From: "Wolfgang Nádasi-Donner" <wonado@...>
Date: 2007-01-25T21:00:10+09:00
Subject: Re: Array and instance variable problem?

Maxime Guilbot schrieb:
> What I meant in my second message, is that when @indexes is a FixNum 
> (not an Array).
> This code is working as expected:

In the code

   def get_next
     @indexes[0] = @indexes[0]*2
     @indexes[1] = @indexes[1]*2

     @indexes
   end

the contents of "@indexes" will be changed, but the "Array" object will be the 
same. In

   def get_next
     @indexes = @indexes*2

     @indexes
   end

"@indexes*2" will create a new object, which will be assigned to "@indexes" 
afterwards.

Conclusion: in case of the Array object you will end with the same object with 
changed contents (the same is valid for Hash objects, and may be valid for 
strings - see below), in case of the Fixnum object a new object will be created 
and referenced.

An example for class String to clarify this.

 >>>>> Code >>>>>

require 'pp'
class Dummy
    def initialize
      @indexes = "a"
    end

    def get_next
      @indexes[0,1] = @indexes[0,1].succ

      @indexes
    end

    def get_next_10
      all = []
      for i in 0..9
        all << get_next
      end
      all
    end

    def show_id
      puts @indexes.object_id
    end
end
d = Dummy.new

x = d.get_next_10
puts '##### show contents #####'
pp x
puts '##### show @indexes-id #####'
d.show_id
puts '##### show ids of Array elements #####'
x.each{|e|puts e.object_id}

 >>>>> Output >>>>>

##### show contents #####
["k", "k", "k", "k", "k", "k", "k", "k", "k", "k"]
##### show @indexes-id #####
24861400
##### show ids of Array elements #####
24861400
24861400
24861400
24861400
24861400
24861400
24861400
24861400
24861400
24861400

 >>>>> EoE >>>>>

If you change a line of code a little bit, something completely different will 
happen.

 >>>>> Parts of Code >>>>>

    def get_next
#    @indexes[0,1] = @indexes[0,1].succ
      @indexes = @indexes.succ # <<<<<< Here is the minor change

      @indexes
    end

 >>>>> Output >>>>>

##### show contents #####
["b", "c", "d", "e", "f", "g", "h", "i", "j", "k"]
##### show @indexes-id #####
24861380
##### show ids of Array elements #####
24861470
24861460
24861450
24861440
24861430
24861420
24861410
24861400
24861390
24861380

 >>>>> EoE >>>>>

Wolfgang Nádasi-Donner