Ruby Bits: Enumerators

By Luis FerreiraOn April 12, 2016

Welcome back to the world of Enumerable my friend. As Rubyists we are used to spend a lot of time looking at and working with this module (even if you're not entirely aware). This time we will be looking at a somewhat hidden feature of most of its methods, they can be used without a block and have a return value that is itself an enumerator.

Here's how you're probably used to iterating over an Array in Ruby:

[1, 2, 3].each do |num|
  puts num * 2

## => 2
## => 4
## => 6

This is great and definitely should be your first solution for this problem, but in some situation you might find it useful to break this process apart. You might want to do part of iteration and continue it later, or have the array in one place and the method that operates on it somewhere else.

Regardless of your reasons, you can also iterate over an Array like so:

iter = [1, 2, 3].each

def double(num)
  num * 2

puts double( # => 2
puts double( # => 4
puts double( # => 6
puts double( 
  # => StopIteration: iteration reached an end

StopIteration is an exception that gets raised when there is nothing else in the enumerator. This is something that gets hidden away by the block syntax, so that you don't have to deal with it every time.

Other enumerators

What we did with each is applicable to many other methods on Enumerable, such as map, find, max_by and sort_by. I feel that this is a feature in the language that you may never use, but it is good to be aware of.

Real World™ example

In Ruby map is a method that can be used to go over each element of an Enumerable, perform some operation on it and return a new Array with the results of performing that operation. It sounds more complicated that it really is. Here's an example:

result = [1, 2, 3].map do |num|
  num * 2

p result # => [2, 4, 6]

The difference between map and each is that each always returns the object in its initial state, but map returns a new Array with the modifications we wanted done to it.

Now, what if we wanted to do something similar to what we've just done but instead of always multiplying the number by 2, we wanted to multiply it by its index in the Array?

We know there's a method called each_with_index, we could try to use that with an auxiliary Array to store the result of each of the steps. On the other hand, that sound a lot like map, I bet there's a map_with_index we can use! Well, unfortunately there isn't... What if we create our own "map_with_index" by composing map and with_index (another Enumerable method)?

result = [1, 2, 3].map.with_index do |num, index|
  num * index

p result # => [0, 2, 6]

Wow, it worked! And it reads pretty well, but how come we can do this? If we open irb and run this:

> [1, 2, 3].map.class.ancestors

## => [Enumerator, Enumerable, Object, Kernel, BasicObject]

We can see that Enumerator is itself Enumerable, which means it has the with_index method, but more importantly than that, it means we can compose this methods at will. And that my friends is why there's no way not to love the Enumerable module.

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