From: "Florian Groß" <florgro@...>
Date: 2005-06-18T09:56:00+09:00
Subject: Re: Why do arrays work this way?

Nigel Wilkinson wrote:

> irb(main):058:0> a=[1,2,3,4,5]; b = a; b[3] = 'w'; a
> => [1, 2, 3, "w", 5]
> 
> so what's the logic behind 'a' & 'c' changing when I change 'b'.
> 
> Is this a bug? and if not what's the reasoning behind this behaviour 
> please.

In Ruby variables are just names for objects. If you do
   my_array = [1, 2, 3]
you name an array with the elements 1, 2 and 3 as my_array.

If you then do
   my_numbers = my_array
you just give the object named my_array another name (my_numbers). So 
what you did was just variable aliasing and nothing else.

Please note that in Ruby
   var += 5
is exactly the same as
   var = var + 5
which binds another name to the variable. This means that
   a = 5
   b = a
   b += 1
   [a, b] # => [5, 6]
because the objects didn't change at all -- you just decided to rename 
things.

Other languages with value semantics typically create a copy of the 
original object when you assign them to anything. This is typically done 
in Ruby like
   my_array = [1, 2, 3]
   my_copy = my_array.clone
which will then give you the behavior you intended:
   my_array[0] = :changed
   my_array # => [:changed, 2, 3]
   my_copy # => [1, 2, 3]

I think that Ruby's behavior is correct because languages where you 
explicitly have to specify that you want a reference to an object 
instead of a copy of it (like PHP used to do it even for values that are 
typically automatically passed by reference in the traditional languages 
like C++ and Java) cause more confusion. In Ruby you just give names to 
objects and ask them for a copy when you want one. It takes a short 
introduction (simply because languages that use pass-by-value somewhere 
are more common), but leads to less trouble later.