From: Robert Klemme <bob.news@...>
Date: 2004-05-11T20:43:59+09:00
Subject: Re: [Q] Array.new - strange behavior


"E.-R. Bruecklmeier" <unet01@radio-eriwan.de> schrieb im Newsbeitrag
news:c7q5it$jc5$1@news.BelWue.DE...
> dear rubyists,
>
> today i noticed a strange bahavior in Array.new while defining multi
> dimentional arrays:
>
> p "P1"
> foo = Array.new(3,[0,0])
> foo.each do |foo1|
>    p foo1.id
> end
>
> p "P2"
> foo = Array.new
> 3.times {foo.push [0,0]}
> foo.each do |foo1|
>    p foo1.id
> end
>
>
> produces:
>
> "P1"
> 19645868
> 19645868
> 19645868
> "P2"
> 19645760
> 19645748
> 19645736
> Completed(0)
>
> Is it intentional that the new method fills the array with just one
> instance of [0,0]? In my sense this is a violation of the PoLS.

Well, I'd say no, if you aware of how Ruby deals with instances and
variables.  All variables hold references - there is no distinction
between pass by value and pass by reference as it is in C++.  Then you'll
see that a single instance is handed over to the constructor in your first
example.  That instance is used for each array element, which is fine for
immutable classes (integers for example).  If you need different
instances, you'll need the block form as shown by you or even better the
other one:

foo = Array.new( 3 ) {[0,0]}

irb(main):002:0> foo = Array.new( 3 ) {[0,0]}
=> [[0, 0], [0, 0], [0, 0]]
irb(main):003:0> foo.each {|o| puts o.id}
135011840
135011828
135011816
=> [[0, 0], [0, 0], [0, 0]]

Regards

    robert