From: Robert Klemme <bob.news@...>
Date: 2005-02-17T20:59:48+09:00
Subject: Re: A little bit confused about array+array addition (bug or expected behavior?)


"Assaph Mehr" <assaph@gmail.com> schrieb im Newsbeitrag
news:1108596235.249240.166770@z14g2000cwz.googlegroups.com...
>
> gga wrote:
> >
> > irb(main):082:0> def add(x)
> > irb(main):084:1>    x += [5,6]
> > irb(main):085:1> end
> >
> > irb(main):087:0> add(a)
> > => [1, 2, 3, 4, 5, 6]
> > irb(main):088:0> a
> > => [1, 2, 3, 4]        ===>>> uh?  am I not passing stuff by
> reference?
> >  where is 5, 6?
>
> In the method body you are reassing to the local variable x.
> . x += [...]
> is translated to:
> . x = x + [...]
> and #+ as we all know returns a copy of the object.
>
> You can see this with:
>
> . def add(x)
> .   puts "Before #{x.object_id}"
> .   x += [5,6]
> .   puts "After #{x.object_id}"
> . end
>
> So you ARE passing by ref, but then changing the ref to point to
> anotehr object.
>
> > irb(main):089:0> def add2(x)
> > irb(main):089:0>   x << [5,6]
> > irb(main):089:0> end
> >
> > irb(main):090:0>  add2(a)
> > => [1, 2, 3, 4, [5, 6]]
> > irb(main):091:0>  a
> > => [1, 2, 3, 4, [5, 6]]  ==> seems logical, but then .... why +=
> > behaves differently?
>
> The METHOD #<< modifies the object in place. The SYNTACTIC SUGAR += is
> changing a objeced referred to.

Alternative is #concat

>> a=[1,2]
=> [1, 2]
>> def add3(a) a.concat [3,4] end
=> nil
>> add3 a
=> [1, 2, 3, 4]
>> a
=> [1, 2, 3, 4]


Regards

    robert