Class For Calculating the Standard Deviation of Arrays

standard_deviation.rb

require 'minitest/autorun'

# Test case provided by: http://en.wikipedia.org/wiki/Standard_deviation
class TestStandardDeviation < MiniTest::Unit::TestCase
  def setup
    @values = [2, 4, 4, 4, 5, 5, 7, 9]
  end

  def test_average_is_calculated
  	average = @values.inject{ |memo, element| memo + element} / @values.length
    assert_equal 5, average
  end

  def test_mean_is_calculated
  	average = @values.inject{ |memo, element| memo + element} / @values.length
	mean = @values.inject(0){ |memo, element|  memo + ((element - average) ** 2).abs} / @values.length

	assert_equal 4, mean
  end

  def test_standard_deviation_is_calculated
  	average = @values.inject{ |memo, element| memo + element} / @values.length
	mean = @values.inject(0){ |memo, element|  memo + ((element - average) ** 2).abs} / @values.length

	standard_deviation = Math.sqrt(mean)

	assert_equal 2, standard_deviation
  end

  def test_standard_deviation
  	standard_deviation = StandardDeviation.new(@values)
  	assert_equal 2, standard_deviation.calculate
  end
end

class StandardDeviation
	def initialize(values)
		@values = values
	end 

	def calculate
		average = @values.inject{ |memo, element| memo + element} / @values.length
		mean = @values.inject(0){ |memo, element|  memo + ((element - average) ** 2).abs} / @values.length

		Math.sqrt(mean)
	end
end