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Understanding Ruby - Enumerable - Grouping

baweaver profile image Brandon Weaver ・6 min read


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.


Ruby is an interesting group of folks and code, so interesting that they even have methods to group collections as well.


partition will split elements into two distinct groups depending on a condition:

[1, 2, 3, 4].partition(&:even?)
# => [[2, 4], [1, 3]]
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It's very useful to use when group_by doesn't quite make sense and you only have two distinct groups to group elements against. Combined with destructuring assignments and that can be really useful:

evens, odds = [1, 2, 3, 4].partition(&:even?)

{ evens: evens, odds: odds }
# => {:evens=>[2, 4], :odds=>[1, 3]}
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Remember, in Ruby use the method with the least power to get the result you want. partition is a good example of that.


chunk is somewhat like group_by or partition except in that it returns back chunked groups of elements:

[3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5].chunk(&:even?).to_a
# => [[false, [3, 1]], [true, [4]], [false, [1, 5, 9]], [true, [2, 6]], [false, [5, 3, 5]]]
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It's more useful in the context of a sorted list, but in that case partition makes much more sense for Boolean like conditions, and group_by more for multiple.

chunk is a method that I do not encounter frequently in my programming, but has exceptional use when dealing with output logs like Subversion produces, or other unix-like formats.

Note: You might notice to_a on a few of these methods, that's because they return an Enumerator rather than an Array, and tend to be used by chaining together methods rather than by themselves.


chunk_while, on the other hand, I do find uses for, especially in interviews:

[1,2,4,9,10,11,12,15,16,19,20,21].chunk_while { |i, j| i+1 == j }.to_a
# => [[1, 2], [4], [9, 10, 11, 12], [15, 16], [19, 20, 21]]
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In the case of the example it can find contiguous chunks of numbers. The Block Function it takes will expose the element before and after the current element, giving it more power than chunk to decide on how to chunk elements together.

Problems like greatest ascending chain of numbers, contiguous groups, and others can be very easy to solve if you remember chunk_while.


each_cons is short for each consecutive:

# => [[1, 2, 3], [2, 3, 4], [3, 4, 5], [4, 5, 6], [5, 6, 7], [6, 7, 8], [7, 8, 9], [8, 9, 10]]
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This is exceptionally useful for sliding window type algorithms, and very frequently I find myself using it in interviews so I practice a lot with Enumerable before I have to go back on the hunt.

It can take a block, but like some others it returns nil afterwards. If you want to immediately print it, sure, but otherwise it doesn't make too much sense to use without chaining to another Enumerable method.


each_slice is similar to each_cons except in that it will return back distinct slices rather than a sliding window of elements like each_cons:

# => [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10]]
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It's useful for getting elements in groups of a certain size to work with. Sometimes you can even pair it with to_h or zip to do interesting things:

(1..10).each_slice(3).to_h { |k, *vs| [k, vs] }
# => {1=>[2, 3], 4=>[5, 6], 7=>[8, 9], 10=>[]}
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After all, Enumerable is very much about how you combine the methods together to make something greater.


slice_before will slice a collection right before a condition that returns true is met:

gemfile = <<~GEMFILE
  # frozen_string_literal: true

  source ""

  # Specify your gem's dependencies in matchable.gemspec

  # Other gems
  gem "rake", "~> 13.0"

  # Testing
  gem "rspec", "~> 3.0"
  gem "guard-rspec"
  gem "benchmark-ips"

  .slice_before { |v| v.start_with?('#') }
# => ["# frozen_string_literal: true\n", "# Specify your gem's dependencies in matchable.gemspec\n", "# Other gems\n", "# Testing\n"]
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In this case we're looking at a Gemfile for Ruby and trying to split right before each comment line, and then just grabbing the commend line that started it off. Sometimes text files don't quite have a nice format like JSON, and this is a good way to break it into something you can use.

It also takes a pattern (responds to ===), which might make the above code more concise:

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slice_after is like slice_before, except it slices after a condition is true:

text = <<~TEXT
  Some paragraphs and content here.

    ...and then a few more after that

=> [["  Some paragraphs and content here.\n", "\n"], ["...and then a few more after that\n"]]
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Like slice_before you can use a pattern or a Block Function:

(1..10).slice_after { |e| e.even? }.to_a
# => [[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]]
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slice_when is effectively the opposite of chunk_while in that it will slice when the Block Function returns true rather than when it returns false:

[1,2,4,9,10,11,12,15,16,19,20,21].slice_when { |i, j| i+1 == j }.to_a
# => [[1], [2, 4, 9], [10], [11], [12, 15], [16, 19], [20], [21]]

[1,2,4,9,10,11,12,15,16,19,20,21].chunk_while { |i, j| i+1 == j }.to_a
# => [[1, 2], [4], [9, 10, 11, 12], [15, 16], [19, 20, 21]]
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In the case of slice_when it will slice when a condition is met rather than right after. Like chunk_while it takes a before and after element.


group_by allows you to group elements by a Block Function, and that function defines the key. Consider our text from earlier:

%w(a fresh lively lemur jumps over a tea kettle).group_by { |w| w[0] }
# => {"a"=>["a", "a"], "f"=>["fresh"], "l"=>["lively", "lemur"], "j"=>["jumps"], "o"=>["over"], "t"=>["tea"], "k"=>["kettle"]}
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We can group by the first character of every word, getting the groups back as Arrays. This is a method I use very frequently when I'm not after the count with tally.

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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