From: Adam Shelly <adam.shelly@...>
Date: 2008-08-20T08:53:18+09:00
Subject: Re: Nested Arrays in Method Definitions--a puzzle

On 8/19/08, Jonah Bloch-Johnson <jsb2142@columbia.edu> wrote:

> I am writing because I am somewhat stumped by why the following discrepency occurs...

Variables and Array entries hold references to objects.

irb(main):001:0> x=[1]
=> [1]
irb(main):002:0> x.object_id
=> 20774910

irb(main):003:0> y=[];y<<x
=> [[1]]
irb(main):004:0> y[0].object_id
=> 20774910    #the same object that x referrs to

irb(main):005:0> x=[2]
=> [2]
irb(main):006:0> x.object_id
=> 21222360   #now x refers to the newly created array

irb(main):008:0> y
=> [[1]]
irb(main):007:0> y[0].object_id
=> 20774910  #but y still holds a reference to the old one.

irb(main):009:0> y=[x]
=> [[2]]
irb(main):010:0> y[0].object_id
=> 21222360   #y[0] is now a reference to the same Array as x

irb(main):011:0> x[0]=4
=> 4
irb(main):013:0> y
=> [[4]]    #so changes to the content of that Array are visible
through y's reference.


hope this helps,
-Adam



























































>        x = [1]

[1] is an Array object, x is a variable referring to that array
>        y = []
y referrs to  a 2nd Array
>        y << x

And now that array contains a reference to

>        x = [2] # (compare this)

Here you are creating an array, labeling it 'x', and storing a
reference to it in the array labeled y.  Then you reassign the 'x'
label to yet another array (the one containing 2).

>  end
>  def havenots
>        x = [1]
>        y = []
>        y << x
>        x[0] = 2 # (to this)
>        y
>  end
> end
>
> I get this in irb:
>
> >
> > > load 'snah.rb'
> > >
> >
> => true
> >
> > > a = Warfare.new
> > >
> >
> => #<Warfare:0x33a93c>
> >
> > > a.haves
> > >
> >
> => [[1]]
> >
> > > a.havenots
> > >
> >
> => [[2]]
>
> Why does meddling with the innards of the "x" have retroactive
> repercussions, whereas redefining it outright does not?
>
> Any insight would be greatly appreciated.
>
> -Jonah
>
>
>