From: Seebs <usenet-nospam@...>
Date: 2009-12-29T15:25:05+09:00
Subject: Re: PLEASE HELP...dup is not working correctly in the following code

On 2009-12-29, timr <timrandg@gmail.com> wrote:
> You are right about the strings having the same object_ids in each
> object, which explains why the modification on the dupped object
> affects the original strings. That helps. But the suggested strategy
> doesn't solve the problem:

Hmm.

>>> a = [['a', 'b'], ['c','d']]
>=> [["a", "b"], ["c", "d"]]
>>> a.each{|r| r.each{|letter| p letter.object_id}}
> 4442750
> 4442740
> 4442710
> 4442700
>=> [["a", "b"], ["c", "d"]]
>>> b = a.map{|r| r.each{|letter| letter.dup}}

This doesn't seem right.  The inner loop (which is the one I think
we care about) is using each, not map.  So we never actually return
the array consisting of all the duplicated letters.

If you have an array of arrays, I think you need to use map at both
levels.

irb(main):001:0> a = [['a', 'b'], ['c','d']]
=> [["a", "b"], ["c", "d"]]
irb(main):002:0> b = a.map {|r| r.map{|l| l.dup}}
=> [["a", "b"], ["c", "d"]]
irb(main):003:0> a.each{|r| r.each{|letter| p letter.object_id}}
2152912244
2152912216
2152912132
2152912104
=> [["a", "b"], ["c", "d"]]
irb(main):004:0> b.each{|r| r.each{|letter| p letter.object_id}}
2152886456
2152886316
2152886232
2152886176
=> [["a", "b"], ["c", "d"]]

What's throwing you off here is that .each returns the array object
it started with.

So "r.each {...}" always returns r, unmodified.

-s
-- 
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