From: "Älphä Blüë" <jdezenzio@...>
Date: 2009-07-22T07:32:47+09:00
Subject: Standard Deviation question regarding EDOM errors

Okay,

Take a very close look at the two (one is a class, the other is a
method):

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

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

  def product
    @arr.inject {|a,b| a * b }
  end

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

end

def stddev(arr)
  size = arr.to_a.size
  product, sum = 0.0, 0.0
  arr.each do |a|
    sum += a
    product += a*a
  end
  std = Math.sqrt(((product - (sum * sum)/size)/(size-1)))
  return std
end

Testing with both:

a = [123.369, 68.6179]

Stdev.new(a).calculate
=> Errno::EDOM: Domain error - sqrt

stddev(a)
=> 38.7148740874228

Test a web calculator for the same thing:

http://www.easycalculation.com/statistics/standard-deviation.php

The method will pan out.

So, to save suspense the reason why the class is erroring out is because
the sqrt is being attempted on a negative number.  Therefore, in order
to continue to use that I would have to require 'complex' and the return
values from complex return two values in an array.

I'm just curious, why, given both of these being so very similar - the
method passes but the class fails?
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