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Brandon Weaver
Brandon Weaver

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Understanding Ruby - Enumerable - Combining


Enumerable. Debatably one of, if not the, most powerful features in Ruby. As a majority of your time in programming is dealing with collections of items it's no surprise how frequently you'll see it used.



Some knowledge required of functions in Ruby. This post focuses on foundational and fundamental knowledge for Ruby programmers.

Prerequisite Reading:


Enumerable is an interface module that contains several methods for working with collections. Many Ruby classes implement the Enumerable interface that look like collections. Chances are if it has an each method it supports Enumerable, and because of that it's quite ubiquitous in Ruby.

Note: This idea was partially inspired by Lamar Burdette's recent work on Ruby documentation, but takes its own direction.


When grouping and counting won't quite do it's time to start combining us some elements. Ruby provides various methods for combining Enumerables together, or perhaps even combining them all down into one element. Either way, some of my favorite methods are in here.


chain allows you to combine two Enumerators, which can be useful if you want to combine two Enumerables, like say we had our range above for card ranks:

RANKS = %w(2 3 4 5 6 7 8 9 10 J Q K A).freeze
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Instead of typing that all out we could do this:

('2'..'10').chain(%w(J Q K A))
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This can also take multiple Enumerables as arguments.

I haven't often used chain, but have found a few minor usages of it on occasion when dealing with a lot of Enumerators.


cycle lets you take a single collection and make it loop infinitely:

[1, 2, 3].cycle.first(20)
# => [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2]
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If given an argument it will only cycle that many times:

[1, 2, 3].cycle(3).to_a
# => [1, 2, 3, 1, 2, 3, 1, 2, 3]
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If you don't give it a Block Function it'll return an Enumerator so you won't actually be allocating infinite cycles. We'll be getting more into Enumerator in another post.

cycle is another function I don't frequently use, but can be useful for zip.

#reduce / #inject

reduce is interesting in that it's so powerful you could literally write any other Enumerable method using it. I even did a conference talk on this once. We won't get into that for now, but the idea is that it allows you to reduce a collection into one element. It's also called fold or foldLeft in other programming languages.

Its usual example is a sum or product:

[1, 2, 3].reduce(0) { |a, i| a + i }
# => 6

[1, 2, 3].reduce(1) { |a, i| a * i }
# => 6
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reduce takes an optional argument as an initial accumulator, and if one isn't given it uses the first item of the collection. It then takes a Block Function which takes two arguments, the accumulator and the item. The result of each call to reduce becomes the new accumulator the next loop. Sound like a mouthful? It is, let's look at an example:

[1, 2, 3].reduce(0) do |a, i|
  puts(a: a, i: i, new_a: a + i)
  a + i
# STDOUT: {:a=>0, :i=>1, :new_a=>1}
# STDOUT: {:a=>1, :i=>2, :new_a=>3}
# STDOUT: {:a=>3, :i=>3, :new_a=>6}
# => 6
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Note: puts(k: value) is one of my favorite debugging and example tricks. The Hash braces are implied, and it gives extra information to help finding issues.

So we can see that when the reduce function starts it has an accumulator of 0, a first item of 1, and that function returns 1 which becomes the next accumulator for the next loop. Eventually it runs out of items and 6 was the last value of the accumulator.

An interesting observation is that empty value doesn't need to be a number. What if it were a String? Hash? Boolean, Array, etc etc. Then it gets real interesting. Consider map reimplemented in terms of reduce:

def map(collection, &fn)
  collection.reduce([]) { |a, v| a << }
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reduce is insanely powerful, but at the same time there are less powerful methods which do the same job with less effort. Prefer methods which are more tailored for your task, like sum or tally.


each_with_object is like a reversed reduce, except that the return value of each call to the Block Function is ignored and it only cares about the object it was iterating with:

[1, 2, 3].each_with_object({}) { |i, a| a[i.to_s] = i }
# => {"1"=>1, "2"=>2, "3"=>3}
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Oh, and make sure to mutate a because otherwise nothing will happen. Notice as well that the arguments are reversed. The way I remember this is the with_object after each, implying the object is the second argument. I still get that backwards more often than I'd care to admit.


zip allows us to combine two or more collections into one:

a = [1, 2, 3]
b = [2, 3, 4]
c = [3, 4, 5], c)
# => [[1, 2, 3], [2, 3, 4], [3, 4, 5]]
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zip can be useful for merging multiple collections into one, especially when you have things like keys and values as separate variables you need to put together.

It can also take a Block Function which specifies how to zip values:

a = [1, 2, 3]
b = [2, 3, 4]
c = [3, 4, 5], c) { |x, y, z| [z, y - x] }
# => nil
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Oddly this returns nil and you have to use an outside array to capture these values. I cannot say I understand this as I might have expected this to behave like map, but it is as it is. Given that I would suggest avoiding this syntax, as it may be confusing.

Wrapping Up

The next few articles will be getting into the various parts of Enumerable, grouped by functionality:

  1. Transforming
  2. Predicate Conditions
  3. Searching and Filtering
  4. Sorting and Comparing
  5. Counting
  6. Grouping
  7. Combining
  8. Iterating and Taking
  9. Coercion

While lazy is part of Enumerable that deserves a post all its own, and we'll be getting to that one soon too.

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