From: Tom Stuart <tom@...>
Date: 2009-11-25T00:26:02+09:00
Subject: Re: Difference between << and += for Strings and Arrays. Bug?

2009/11/24 Pieter Hugo <pieter@internext.co.za>:
> Hi
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]????
b=a means 'the variable b points to the same object as the variable
a'. So calling a method on b which changes the underlying object means
that the underlying object is changed, regardless of whether you call
it a or b. The << method changes the array. As the docs describe it:
"Append—Pushes the given object on to the end of this array."
(http://ruby-doc.org/core/classes/Array.html#M002167).

> 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.
'b += [3]' is syntactic sugar, or shorthand, for 'b = b + [3]'. i.e.
This calls the '+' method on b 'under the hood'. '+' does not change
the array, but returns a new array. From the docs again:
"Concatenation—Returns a new array built by concatenating the two
arrays together to produce a third array."
(http://ruby-doc.org/core/classes/Array.html#M002209). So here you're
getting a completely separate object and saying that the variable b
should point to this new object. The old object pointed to by b (and
also by a) is unaffected.

> The same happens with Strings.
As per the docs for Strings on the equivalent methods.

> Why would the append
> operator (<<) change the original variable, but other operators dont
> seem to. PS What I really want to know is why the original variable is
> changed at all?
I hope my explanations make some kind of sense and help answer these questions?

Cheers,

Tom