From: Stefano Crocco <stefano.crocco@...>
Date: 2009-06-12T17:03:59+09:00
Subject: Re: About 'dup'

On Friday 12 June 2009, Wu Xiaoyi wrote:
> |Hello,
> |
> |mainarray=Hash.new(0)
> |subarray=Hash.new(0)
> |
> |thirdarray={
> | 'a' => '0',
> | 'b' => '0',
> | 'c' => '0',
> |}
> |
> |subarray['first']=thirdarray.dup
> |subarray['second']=thirdarray.dup
> |
> |mainarray['x']=subarray.dup
> |mainarray['y']=subarray.dup
> |
> |mainarray['x']['first']['c']=56
> |mainarray['y']['first']['c']=73
> |p mainarray
> |
> |
> |the result is:
> |{"x"=>{"second"=>{"a"=>"0", "b"=>"0", "c"=>"0"}, "first"=>{"a"=>"0",
> |"b"=>"0", "c"=>73}},
> | "y"=>{"second"=>{"a"=>"0", "b"=>"0", "c"=>"0"}, "first"=>{"a"=>"0",
> |"b"=>"0", "c"=>73}}}
> |
> |Why did the mainarray['x']['first']['c'] become 73,too?
> |dup does not work if I use it twice?

The problem is that dup only does a shallow copy of the object. In your case, 
this means it creates a new hash but fills it with the same contents of the 
original one. This means that mainarray['x']['first'] contains the same object 
as mainarray['y']['first'] (you can see this by checking the object_id of the 
two). If you want the hashes in mainarray['x'] and mainarray['y'] to have 
different contents, you need to do a deep copy of subarray. The simpler way to 
do this is to use Marshal.dup and Marshal.load:

mainarray['x'] = Marshal.load(Marshal.dump(subarray))
mainarray['y'] = Marshal.load(Marshal.dump(subarray))

Of course, this will fail if subarray contains an object which can't be 
marshalled (see the documentation for the Marshal module to see which they 
are). In this case you'll need to define a more specific method to do this.

I hope this helps

Stefano