From: Robert Klemme <shortcutter@...>
Date: 2009-04-20T20:01:57+09:00
Subject: Re: Hashed matrix

2009/4/20 Yoann Moreau <yoni612@hotmail.com>:
> Hello, I'd like to use a specific matrix with string as key for lines
> and integer index for columns. I could also use string for columns (a
> string version of integer), it doesn't matter. I use the matrix to store
> a small object (named State) I just need to access data like that :
> matrix[a_string][a_number]
>
> Here is how I tried to do it :
> matrix = Hash.new{ |hash, key| hash[key] = Array.new(5, State.new(0,
> "")) }

You want the block form of Array.new because otherwise all array
entries point to the same object:

matrix = Hash.new{ |hash, key| hash[key] = Array.new(5) { State.new(0,"") } }

Alternative approach: use an Array of String and Fixnum as Hash key:

class Matrix
  def initialize
    @hash = Hash.new {|h,k| h[k] = State.new(0, "")}
  end

  def [](row,col)
    @hash[[row, col]]
  end

  def []=(row,col, val)
    raise ArgumentError, "Not a state %p" % val
    @hash[[row, col]] = val
  end
end

This is likely more efficient if your matrices are large and sparse.

Kind regards

robert

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