From: Chris Shea <cmshea@...>
Date: 2008-08-26T08:10:40+09:00
Subject: Re: Dumb "array of arrays" question

On Aug 25, 4:57 pm, RichardOnRails
<RichardDummyMailbox58...@uscomputergurus.com> wrote:
> Hi,
>
> I wrote the following intending to create an array of 3 element, each
> being itself an array of two elements initialized to zero;  Judging by
> "inspect", I seem to have that:
>
> a = Array.new(3, [0,0]) => [[0, 0], [0, 0], [0, 0]]
>
> I tried the following,  which confirms that a[0] is indeed a sub-
> array.
>
> a[0].inspect + "; " + a[0].class.to_s => [0, 0]; Array
>
> Then I tried to assign a number (5) to the first element of the first
> sub-array. What I got was more 5's than I bargained for:
>
> a[0][0]=5 => [[5, 0], [5, 0], [5, 0]]
>
> What am I missing here?
>
> Thanks in Advance,
> Richard

This is a gotcha that trips up everyone the first time they come
across it. Array.new(size, obj) creates a new array with _size_ copies
of _obj_. That is, your array has three copies of the array [0,0].
Each one is the same as the other. So when you assign to the first
element of that array, all copies get updated. You'll want to use
Array.new with a block to get different objects:

a = Array.new(3, [0,0])
a   # => [[0, 0], [0, 0], [0, 0]]
a.map {|element| element.object_id} # => [78860, 78860, 78860]

a2 = Array.new(3) {[0,0]}
a2  # => [[0, 0], [0, 0], [0, 0]]
a2.map {|element| element.object_id}  # => [77710, 77700, 77690]

a2[0][0] = 5
a2  # => [[5, 0], [0, 0], [0, 0]]

HTH,
Chris