From: Robert Klemme <shortcutter@...>
Date: 2007-02-20T18:55:13+09:00
Subject: Re: Help with Class design

On 20.02.2007 10:38, Chris Lowis wrote:
> 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.

You just need to change the line to

m = mean

or - if you want to be more verbose -

m = self.mean

i.e. you just picked the wrong receiver for the call.

Kind regards

	robert