From: Stefano Crocco <stefano.crocco@...>
Date: 2008-12-05T18:32:07+09:00
Subject: Re: Array#shift weird problem

Alle Friday 05 December 2008, Hamza Haiken ha scritto:
> Hi
>
> Can someone explain to me why is this happening ?
>
> >> matrix1 = ["One","Two","Three"]
>
> => ["One", "Two", "Three"]
>
> >> matrix2 = matrix1
>
> => ["One", "Two", "Three"]
>
> >> matrix2.shift
>
> => "One"
>
> >> matrix2
>
> => ["Two", "Three"]
>
> >> matrix1
>
> => ["Two", "Three"]
>
> According to my logic, when I do matrix2.shift, it should have nothing
> to do with matrix1. Why's this one shifted as well ?

Because matrix1 and matrix2 are only different names used to refer to the same 
thing, as you can see by checking the values returned by matrix1.object_id and 
matrix2.object_id. If you want the two variables to contain different objects, 
you need to write

matrix2 = matrix1.dup

This will set the contents of matrix2 to a duplicate of matrix1, so that 
changing one won't change the other. Note, however, that this is only a 
shallow copy, that is: the array itself is duplicated, but its contents 
aren't. This means that any method which changes one of the contained strings 
in-place will affect both array. For example:

matrix2 = matrix1.dup
matrix1[0].sub!('O','o')
p matrix1
#=> ["one", "Two", "Three"]
p matrix2
#=> ["one", "Two", "Three"]

If you also need to be sure that the arrays contain different objects, you 
need to do a deep copy, which can be achieved using marshal:

matrix2 = Marshal.load(Marshal.dump(matrix1))

I hope this helps

Stefano