Ruby script to calculate a simple regression curve.(power)

regression_curve_pow.rb

#! /usr/local/bin/ruby
#*********************************************
# Ruby script to calculate a simple regression curve.
# : y = a * x**b
# : 連立方程式を ガウスの消去法で解く方法
#*********************************************
#
class Array
  def reg_curve_pow(y)
    # 以下の場合は例外スロー
    # - 引数の配列が Array クラスでない
    # - 自身配列が空
    # - 配列サイズが異なれば例外
    raise "Argument is not a Array class!"  unless y.class == Array
    raise "Self array is nil!"              if self.size == 0
    raise "Argument array size is invalid!" unless self.size == y.size

    sum_lx   = self.inject(0) { |s, a| s += Math.log(a) }
    sum_lx2  = self.inject(0) { |s, a| s += Math.log(a) * Math.log(a) }
    sum_ly   = y.inject(0) { |s, a| s += Math.log(a) }
    sum_lxly = self.zip(y).inject(0) { |s, a| s += Math.log(a[0]) * Math.log(a[1]) }
    mtx = [
      [self.size,  sum_lx,   sum_ly],
      [   sum_lx, sum_lx2, sum_lxly]
    ]
    ans = solve_ge(mtx)
    {a: Math.exp(ans[0][-1]), b: ans[1][-1]}
  end

  private

  # 連立方程式の解（ガウスの消去法）
  def solve_ge(a)
    n = a.size
    # 前進消去
    (n - 1).times do |k|
      (k + 1).upto(n - 1) do |i|
        d = a[i][k] / a[k][k].to_f
        (k + 1).upto(n) do |j|
          a[i][j] -= a[k][j] * d
        end
      end
    end
    # 後退代入
    (n - 1).downto(0) do |i|
      d = a[i][n]
      (i + 1).upto(n - 1) do |j|
        d -= a[i][j] * a[j][n]
      end
      a[i][n] = d / a[i][i].to_f
    end
    return a
  end
end

# 説明変数と目的変数
#ary_x = [107, 336, 233, 82, 61, 378, 129, 313, 142, 428]
#ary_y = [286, 851, 589, 389, 158, 1037, 463, 563, 372, 1020]
ary_x = [83, 71, 64, 69, 69, 64, 68, 59, 81, 91, 57, 65, 58, 62]
ary_y = [183, 168, 171, 178, 176, 172, 165, 158, 183, 182, 163, 175, 164, 175]
puts "説明変数 X = {#{ary_x.join(', ')}}"
puts "目的変数 Y = {#{ary_y.join(', ')}}"
puts "---"

# 単回帰曲線算出
reg_line = ary_x.reg_curve_pow(ary_y)
puts "a = #{reg_line[:a]}"
puts "b = #{reg_line[:b]}"