From: Roland Mai <roland.mai@...>
Date: 2007-05-27T04:26:38+09:00
Subject: Re: Can you make sense of this Array issue

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O I think I did figure this out. It seems the * operator just duplicates the
pointers in some optimized fashion. If the nxn array is constructed by
setting each m[i][j] element everything works fine.. I should have thought
of that before.

irb(main):001:0> n = 5
=> 5
irb(main):002:0> m = []
=> []
irb(main):003:0> for i in 0..(n-1)
irb(main):004:1>   m[i] ||= []
irb(main):005:1>   for j in 0..(n-1)
irb(main):006:2>     m[i][j] = nil
irb(main):007:2>     end
irb(main):008:1>   end
=> 0..4
irb(main):009:0> m
=> [[nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil,
nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]
irb(main):010:0> m[0][0] = 0
=> 0
irb(main):011:0> m
=> [[0, nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil,
nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]


On 5/26/07, Roland Mai <roland.mai@gmail.com> wrote:
>
> I want to have an array, say 5x5 with all nils unless set to something
> else.
> However, when I set m[0][0] to 0 .. all 0 indices are set to zero.. Hmm..
> never asked for that to be the case.
> There's some funky stuff going on here.
>
> irb(main):001:0> n = 5
> => 5
> irb(main):002:0> m = [[nil]*n]*n
> => [[nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil,
> nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]
> irb(main):003:0> m[0][0] = 0
> => 0
> irb(main):004:0> m
> => [[0, nil, nil, nil, nil], [0, nil, nil, nil, nil], [0, nil, nil, nil,
> nil], [0, nil, nil, nil, nil], [0, nil, nil, nil, nil]]
>
>
> Any ideas?
>

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