reynolds and Gries in related books pays off. You want to inspect all the characters of the string once each in a single left-to-right loop. On Wed, 2 Sep 2020 at 18:19, Roelof Wobben via Pharo-users < pharo-users(a)lists.pharo.org> wrote: > Hello, > > I have now a challenge where I have to validate a ISBN number. > > Can I do something like this on the class side : > > (string isEmpty) > ifTrrue: [ ^ false] > ifFalse: [ digits:= something. > controlDigit := something. > self validateISBNNumber] > > where on the validateISBNNumber I use the instance variables digits and > controlDigit to validate the ISBN number on the instance side. > > Roelof > > > > > > > >

Sept. 2, 2020

Re: [Pharo-users] Can it do this way ?

by Richard O'Keefe

Everything Joachim Tuchel wrote about this is fine EXCEPT that this is an Exercism task, where Roelof has to implement an API that he is given and is not at liberty to do it right. Thanks to the way that IsbnVerifierTest is structured, - the IsbnVerifier class has NO use for ANY variables of ANY kind: no instance variables, no class variables, and no class instance variables. - the IsbnVerifier class has only ONE method that is called, and that is on the INSTANCE side. That is, it's a simple pure function disguised as an object. A good design might have an ISBN class, and would put #isValidISBN10: and #isValidISBN13: on the class side. But to solve the Exercism exercises, you HAVE to implement the API that the test class is expecting, not something better. If it passes the tests, it is by definition right. If it doesn't, it is by definition wrong, even if it is a far better design. So - one class, IsbnVerifier - with no variables of any kind - with one and only one method, #isValidIsbn, on the instance side. It all gets so much clearer when you realise that the Exercism tasks are NOT designed to teach Object-Oriented Design. The problems have to serve many programming languages, some of which, like bash and x86 assembly code, are not object-oriented. On Wed, 2 Sep 2020 at 18:42, jtuchel(a)objektfabrik.de < jtuchel(a)objektfabrik.de> wrote: > Roelof, > > I don't think so. > > A Class cannot access instance variables. Why? because a Class doesn't > know which of its instances to ask for it. > Ask yourself: who is "self" in a Class method? Is it the Class or is it an > individual instance of the Class? > > (Hint: in a Class, #self is the Class, while in an instance, #self is the > instance) . > > > "Who" do you want to validate the ISBN number? An individual instance of > your Class (like Book)? Or is validating an ISBNNumber a Task that is > independent of an individual item? If it is independent, then why would you > want to use semething specific in that method? > > You need to understand the differences between Class and Instance > variables. I suggest reading Chapter 4 of "The Art and Science of > Smalltalk > <http://stephane.ducasse.free.fr/FreeBooks/Art/artAdded174186187Final.pdf>" > or better: read the whole book. It's easy to read and sparks quite a few > A-HA's per hour. > > > HTH > > Joachim > > > Am 02.09.20 um 08:18 schrieb Roelof Wobben: > > Hello, > > I have now a challenge where I have to validate a ISBN number. > > Can I do something like this on the class side : > > (string isEmpty) > ifTrrue: [ ^ false] > ifFalse: [ digits:= something. > controlDigit := something. > self validateISBNNumber] > > where on the validateISBNNumber I use the instance variables digits and > controlDigit to validate the ISBN number on the instance side. > > Roelof > > > > > > > > > > -- > ----------------------------------------------------------------------- > Objektfabrik Joachim Tuchel mailto:jtuchel@objektfabrik.de <jtuchel(a)objektfabrik.de> > Fliederweg 1 http://www.objektfabrik.de > D-71640 Ludwigsburg http://joachimtuchel.wordpress.com > Telefon: +49 7141 56 10 86 0 Fax: +49 7141 56 10 86 1 > > > >

Sept. 2, 2020

Re: [Pharo-users] Can it do this way ?

by jtuchel＠objektfabrik.de

Roelof, I don't think so. A Class cannot access instance variables. Why? because a Class doesn't know which of its instances to ask for it. Ask yourself: who is "self" in a Class method? Is it the Class or is it an individual instance of the Class? (Hint: in a Class, #self is the Class, while in an instance, #self is the instance) . "Who" do you want to validate the ISBN number? An individual instance of your Class (like Book)? Or is validating an ISBNNumber a Task that is independent of an individual item? If it is independent, then why would you want to use semething specific in that method? You need to understand the differences between Class and Instance variables. I suggest reading Chapter 4 of "The Art and Science of Smalltalk <http://stephane.ducasse.free.fr/FreeBooks/Art/artAdded174186187Final.pdf>" or better: read the whole book. It's easy to read and sparks quite a few A-HA's per hour. HTH Joachim Am 02.09.20 um 08:18 schrieb Roelof Wobben: > Hello, > > I have now a challenge where I have to validate a ISBN number. > > Can I do something like this on the class side : > > (string isEmpty) > Â Â ifTrrue: [ ^ false] > Â ifFalse:Â [ digits:= something. > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â controlDigit := something. > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â self validateISBNNumber] > > where on the validateISBNNumber I use the instance variables digits > and controlDigit to validate the ISBN number on the instance side. > > Roelof > > > > > > > > -- ----------------------------------------------------------------------- Objektfabrik Joachim Tuchel mailto:jtuchel@objektfabrik.de Fliederweg 1 http://www.objektfabrik.de D-71640 Ludwigsburg http://joachimtuchel.wordpress.com Telefon: +49 7141 56 10 86 0 Fax: +49 7141 56 10 86 1

Sept. 2, 2020

Can it do this way ?

by Roelof Wobben

Hello, I have now a challenge where I have to validate a ISBN number. Can I do something like this on the class side : (string isEmpty) Â Â ifTrrue: [ ^ false] Â ifFalse:Â [ digits:= something. Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â controlDigit := something. Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â self validateISBNNumber] where on the validateISBNNumber I use the instance variables digits and controlDigit to validate the ISBN number on the instance side. Roelof

Sept. 2, 2020

Re: [Pharo-users] mentoring contined I hope

by Richard O'Keefe

Richard Sargent is giving good advice that you cannot use in Exercism. Your code MUST pass the exercism tests and that means it MUST NOT limit the hour or minute ranges. On Tue, 1 Sep 2020 at 08:58, Roelof Wobben via Pharo-users < pharo-users(a)lists.pharo.org> wrote: > > I would argue against that approach. Make it a requirement that a given > API must be used with correct values. > e.g. #hours:minutes: requires hours between 00 and 23 and minutes between > 00 and 59. > > If you want to convert equivalent time values, use a specific API. For > example, many time implementations publicly define a seconds-based API, > such as Time class>>#fromSeconds:. You can do the same with your > implementation with a class-side #fromMinutes: method. (The corresponding > instance methods would be #asSeconds and #asMinutes or something similar.) > > > > Moment, I do not know if I understand you right. > > so I would use the given method and use on the instance another function > that takes care of the converting to or a valid time object or for example > convert it to minutes and on display take care that a valid time is shown. > > Do I understand you right ? > > I send this personal because I cannot send to the mailing list according > to some spam list >

Sept. 2, 2020

Re: [Pharo-users] mentoring continued I hope

by Richard O'Keefe

Roelof explicitly said that this is for an Exercism exercise. That exercise defines the API (implicitly) in the test cases. Roelof has no choice about what the API is, his job is just to make the tests pass. Your ideas about how to design an API are good ones, it's just that Exercism says what to do. On Tue, 1 Sep 2020 at 05:01, Richard Sargent < richard.sargent(a)gemtalksystems.com> wrote: > On Sun, Aug 30, 2020 at 10:04 PM Roelof Wobben via Pharo-users < > pharo-users(a)lists.pharo.org> wrote: > >> Hello, >> >> I have this challenge from exercism : >> https://github.com/exercism/pharo-smalltalk/tree/master/exercises/clock >> >> and this function is given : >> >> hour: anInteger minute: anInteger2 >> self shouldBeImplemented >> >> >> Schould I make it on this class method work that the minutes will not >> exceed the 59 minutes >> So when for example when 70 min are given so hour:0 minute:70 it will >> be converted to hour: 1 minutes: 10 >> > > I would argue against that approach. Make it a requirement that a given > API must be used with correct values. > e.g. #hours:minutes: requires hours between 00 and 23 and minutes between > 00 and 59. > > If you want to convert equivalent time values, use a specific API. For > example, many time implementations publicly define a seconds-based API, > such as Time class>>#fromSeconds:. You can do the same with your > implementation with a class-side #fromMinutes: method. (The corresponding > instance methods would be #asSeconds and #asMinutes or something similar.) > > > or can I better do this on the instance side. >> >> Regards, >> >> Roelof >> >> >>

Sept. 2, 2020

Re: [Pharo-users] mentoring continued I hope

by Richard O'Keefe

I have always found it annoying that the exercism specifications do not spell things out, so you HAVE to read the tests to find out what you are supposed to do, which pretty much spoils the point of them BEING tests. In this case, we note that there is a test case where the number of hours is 100, and it is to be accepted as meaning '04'. There is also a test case where the number of minutes is 160 and it is to be taken as 40 minutes plus 2 hours. So the only way you can pass the tests is something like class methods for: 'instance creation' hours: h minutes: m ^self new setMinutesSinceMidnight: (h * 60 + m bitOr: 0) \\ 1440 methods for: 'private' setMinutesSinceMidnight: m minutesSinceMidnight := m. methods for: 'accessing' hour ^minutesSinceMidnight // 60 minute ^minutesSinceMidnight \\ 60 The only *checking* that is involved is that the hour and minute counts must be integers, which I used " bitOr: 0" to do. What's the best way to do thisE? As for where to do checking in general, put it as close to the encapsulation boundary as you reasonably can. If there is a PUBLIC method for changing the time on a Clock, you need to put the checking in that method. If (as here) a clock value is supposed to be immutable, put it in the instance creation method. On Mon, 31 Aug 2020 at 18:18, Roelof Wobben via Pharo-users < pharo-users(a)lists.pharo.org> wrote: > Hello, > > I have this challenge from exercism : > https://github.com/exercism/pharo-smalltalk/tree/master/exercises/clock > > and this function is given : > > hour: anInteger minute: anInteger2 > self shouldBeImplemented > > > Schould I make it on this class method work that the minutes will not > exceed the 59 minutes > So when for example when 70 min are given so hour:0 minute:70 it will > be converted to hour: 1 minutes: 10 > or can I better do this on the instance side. > > Regards, > > Roelof > > >

Sept. 2, 2020