From: khaines@...
Date: 2008-04-25T01:41:31+09:00
Subject: Re: Pointer concept in Ruby ?

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On Fri, 25 Apr 2008, [UTF-8] I=C3=B1aki Baz Castillo wrote:

> Yes, I know that since object_id is the same **until** a write
> operation is done.
>
>
>>  If you however call a self-modifying method on a1 while it's pointing a=
t
>> the array string, it'll also change in the array, as it's still pointing=
 at
>> the same object.
>
> No, that's not true. Demostration:
>
> array=3D[000,111,222,33]
> =3D> [0, 111, 222, 33]
>
> a1=3Da[1]
> =3D> 1
>
> a[1].object_id
> =3D> 345
>
> a1.object_id
> =3D> 345
>
> a1=3D"NEW VALUE"
> =3D> "NEW VALUE"
>
>> a1.object_id
> =3D> -606081098   <----------- Has changed !!!

Because you changed the object that a1 points to.

> a[1].object_id   <-------- Remains the same
> =3D> 345

a[1] points to an object.
a1 points to an object.

They were the same object, until you pointed a1 at a different object.

This is the important distinction that you are missing.

To recap:

a =3D [1,2,3,4]

a points to an Array that contains 4 objects, the Fixnums 1, 2, 3, and 4.

a[1] calls a method [] on the object pointed to by 'a'.  That method=20
returns the object in the 1 position in the array.

a1 =3D a[1] points 'a1' at that object.  a1 is just a pointer, though, to a=
n=20
object.

You can't change what the Array pointed to by 'a' contains by pointing=20
'a1' at a different object, which is what your assignment of "NEW STRING"=
=20
to 'a1' did.

If you want to change what is stored in the 1 index position in the Array=
=20
pointed to by 'a', you need to be operating on that Array.

b =3D a
b[1] =3D "another new string"
puts a[1]

another new string


Make sense?


Kirk Haines

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