Add Enumerable#average

README.md

@@ -25,6 +25,7 @@ Or install it yourself as:
     * [#butlast](http://rdoc.info/github/bbatsov/powerpack/Array#butlast-instance_method)
     * [#tail](http://rdoc.info/github/bbatsov/powerpack/Array#tail-instance_method)
 * [Enumerable](http://rdoc.info/github/bbatsov/powerpack/Enumerable)
+    * [#average](http://rdoc.info/github/bbatsov/powerpack/Enumerable#average-instance_method)
     * [#drop_last](http://rdoc.info/github/bbatsov/powerpack/Enumerable#drop_last-instance_method)
     * [#drop_last_while](http://rdoc.info/github/bbatsov/powerpack/Enumerable#drop_last_while-instance_method)
     * [#exactly?](http://rdoc.info/github/bbatsov/powerpack/Enumerable#exactly?-instance_method)

lib/powerpack/enumerable.rb

@@ -1,3 +1,4 @@
+require_relative 'enumerable/average'
 require_relative 'enumerable/drop_last'
 require_relative 'enumerable/drop_last_while'
 require_relative 'enumerable/exactly'

lib/powerpack/enumerable/average.rb

@@ -0,0 +1,20 @@
+unless Enumerable.method_defined? :average
+  module Enumerable
+    # Calculates the average of a numeric collection.
+    #
+    # @param default [Object] an optional default return value if there are no elements.
+    #   It's nil by default.
+    # @return The average of the elements or the default value if there are no
+    #   elements.
+    #
+    # @example
+    #   [1, 2, 3].average #=> 2
+    #   [1, 2, 3, 4].average #=> 2.5
+    #   [].average #=> nil
+    #   [].average(0) #=> 0
+    def average(default = nil)
+      coll_size = size
+      coll_size > 0 ? reduce(&:+) / coll_size.to_f : default
+    end
+  end
+end

spec/powerpack/enumerable/average_spec.rb

@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe 'Enumerable#average' do
+  it 'calculates the average of an enum' do
+    expect((1..3).average).to eq(2)
+  end
+
+  it 'calculates the average of an array' do
+    expect([1, 2, 3, 4].average).to eq(2.5)
+  end
+
+  it 'returns nil when invoked on an empty collection' do
+    expect([].average).to be_nil
+  end
+
+  it 'returns default value when invoked on an empty collection' do
+    expect([].average(0)).to be_zero
+  end
+end