Separate array and object extensions from id3_tree

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