From: "Älphä Blüë" <jdezenzio@...>
Date: 2009-07-22T08:21:33+09:00
Subject: Re: Standard Deviation question regarding EDOM errors

Hi David,

Thanks for showing me my error.  So, I implemented a change in the class 
for testing purposes:

  class Stdev
    def initialize(arr)
      @arr = arr
      @size = @arr.to_a.size
    end

    def sum
      @arr.inject {|a,b| a + b }
    end


    def product
      total = 0.0
      @arr.each {|a| total += a * a }
      total
    end

    def calculate
      Math.sqrt(((product - (sum * sum)/@size)/(@size-1)))
    end

  end

Running the same test with the same things:

a = [1,2,3,4,5]
b = [1,2]
c = [123.369, 68.6179]

stddev(a)
=> 1.58113883008419  (correct)

stddev(b)
=> 1.0          (NOT expected)

stddev(c)
=> 38.7148740874228  (correct)

So, compared to the class now, versus the method I created, do you see 
any anomolies that would change this behavior?


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