diff --git a/lib/core_extensions/array.rb b/lib/core_extensions/array.rb
new file mode 100644
index 0000000..a6efeee
--- /dev/null
+++ b/lib/core_extensions/array.rb
@@ -0,0 +1,25 @@
+class Array
+  def classification
+    collect(&:last)
+  end
+
+  # calculate information entropy
+  def entropy
+    return 0 if empty?
+
+    info = {}
+    total = 0
+    each do |i|
+      info[i] = !info[i] ? 1 : (info[i] + 1)
+      total += 1
+    end
+
+    result = 0
+    info.each do |_symbol, count|
+      if count > 0
+        result += -count.to_f / total * Math.log(count.to_f / total) / Math.log(2.0)
+      end
+    end
+    result
+  end
+end
diff --git a/lib/core_extensions/object.rb b/lib/core_extensions/object.rb
new file mode 100644
index 0000000..0b79fd9
--- /dev/null
+++ b/lib/core_extensions/object.rb
@@ -0,0 +1,9 @@
+class Object
+  def save_to_file(filename)
+    File.open(filename, 'w+') { |f| f << Marshal.dump(self) }
+  end
+
+  def self.load_from_file(filename)
+    Marshal.load(File.read(filename))
+  end
+end
diff --git a/lib/decisiontree.rb b/lib/decisiontree.rb
index 99f893f..3da0b47 100644
--- a/lib/decisiontree.rb
+++ b/lib/decisiontree.rb
@@ -1,2 +1,3 @@
 require File.dirname(__FILE__) + '/decisiontree/id3_tree.rb'
-require 'core_extension/array.rb'
+require 'core_extensions/object'
+require 'core_extensions/array'
diff --git a/lib/decisiontree/id3_tree.rb b/lib/decisiontree/id3_tree.rb
index 187b97e..4e4c340 100755
--- a/lib/decisiontree/id3_tree.rb
+++ b/lib/decisiontree/id3_tree.rb
@@ -3,16 +3,6 @@
 ### Copyright (c) 2007 Ilya Grigorik <ilya AT igvita DOT com>
 ### Modifed at 2007 by José Ignacio Fernández <joseignacio.fernandez AT gmail DOT com>
 
-class Object
-  def save_to_file(filename)
-    File.open(filename, 'w+') { |f| f << Marshal.dump(self) }
-  end
-
-  def self.load_from_file(filename)
-    Marshal.load(File.read(filename))
-  end
-end
-
 module DecisionTree
   Node = Struct.new(:attribute, :threshold, :gain)