From: Matthew Kerwin <lists@...>
Date: 2013-04-17T13:10:43+09:00
Subject: Re: Delete method also modifies the original variable

Vincent Stowbunenko wrote in post #1105954:
> How do I make sure the first array stays original after the modification
> of second array?

Variables in ruby hold references to objects.  This can be evidenced by 
viewing the object_ids:

irb:001> array1 = [1,2,3,4,5]
irb:002> array2 = array1
irb:003> array2.object_id
=> 70171269757120
irb:004> array1.object_id
=> 70171269757120

To de-duplicate the reference you need to create a second object, which 
is a copy of the first.  The simplest way is to use `dup`

irb:005> array2 = array1.dup
irb:006> array1.object_id
=> 70171269757120
irb:007> array2.object_id
=> 70171269775520
#           ^^^^^

irb:008> array1.delete(1)
irb:009> array1
=> [2, 3, 4, 5]
irb:010> array2
=> [1, 2, 3, 4, 5]

The thing to remember is that Array#dup performs a shallow copy.  For 
example:

irb:011> array1 = [[1,2,3],[4,5]]
irb:012> array2 = array1.dup
irb:013> array1[0].delete(1)
irb:014> array1
=> [[2, 3], [4, 5]]
irb:015> array2
=> [[2, 3], [4, 5]]

The variables `array1` and `array2` point to different Array objects, 
but each of _those_ independently maintains references to the _same_ 
inner-arrays.  In other words, array1[0] and array2[0] both reference 
the same object.

It's not only limited to arrays, either:

irb:016> array1 = ['a']
irb:017> array2 = array1.dup
irb:018> array1[0] << 'b'
irb:019> array1
=> ["ab"]
irb:020> array2
=> ["ab"]

There is no general-form solution to creating a deep-copy of a 
datastructure in ruby.

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