From: benjohn@...
Date: 2007-02-20T19:38:28+09:00
Subject: Re: Help with Class design

> Thank you all for your help, knowing exactly what "self" refers to in
> each context seems to be the stumbling block for me at the moment. I'll
> have to read about this some more, along with the documentation for some
> of the Enumerables .
>
>> I suppose that you variance method should read something like
>>
>>    def variance
>>       m = mean
>>       @signal.inject(0) { |sum, x| sum + (x-m)**2 } / @signal.length
>>    end
>>
>> Note that I prefer not using meaningless assignment in the inject
>> block.
>
> And to that end, using the suggestions above, I suppose I could equally
> well write :
>
>   def variance()
>     @signal.inject(0) { |var, x| var += (x - mean) ** 2 } /
> (@signal.length-1)
>   end
>
> removing the assignment "m = mean" ?

You can, and I probably would, but you will then be calling the method
"mean" a lot. In fact, for a signal of size n, you will find the mean n
times, which makes your complexity scale n**2.

However, you can make the method "mean" cache, or memoize the value it
calulates:

def mean
  @mean ||= calculate_mean
end

def calculate_mean
  @signal.inject {|s,x| s+x} / @signal.size
end



>
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> Posted via http://www.ruby-forum.com/.
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>