From: brabuhr@...
Date: 2009-11-25T00:25:10+09:00
Subject: Re: Difference between << and += for Strings and Arrays. Bug?

On Tue, Nov 24, 2009 at 9:46 AM, Pieter Hugo <pieter@internext.co.za> wrote:
> Hi
> I have been programming with Ruby for about 1 year and am loving it. I
> cam across something that I can't figure out why it works the way it
> does:
>
> Eg:
> a=[1,2]
> b=a
> b << 3
>
> The value of b is now [1,2,3], but the value of a is also [1,2,3]????

http://ruby-doc.org/core/classes/Array.html#M002167
"Append—Pushes the given object on to the end of this array."

> And yet:
>
> a=[1,2]
> b=a
> b += [3]
>
> The value of b is still [1,2,3] but a remains [1,2]. This just doesnt
> make sense to. The same happens with Strings. Why would the append
> operator (<<) change the original variable, but other operators dont
> seem to.

http://ruby-doc.org/core/classes/Array.html#M002209
"Concatenation—Returns a new array built by concatenating the two
arrays together to produce a third array."

> PS What I really want to know is why the original variable is
> changed at all?

After b=a, the variables 'a' and 'b' both reference the same object.
+= creates a new array (the + part) and changes b to reference that
new array (the = part).  With << the object referenced by b is
directly modified, it appears that a is also changed because a
references the same object as b.  It's not really that b was changed
by <<, the object referenced by b is what changed.