Added linear regression

README.md

@@ -45,7 +45,7 @@ To implement
 
 ### Regressions ###
 
-- Linear regression
+- <del>Linear regression</del>
 - Multiple linear regression
 - Pearson's correlation
 - Spearman correlation

lib/stats.rb

@@ -5,3 +5,4 @@
 require 'stats/distribution'
 require 'stats/significance'
 require 'stats/assumption'
+require 'stats/regression'

lib/stats/regression.rb

@@ -0,0 +1,26 @@
+module Stats
+  module Regression
+    class << self
+
+      # Performs linear regression on the supplied data using least
+      # squares fit.
+      # 
+      # Returns the results as Hash with keys 'gradient' and 'intercept' which
+      # map to the gradient and intercept respectively.
+      def linear_regression(x, y)
+        return nil if x.length < 2 || y.length < 2 || x.length != y.length
+        sum_x = x.reduce(&:+)
+        sum_y = y.reduce(&:+)
+        sum_xy = x.zip(y).map { |x1, y1| x1 * y1 }.reduce(&:+)
+        sum_x_squared = x.map { |x1| x1 * x1 }.reduce(&:+)
+
+        gradient = (sum_xy - (sum_x * sum_y).to_f / x.length).to_f /  (sum_x_squared - sum_x * sum_x / x.length)
+        intercept = (sum_y - gradient * sum_x).to_f / x.length
+
+        ['gradient' => gradient, 'intercept' => intercept]
+      end
+    end
+  end
+end
+
+

spec/regression_spec.rb

@@ -0,0 +1,36 @@
+require 'spec_helper'
+
+module Stats
+  describe Regression do
+    it "performs linear regression using least squares fit" do
+      x = []
+      y = [1, 2, 3, 4, 5]
+      Regression.linear_regression(x, y).should == nil
+
+      x = [1, 2, 3, 4, 5]
+      y = []
+      Regression.linear_regression(x, y).should == nil
+
+      x = [1]
+      y = [1]
+      Regression.linear_regression(x, y).should == nil
+
+      x = [1, 2, 3, 4, 5]
+      y = [1, 2, 3, 4, 5, 6]
+      Regression.linear_regression(x, y).should == nil
+
+      x = [2, 4, 6, 8]
+      y = [2, 5, 5, 8]
+      Regression.linear_regression(x, y).should == ['gradient' => 0.9, 'intercept' => 0.5]
+
+      x = [1, 2, 3, 4, 5]
+      y = [2, 4, 6, 8, 10]
+      Regression.linear_regression(x, y).should == ['gradient' => 2, 'intercept' => 0]
+
+      x = [0, 1, 2, 3, 4]
+      y = [3, 6, 7, 8, 11]
+      Regression.linear_regression(x, y).should == ['gradient' => 1.8, 'intercept' => 3.4]
+    end
+  end
+end
+