From: David Alan Black <dblack@...>
Date: 2001-04-18T00:38:47+09:00
Subject: [ruby-talk:13761] Re: Odd Hash Behavior (long)

Hello --

On Wed, 18 Apr 2001, Steve Tuckner wrote:

> Could someone explain the following behavior?
>
> The following code
> -------------------------------
> class A
>   attr_reader :value
>   def initialize
>     @value = 0
>   end
>   def add_one
>     @value += 1
>   end
> end
>
> aTestHash = Hash.new(A.new)
>
> aTestHash['A'] = A.new
> aTestHash['A'].add_one
> aTestHash['B'].add_one
> print "aTestHash['A'] = #{aTestHash['A'].value}\n"
> print "aTestHash['B'] = #{aTestHash['B'].value}\n"
>
> print "\nAll Hash values:\n"
> aTestHash.each { |aKey, aA| print "#{aKey} = #{aA.value}\n" }
... =>
> aTestHash['A'] = 1
> aTestHash['B'] = 1
>
> All Hash values:
> A = 1

Your hash will use the default value you give it in place of nil for
non-existent keys -- but the keys are still non-existent :-)  They
don't get created automatically.

Also note that when you do this:

  h = Hash.new(some_method)

the method get called only once, and the return value of that call is
used for all subsequent non-existent key fetches.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav