From: Robert Klemme <shortcutter@...>
Date: 2006-07-10T02:30:14+09:00
Subject: Re: Definition of methods: self

Damaris Fuentes wrote:
> Ok, thanks for the explanations!
> But...now that you mention the concatenator... I will comment another 
> problem in this entry. (I write all the code, ven though perhaps you 
> don't need it)
> 
> The problem here is that the re- definition ot a class (LotteryDraw) 
> doesn't seem to see the attributes of the first definition.
> 
> I have the class ot the lottery ticket:
> ***********************
> class LotteryTicket
>   NUMERIC_RANGE = 1..25
>   attr_reader :picks
> 
>   def initialize( *picks )
>     [...]
>     @picks = picks
>   end
> 
>   def score( final )
>     count = 0
>     final.picks.each do |note|
>       count +=1 if picks.include? note
>     end
>     count
>   end
> 
> end
> 
> *****************************
> Now I have the lottery draw, where I store the buyers of the tickets:
> **********************
> class LotteryDraw
>   @@tickets = {}
>    def LotteryDraw.buy( customer, *tickets )
>      unless @@tickets.has_key?( customer )
>        @@tickets[customer] = []
>      end
>      @@tickets[customer] += tickets
>    end
> end
> *******
> And NOW, I REDEFINE THE CLASS with the concatenator, adding a new class 
> method:
> ********************
> class << LotteryDraw
>    def play
>      final = LotteryTicket.new_random
>      winners = {}
>      @@tickets.each do |buyer, ticket_list|
>        ticket_list.each do |ticket|
>          score = ticket.score( final )
>          next if score.zero?
>          winners[buyer] ||= []
>          winners[buyer] << [ ticket, score ]
>        end
>      end
>      @@tickets.clear
>      winners
>    end
>  end
> ********
> 
> And now I execute the following:
> ************************
> LotteryDraw.buy('JuanMi',
>      LotteryTicket.new( 12, 6, 19 ),
>      LotteryTicket.new( 5, 1, 3 ),
>      LotteryTicket.new( 24, 6, 8 ))
> 
> LotteryDraw.buy('Da',
>      LotteryTicket.new( 18, 4, 7 ),
>      LotteryTicket.new( 21, 25, 3 ),
>      LotteryTicket.new( 12, 17, 19 ))
> 
> LotteryDraw.play.each do |winner, tickets|
>   puts winner + "won on " + tickets.length + " ticket(s)!"
>     tickets.each do |ticket, score|
>       puts "\t" + ticket.picks.join( ', ' ) + ": " + score
>     end
>  end
> ************************************
> 
> And an exception arises:
> in `play': uninitialized class variable @@tickets in Object.
> This exception appears in the trace:
> - LotteryDraw.play.each do |winner, tickets|,
> - @@tickets.each do |buyer, ticket_list| (in LotteryDraw.play)
> 
> What is the problem? Thanks (and sorry if this is very long :()
> 

Without looking too closely at your code I'm guessing that it's because 
your class methods are in the singleton class of LotteryDraw - and so 
lookups for class variables are done in *that* hierarchy and not in the 
one of LotteryDraw.

I generally recommend to not use class variables because they introduce 
all sorts of weirdnesses.  I'd rather use a normal instance variable of 
the class instance.  That's accessible for sure in the singleton method.

Kind regards

	robert