From: "David A. Black" <dblack@...>
Date: 2009-07-21T22:27:11+09:00
Subject: Re: Mean method

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Hi --

On Tue, 21 Jul 2009, Älphä Blüë wrote:

> I'm working on a lot of math in my projects so I thought I would convert
> over a few methods I used in other languages that I created and port
> them to ruby.
>
> Here's a mean method which allows you to return either:
>
> a) arithmetic mean
> b) geometric mean
> c) harmonic mean
>
> http://pastie.org/553287
>
> Enjoy.

Thanks for sharing the code. I've got a few suggestions for
refactoring it, which I hope you won't mind my sharing with you.

First of all, if you want to call #sum on an array, you'll need a #sum
method :-) I've thrown in #product too.

   class Array
     def sum
       inject {|a,b| a + b }
     end

     def product
       inject {|a,b| a * b }
     end
   end

And here's a somewhat more concise version of your mean method. Note
that I don't check the classes of the arguments. I'd rather try to
make them work, and if they don't then an exception will be raised at
some point anyway. I do, however, add a check to make sure the mean
type is a known one.

   def mean(array, i_type = 1)
      case i_type
      when 1 then array.sum / array.size
      when 2 then array.product ** (1.0/array.size)
      when 3 then array.size / array.inject {|a,b| a + 1.0 / b }
      else
        raise ArgumentError, "Unknown mean type '#{i_type}'"
      end
    end

It might be user-friendly to wrap the various types in methods of
their own:

   def arithmetic_mean(array)
     mean(array, 1)
   end

etc.


David

-- 
David A. Black / Ruby Power and Light, LLC
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