From: Chris Lowis <chris.lowis@...>
Date: 2007-02-20T18:38:54+09:00
Subject: Help with Class design

I'm quite new to object-orientated programming and have a problem with a
class I am trying to design. At the moment I have

#!/usr/local/bin/ruby

require 'mathn'

class MySignal

  def initialize(input)
    @signal = input
  end

  def mean()
    sum = 0;
    @signal.each {|i| sum+=i}
    mean = sum/@signal.length
  end

  def variance()
    m = @signal.mean
    var = @signal.inject(0) { |var, x| var += (x - m) ** 2 }
    var = var/(@signal.length-1)
  end
end

I can create a new "signal" with :

   a = MySignal.new([1,2,3])

and calculate the mean with :

   puts a.mean

But if I try to calculate the variance:

   puts a.variance

./signal.rb:18:in `variance': undefined method `mean' for [1, 2,
3]:Array (NoMethodError)

Which makes sense, as @signal is an object with the Array class. I don't
want to add methods to Ruby's array class as later my methods may be too
specific to my problem.

How do I write a class such that methods defined within it can "refer"
to each other, for example "variance" can call and use "mean" ? I am
sure this question arises from my lack of familiarity with
object-orientated programming. In addition to suggestions for the
specific problem above, any pointers to references that could help me
learn more general OO-design would also be appreciated.

Kind regards,

Chris

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