Re: [Pharo-users] can I make this so the vm would not be not responsibe when running the tests

[Roelof Wobben via Pharo-users]

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There you are right but the hint we get was thinking about recursion

but your lessons showed me other ways to do it which I think are more the way smalltalk works,

Thanks for that.

Roelof




Op 5-1-2020 om 14:24 schreef Richard O'Keefe:

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

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