"Enumerable" is Ruby's way of saying that we can get elements out of a collection, one at a time.

e·nu·mer·ate /i?n(y)o?om??r?t/ Verb:

1. Mention (a number of things) one by one.
2. Establish the number of.

An "enumerator", then, a the tool we can use to get each element out of a collection in this way. Before we go ahead, it's important to understand what Enumerable and Enumerator are in Ruby terms. Let's look at some examples.

First, calling each on the Array [4, 8] in this next example, returns an Enumerator object.

This is different from how we've been usually using each.

Enumerable is a module used as a mixin in the Array class. It provides a number of enumerators like map, select, inject. The Enumerable module itself doesn't define the each method. It's the responsibility of the class that is including this module to do so.

This Array class, consequently, defines the each method. It returns an object of the type Enumerator when no block is given (like our first example). It yields a value of the type self when there is one (the original array in the next example).

What's the point of returning the Enumerator then?

Enumerator is an objectification of enumeration. The point of these methods returning these enumerators is to allow us to chain operations indefinitely and make more heavy-duty collections.

Here we chain the initial each_with_index enumerator with select to build an enumerator that returns an array of elements whose values are smaller than their index positions.

Try and implement a simplemap_with_index on the Array class through which you can call a block with two arguments: the element and its index. It should return an Enumerator object if no block is given, an Array otherwise. You are good to delve deeper into the Enumerable class and learn how and when to make use of these collections.