From: Stefano Crocco <stefano.crocco@...>
Date: 2009-01-12T17:58:33+09:00
Subject: Re: Dynamically created attributes

Alle lunedì 12 gennaio 2009, Peter Marks ha scritto:
> I'd like to specify a quantity of object attributes when creating an
> instance of an object. For example, I would like to create a Grid object
> with 4 different 'row' attributes like this: Grid.new(4). Here's some
> dirty code that does that:
>
> class Grid
>   #Creates row1, row2, etc. depending on row arg
>   def initialize(rows)
>     row_count = 1
>     rows.times do
>       instance_eval "@row#{row_count} = 'xxx'"
>       row_count += 1
>     end
>   end
> end
>
> It's dirty, but it creates the attributes. I don't see how to
> dynamically create accessor methods for all these attributes though. Any
> suggestions? I sense there's a better way to do this.

Well, the first thing I can think of is to replace your n instance variables 
with a single instance variable @rows which will be an array and contain the 
rows. Something like this:

class Grid

  attr_reader :rows
  def initialize(rows)
    @rows = Array.new(rows){ 'xxx' }
  end

end

If you don't want to make the array availlable from outside the instance, you
can skip the attr_readr line and provide methods to directly access the rows. 
For example, you can define [] and []= methods as array does:

class Grid

  def [](idx)
    @rows[idx]
  end

  def []=(idx, value)
    @rows[idx] = value
  end

end

If you truly want to use an instance variable for each row, see the 
documentation about instance_variable_set and look at the section "Object 
specific classes" in the "Classes and Objects" chapter of the online edition 
of the Pickaxe book
(http://www.ruby-doc.org/docs/ProgrammingRuby/html/classes.html)

I hope this helps

Stefano