From: Len Lawrence <lcl@...>
Date: 2008-03-22T23:14:52+09:00
Subject: Re: Cloning arrays

On Sat, 22 Mar 2008 08:37:46 -0500, Stefano Crocco wrote:

> On Saturday 22 March 2008, Len Lawrence wrote:
0 
> a = %w[a b c]
> b = a.dup
> puts "The ID of a is #{a.object_id}"
> puts "The ID of b is #{b.object_id}"
> puts "a is the same object as b? #{a.equal? b}"
> 
> puts "The ID of the contents of a: #{a.map{|i| i.object_id}.join(', ')}"
> puts "The ID of the contents of b: #{b.map{|i| i.object_id}.join(', ')}"
> a.each_index do |i|
>   puts "a[#{i}] is the same object as b[#{i}]? #{a[i].equal? b[i]}"
> end
> 
> produces:
> The ID of a is -605837488
> The ID of b is -605837528
> a is the same object as b? false
> The ID of the contents of a: -605837498, -605837508, -605837518
> The ID of the contents of b: -605837498, -605837508, -605837518
> a[0] is the same object as b[0]? true
> a[1] is the same object as b[1]? true
> a[2] is the same object as b[2]? true
> 
> This means that while a and b are not the same object (and so, you can modify 
> one, for example adding or removing an item, leaving the other as is), they 
> contain the same objects. So, calling 'destructive' methods (that is, methods 
> which change the receiver) on the elements of one of the array will also 
> change the corresponding element of the other array (because they contain the 
> same object).
> 
> To achieve what you want, you need deep copy. As someone on this list

> recently pointed out, you can usually achieve this using marshal:
> 
> b = Marshal.load(Marshal.dump(a))
> 
> The problem is that with rubytk you're dealing with a C extension, and
> with objects created in C, which may, or may not, work correctly with
> marshal. If you find out they don't, you can try to manually
> deep-duplicate the array. In the simple case of my example above, you
> can do:
> 
> b  = a.map{|i| a.dup}
> 
> If your array is a nested array, things become more complicated, as

> you'd need nested loops. You also need to keep in mind that object
> created from C extensions may not be clonable. In this case, there's
> nothing to do.
> 
> I hope this helps
 
Thanks Stefano; it sure does.  I missed the earlier references to
Marshalling - not enough time to keep up with the newsgroup.  I can cope
with nested arrays.  Anyhow, many thanks for the education.

Len