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Ruby Enumerable Built-ins



By the end of this, developers should be able to:

  • Define “list” and give two examples of lists in Ruby.
  • Diagram the flow of Enumerable methods as invoked through concrete classes.
  • Give two examples of methods defined in Enumerable and available on all three of Range, Array, and Hash.


  1. Fork and clone this repository. FAQ
  2. Create a new branch, training, for your work.
  3. Checkout to the training branch.
  4. Install dependencies with bundle install.


A list is an abstract data type (ADT) that represents an ordered collection of items. The list may be empty. If not empty, the list has a first item followed by a list containing the rest of the items. This is not a rigorous definition.

Enumerable is Ruby's implementation of the list abstraction.

What are some types of lists?

What are some things we'd put on those lists?

Arrays as lists

In JavaScript, the concept of list is built into Array. In Ruby, it is built into Enumerable which is included in Ruby's Array.

The following table contains a mapping of some of the methods that potentially touch all the elements in an Array. We've already covered most of these in Ruby Array Methods.

JavaScript Ruby
every all?
filter select
forEach each
map map
reduce reduce
some any?

But, wait. Where is the method all? defined? The method reduce? They're from the Enumerable mix-in, and will be the focus of this talk. "Enumerable" is another word for "iteratable", so we can say that each of Ruby's Array, Range, and Hash types behave as something that can be iterated over.

The Enumerable Module

Ruby's Enumerable module provides many list processing methods relying on a class's each method. Ruby's Array class includes the Enumerable module.


In Ruby, modules serve two purposes. The first is to create name spaces. The second is to supply common behavior to a class.

The Math module hides mathematical functions inside the name-space Math so that short and common names don't pollute the global name-space (e.g. Math::PI or Math.cos).

The Enumerable module contains code implementing list methods in terms of a concrete class's each method.

The connection

Let's diagram the delegation from Array to Enumerable and back.

Ranges as lists

Ruby's Range class provides a convenient way to express a sequence of integers. Range includes Enumerable so we can treat instances as a list.

Demo - Range

rng = 1..10

Code along - Range

Let's explore using Range as an enumerable in bin/range_list.rb.

Lab - Range

In bin/range_lab.rb, use reduce to calculate the sum of all the even numbers in a Range. All the odd numbers. Now use each_with_object to do both at once.

Hint: Better Hash Injection using each_with_object

Hashes as lists

Hash includes Enumerable so we can treat it as a list.

Demo - Hash

hash = {'1':'one','2':'two'}
# => {:"1"=>"one", :"2"=>"two"}

hash.each {|key, value| puts "#{key} and #{value}"}
# 1 and one
# 2 and two
# => {:"1"=>"one", :"2"=>"two"}

Code along - Hash

Let's explore using Hash as an enumerable in bin/hash_list.rb.

Lab - Hash

In bin/hash_lab.rb, use reduce to accumulate all of the keys and values in a Hash as Arrays. Store these keys and values in a memo Hash with the keys :keys and :values.

So if the input hash was:

hash = {:a => 1, :b => 2, :c => 3}

...your ouput should look like:

memo = {:keys => [:a, :b, :c], :values => [1, 2, 3]}

Run the linter after you complete the reduce task. Notice a curious linter warning? Fix all the rest, then continue. Now use each_with_object to do the same.

Additional resources

Two images to give you a sense of the relationships in Ruby.

These images may diverge slightly from the actual relationships, Ruby is an evolving language, but do give a sense of much of what goes on.


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