Hi Richard, Just fyi, the aim of Exercism is not to teach programming. Its aim is for experienced programmers to fast-start in new languages. It just happens to that new programmers also use Exercism and there is some catering in the exercise text for this.
The grains exercise provides a nice vehicle for programmers to familiarize themselves with recursion in Pharo, and the hint for the exercise says "These kinds of problems (where an answer is dependent on a previous) one are often called recursion" which is why Roelof is approaching the problem this way. That said, any solution will do. Your advice on alternative considerations is insightful and always good to read. cheers -ben On Mon, 6 Jan 2020 at 00:25, Richard O'Keefe <rao...@gmail.com> wrote: > I did not ask why you were validating input. > I asked about why you *repeatedly* validated input. > > Think of it this way: > publicMethod: arg1 also: arg2 > ... check arg1 ... > ... check arg2 ... > ^self privateMethod: arg1 also: arg2 > > privateMethod: arg1 also: arg2 > ... trust that arg1 and arg2 are valide ... > ... recursive calls use #privateMethod:andAlso: ... > ... not #publicMethod:andAlso: and must ensure ... > ... that arguments are valid by construction ... > > In my solution to the "Grains" exercism, I have > atSquare: n -- checks its argument > total ^(1 bitShift: 64) - 1 > You are required to implement these two methods, true. > You are NOT required to implement #total by calling #atSquare:. > Not even once. Nor is #atSquare: required to be recursive. > > On Mon, 6 Jan 2020 at 02:05, Roelof Wobben <r.wob...@home.nl> wrote: > > > > Hello Ricard. > > > > You mean when I calcualate the total of a board. > > That is because on when I had to calculate the number of a particular > > field there were tests where the number was lower then zero or higher > > then 64 which makes no sense. > > > > But im open for a solution where on a particular field I could check for > > that and for the total I do not need that part. > > > > Roelof > > > > > > > > Op 5-1-2020 om 13:58 schreef Richard O'Keefe: > > > Time microsecondsToRun: [ > > > |n| > > > n := (2 raisedToInteger: 8 * 8) - 1. > > > Transcript > > > nextPutAll: 'The number of grains on an 8x8 chessboard is '; > > > print: n; cr; endEntry]. > > > <PRINT-IT> > > > On my laptop, this reports 194 microseconds. > > > > > > Why would you use recursion, anyway? > > > > > > Time microsecondsToRun: [ > > > |n| > > > n := (1 to: 8 * 8) inject: 0 into: [:acc :each | acc+acc+1]. > > > Transcript > > > nextPutAll: 'The number of grains on an 8x8 chessboard is '; > > > print: n; cr; endEntry]. > > > <PRINT-IT> > > > On the same laptop, this reports 118 microseconds. > > > > > > One of the lessons of 'functional' languages, promptly adopted by > Smalltalk, is > > > to encapsulate control structures into reusable methods, such as > #inject:into:, > > > more commonly known as `foldl` in functional languages. It's then > none of > > > my business whether such a method works by recursion, iteration, or > gangs > > > of otherwise seasonally unemployed Christmas elves. > > > > > > In my own Smalltalk library, > > > (GeometricSeries new: 64 from: 1 byFactor: 2) sum > > > only takes 15 microseconds. > > > > > > I do note that you are calling validateInput repeatedly. Why? > > > > > > > > > On Sun, 5 Jan 2020 at 07:41, Roelof Wobben via Pharo-users > > > <pharo-users@lists.pharo.org> wrote: > > >> Oke, > > >> > > >> So I can better not use recursion for this problem if I understand > you well, Richard. > > >> > > >> Roelof > > >> > > >> > > >> > > >> Op 4-1-2020 om 19:02 schreef Richard Sargent: > > >> > > >> On Sat, Jan 4, 2020 at 9:47 AM Roelof Wobben via Pharo-users < > pharo-users@lists.pharo.org> wrote: > > >>> Hello, > > >>> > > >>> For a exercism challenge I need to calculate the total grains on a > > >>> chessboard. > > >>> So I did : > > >>> > > >>> atSquare: anInteger > > >>> self validateInput: anInteger. > > >>> ^ anInteger = 1 > > >>> ifTrue: [ 1 ] > > >>> ifFalse: [ 2 * (self atSquare: anInteger - 1) ] > > >>> > > >>> > > >>> but when I run the tests , the vm seems to be not responsive for > some 4 > > >>> - 5 seconds. > > >>> > > >>> Is there a way I can use this code and take care that the vm stays > > >>> responsive. > > >> > > >> What do you want the VM to do in addition to calculating that sum > while it is calculating that sum? > > >> > > >> > > >> The best way to keep the VM responsive is to take a page from Gauss' > notebook and pay attention to the numbers[1]. Let's consider the first four > squares and extrapolate from there. > > >> > > >> In binary, the squares hold the following grains: 2r1, 2r10, r2100, > and 2r1000. When we add them up, we get 2r1111. If we use Gauss' tricks, we > can notice that 2r1111 is equal to 2r10000 - 1. So, the sum of the grains > on the first 4 squares is 2^5 - 1. You can easily generalize that pattern, > of course. Then your program can calculate the answer quickly. > > >> > > >> > > >> [1] The story goes that Gauss' teacher was frustrated with his > student's abilities and set him a challenge to occupy him for some time: > sum the numbers from 1 through 100. Gauss immediately answered 5,050, much > to his teacher's chagrin. > > >> > > >>> Regards, > > >>> > > >>> Roelof > > >>> > > >>> > > > >
