From: Chris Hulan <hulachr@...>
Date: 2006-07-21T04:34:35+09:00
Subject: Re: recursive method and references

Sci000 Lem. wrote:
...
> def tyra(arr, pos2, tmp2, resul, res=0, i=0)
>   (puts "#####"; resul.push(res); return) if i>=13
>   (0..3).each do |y|
>     pos=pos2.dup; tmp=tmp2.dup
...
>     next if pos[tmp[0]][tmp[1]]==1
>     pos[tmp[0]][tmp[1]]=1
>     tyra(arr, pos.dup, tmp.dup, resul, res+arr[tmp[0]][tmp[1]], i+1)
>     print "*", pos2, "\n", "_", pos, "\n"
>   end
> end
> 
> arr, pos, tmp, resul = [], [], [], []
> arr.push([32,80,19,98,01,90,14,85])
> arr.push([66,22,73,52,72,57,83,31])
> arr.push([30,84,41,73,16,74,45,92])
> arr.push([77,06,70,24,00,28,67,11])
> arr.push([32,99,44,81,27,75,42,98])
> arr.push([68,21,72,56,59,42,75,17])
> arr.push([34,87,19,92,05,99,27,88])
> 
> arr[0].length.times{tmp.push(0)}
> arr.length.times{pos.push(tmp.dup)}
> tmp2=[3,4]
> 
> print "#", pos, "\n"
> tyra(arr, pos.dup, tmp2.dup, resul)
> print "#", pos, "\n"
> 
> 
> The output is:
> #00000000000000000000000000000000000000000000000000000000
> *00000000000000000000000000001000000000000000000000000000
> _00000000000000000000000000001000000000000000000000000000
> #00000000000000000000000000001000000000000000000000000000
> 
> 
> The lines marked with a "#" does not equal each other as with the test 
> function…
> What goes wrong, where’s the bug?

Your pos array contains references to other arrays,
and pos.dup is a shallow copy, thus pos2 contains refs
to the same sub-arrays as pos.  Any changes made via
access to pos2 affects the same sub-arrays as viewed
from pos.

Clear as mud? 9^)

Cheers
Chris

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