From: Robert Klemme <shortcutter@...>
Date: 2010-04-23T23:37:16+09:00
Subject: Re: What is wrong with this few lines of code

2010/4/23 Rolf Pedersen <rolfhsp@gmail.com>:
> The code ar = Array.new(n, []) actually makes n copies of the same object
> (empty array), so each time you do either ar[0] << e or ar[1] << e you are
> appending elements to the same array.

That description is inconsistent and might confuse one or the other
reader: there are no copies made of the *object* (Array in this case)
but only of the *reference*!

irb(main):001:0> a = Array.new(3,[])
=> [[], [], []]
irb(main):002:0> a.map {|x| x.object_id}
=> [135000124, 135000124, 135000124]
irb(main):003:0> a.map {|x| x.object_id}.uniq
=> [135000124]

The idiom you describe when you say "copy" is the block form of Arra.new:

irb(main):004:0> a = Array.new(3) { [] }
=> [[], [], []]
irb(main):005:0> a.map {|x| x.object_id}
=> [135669212, 135669198, 135669184]
irb(main):006:0> a.map {|x| x.object_id}.uniq
=> [135669212, 135669198, 135669184]

> What I guess you want is to different objects, and you can do that by e.g.
> ar = Array.new(2){[]}
> (or just ar = [[],[]]   ...or many other ways :o) )

Exactly.

Kind regards

robert

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