From: Rick DeNatale <rick.denatale@...>
Date: 2008-09-18T22:01:35+09:00
Subject: Re: why one array continues to grow after repeated call

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On Thu, Sep 18, 2008 at 8:41 AM, Li Chen <chen_li3@yahoo.com> wrote:

> Hi all,
>
> I write a script for studying purpose:
> create a new array(b1) based on an existing one (a1).
>
> I expect that once an array is set up it should be fixed regardless how
> many times it is called. But I find array b1 continues to grow if I call
> it repeatedly? I can't find an explantion for that. Any idea?
>
> Thank you very much in advance,
>

You seem to have a common misperception that assigning a value to a ruby
variable acts like a declaration, rather than just a, potentially temporary,
binding of the variable to a particular object.

> C:\Users\Alex>irb
> irb(main):001:0> class A
> irb(main):002:1>   def  initialize
> irb(main):003:2>     @a1=[]
> irb(main):004:2>     @b1=[]


If instance variables were constrained by the declaration, then @a1 and @b1
would have to  always refer to empty arrays.

>
> irb(main):005:2>     method1

And method1 could have no effect here.

>
> irb(main):006:2>   end
> irb(main):007:1>
> irb(main):008:1*  def method2
> irb(main):009:2>   (method1.size).times{@b1<<10}


What really happens here is that the array referenced by @b1 has @a1.size
10's added each time the method is called.

>
> irb(main):010:2>    return @b1
> irb(main):011:2>  end
> irb(main):012:1>
> irb(main):013:1*
> irb(main):014:1*   def method1
> irb(main):015:2>    @a1=[1,2,3,4]

And this line replaces the empty array bound to @a1 in the initialize method
with a new array instance.  The original empty array is now subject to
garbage collection.

>
> irb(main):016:2>   return @a1
> irb(main):017:2>  end
> irb(main):018:1>
> irb(main):019:1* end
> => nil
> irb(main):020:0> ob=A.new
> => #<A:0x2f9f7c @b1=[], @a1=[1, 2, 3, 4]>
> irb(main):021:0> ob.method2
> => [10, 10, 10, 10]
> irb(main):022:0> ob.method2
> => [10, 10, 10, 10, 10, 10, 10, 10]
> irb(main):023:0> ob.method2
> => [10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10]
> irb(main):024:0> ob.method1
> => [1, 2, 3, 4]
> irb(main):025:0> ob.method2
> => [10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10]
> irb(main):026:0> exit


Perhaps this two year old article in my blog might give some insight

http://talklikeaduck.denhaven2.com/articles/2006/09/13/on-variables-values-and-objects


-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

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