From: Stefano Crocco <stefano.crocco@...>
Date: 2006-12-08T06:01:51+09:00
Subject: Re: Array changing after concat function

Alle 21:49, gioved狸 7 dicembre 2006, Drew Olson ha scritto:
> WKC CCC wrote:
> > I've found that using:
> >
> > copy = one.map { |el| el.clone }
> >
> > will copy all elements into the new array without referring to the same
> > object.
> > Is there a reason why the clone function has been ommitted from the API
> > doc?
>
> It seems much simpler to just clone the array rather than each element
> individually:
>
> irb(main):001:0> a=[]
> => []
> irb(main):002:0> 10.times{|i| a << i}
> => 10
> irb(main):003:0> a
> => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
> irb(main):004:0> b = a.clone
> => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
> irb(main):005:0> a[0] = 10
> => 10
> irb(main):006:0> a
> => [10, 1, 2, 3, 4, 5, 6, 7, 8, 9]
> irb(main):007:0> b
> => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Your code doesn't solve the problem. If a contains mutable objects, such as 
arrays, what happens is this:

irb(main):001:0> a=[[1,2],[3,4]]
=> [[1, 2], [3, 4]]
irb(main):002:0> b=a.clone
=> [[1, 2], [3, 4]]
irb(main):003:0> b[0].push 5
=> [1, 2, 5]
irb(main):004:0> a
=> [[1, 2, 5], [3, 4]]
irb(main):005:0>

What clone (and dup) produce are shallow copies, that is the object is copied, 
but their contents are not: a contains exactly the same objects than b does. 
Because of this, you can't change one of the elements of b whitout changing 
the element of a: the two are the same object.

Instead, what your example shows is that a and b are not the same object: you 
can add an element to a whithout changing b. But here you're changing the 
arrays, not he objects they contain, which was the problem of the original 
poster.

Stefano