gistfile1.rb

require "test/unit"

class VarianceTest < Test::Unit::TestCase

  def test_it_should_compute_the_average_out_of_an_array
    assert_equal(50, [40, 30, 50, 80].average)
  end

  def test_it_should_compute_variance_out_of_an_array
    assert_equal(350, [40, 30, 50, 80].variance)
  end

end

class Array
  def self.average(array)
    array.reduce(:+).to_f / array.size
  end

  def average
    self.class.average self
  end

  def variance
    self.class.average map { |n| (n - average) ** 2 }
  end
end