From: Robert Klemme <bob.news@...>
Date: 2005-09-08T04:56:29+09:00
Subject: Re: weird result when updating an array element

Nelson Owen <nowen@i-55.com> wrote:
> Given the code below, only one element in @ver should be changed,
> however when I execute the code all the elements are changed. Can
> anyone explain why this is happening?
>
> class SomeClass
>    def initialize
>        @ver  = Array.new(2, {'pos' => Array.new(3), 'col' =>
>        Array.new(4)}) @ver[0]['pos'][0] = 0
>        puts @ver.inspect
>    end
> end
> SomeClass.new
>
> actual result:
> [{"col"=>[nil, nil, nil, nil], "pos"=>[0, nil, nil]}, {"col"=>[nil,
> nil, nil, nil], "pos"=>[0, nil, nil]}]
>
> expected result:
> [{"col"=>[nil, nil, nil, nil], "pos"=>[0, nil, nil]}, {"col"=>[nil,
> nil, nil, nil], "pos"=>[nil, nil, nil]}]

You are creating a two element array where the same hash is referenced for 
each array element:

>> obj = Object.new
=> #<Object:0x101928d0>
>> obj.object_id
=> 135042152
>> a=Array.new(3,obj)
=> [#<Object:0x101928d0>, #<Object:0x101928d0>, #<Object:0x101928d0>]
>> a.map {|o| o.object_id}
=> [135042152, 135042152, 135042152]
>> a.map {|o| o.object_id}.uniq
=> [135042152]

You want the block form of array init:

>> a = Array.new(3) { Object.new }
=> [#<Object:0x1017a528>, #<Object:0x1017a480>, #<Object:0x1017a3f0>]
>> a.map {|o| o.object_id}
=> [134992532, 134992448, 134992376]
>> a.map {|o| o.object_id}.uniq
=> [134992532, 134992448, 134992376]

In your case

@ver = Array.new(2){ {'pos' => Array.new(3), 'col' => Array.new(4)} }

Kind regards

    robert