Stochastic Array Aggregation Methods

stochastic_array_extensions.rb

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

  def median
    values = self.sort
    if length % 2 == 0 # even
      center = length / 2
      (values[center - 1] + values[center]) / 2.0
    else # odd
      center = (length + 1) / 2
      values[center]
    end
  end

  def quantile(percent)
    raise ArgumentError.new 'Out of range' if percent < 0 or percent > 1
    values = self.sort
    index = (percent * length).ceil
    values[index - 1]
  end

  def standard_variance
    Math.sqrt variance
  end

  def variance
    numerator = self.map { |x| x - average }.map { |x| x**2 }.reduce(:+)
    denominator = (length - 1).to_f
    numerator / denominator
  end
end