From: Paul Brannan <pbrannan@...>
Date: 2002-04-25T00:56:57+09:00
Subject: Re: Beginner's question

On Thu, Apr 25, 2002 at 12:24:27AM +0900, Aron Griffis wrote:
>   class TwixtBoard < Matrix
>     def initialize(width = 24)
>       super(width, width) { nil }
>     end
> 
>     def to_s
>       self.collect {|row| "#{row}"}.join "\n"
>     end
> 
>     def []=(column, row, color)
>       self[row][column] = TwixtPeg.new(color)
>     end
>
>     def [](column, row)
>       self[row][column].color
>     end

Here, you have defined your own [] and []= operators that have a
different interface than the base class's [] and []=.  If the base class
implements any methods in terms of these two methods, then your code
will break.  (In Ruby, all methods are virtual; when a base class calls
a method, Ruby searches for that method starting with the most derived
class).

A TwixtBoard, therefore, isn't a Matrix, and probably shouldn't inherit
from it.  If you do want to inherit from Matrix anyway, then you should
use:

  def []=(col, row, color)
    super(row)[col] = ...
  end

>   end
> 
>   board = TwixtBoard.new
>   print board, "\n"
>   tb[0,0] = RED
>   tb[23,23] = BLACK
>   print board, "\n"
>
> When I attempt to execute this code, I get the following error:
> 
>   ./mat.rb:5:in `[]=': wrong # of arguments(2 for 3) (ArgumentError)
>           from ./mat.rb:5:in `initialize'
>           from ./mat.rb:4:in `each_index'
>           from ./mat.rb:4:in `initialize'
>           from ./mat.rb:24:in `initialize'
>           from ./mat.rb:40:in `new'
>           from ./mat.rb:40

And, in fact, the base class does use []=; Matrix#initialize uses it,
and crashes when it tries to pass 2 arguments to a method that is
expecting 3.

Hope this helps,

Paul