standard_deviation.rb

class Array

  # 数列の合計を返す
  def sum
    self.reduce(0){|memo,v|
      memo + v.to_f
    }
  end

  # 数列の平均を返す
  def mean
    return nil if self.size == 0
    self.sum / self.size
  end

  # 数列の偏差の配列を返す
  def deviations
    me = self.mean
    return nil if me.nil?
    self.map {|v|
      v.to_f - me
    }
  end

  # 数列の分散を返す
  def variance
    devs = self.deviations
    return nil if devs.nil? or devs.size == 0
    dev_sum = devs.reduce(0) {|memo, v| memo + (v**2)}
    dev_sum / devs.size
  end

  # 数列の標準偏差を返す
  def standard_deviation
    variance = self.variance
    return nil if variance.nil?
    Math.sqrt variance
  end

end