Ruby Sets – Examples, Operators & Methods

What is a Ruby set?

A set is a class that stores items like an array…

But with some special attributes that make it 10x faster in specific situations!

On top of that:

All the items in a set are guaranteed to be unique.

In this Ruby tutorial you’ll learn:

  • How & when to use a set for your maximum benefit
  • The difference between sets & arrays!
  • A list of useful set methods

Let’s get started!

Code Examples

A set is a Ruby class that helps you create a list of unique items.

Here’s an example of how this is useful:

Let’s say you are going over a big list of products.

But there are duplicated entries in this list & you only want unique products.

You could put them in a set, and the set will make sure your product list is always unique without any extra work.

Here’s how to do that:

require 'set'

products =

products << 1
products << 1
products << 2

# Set: {1, 2}

Another benefit is that searching this list is going to be very fast:


# true

This is so fast because the search is made in constant time.

Set vs Array – Understanding The Difference

Now you may be wondering…

What’s the difference between a set & an array?

A set has no direct access to elements:


# undefined method `[]'

That’s the main difference.

But a set can be converted into an array any time you need:


# [1, 2]

The whole point of using a set is to use its two special attributes:

  • Fast lookup times (with include?)
  • Unique values

If you need these then a set will give you a good performance boost, and you won’t have to be calling uniq on your array every time you want unique elements.

Set vs Array Benchmark

Here’s a benchmark to show you the performance difference between array & set include? methods.

# Ruby 2.5.0

set   include: 8381985.2 i/s
array include: 703305.5  i/s - 11.92x  slower

The reason for this difference is that an array has to check every single element!

If you have a 1 million element array it’s going to be checking 1 million elements every time you call include?.

A set doesn’t have to do that.

Ruby Set Methods

There is a number of operations or methods that can be done between sets, if you have some math background you may be able to recognize them.

The Union Operator

One of the useful set methods is the union operator:

products | (1..10)

# Set: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

This operator allows you to combine all of the unique elements of two sets into one set.

Btw, this works with any Enumerable object, like arrays, ranges & hashes, not only objects from the Set class.

The Difference Operator

If you want to remove elements of one set from the other set then this is your method.

Here’s an example:

products - (3..4)

# Set: {1, 2, 5, 6, 7, 8, 9, 10}

The Intersection Operator

Here’s the set intersection operator:

A =
B =

A & B # {2, 3}

This gives you the elements common in both sets. These 3 operators (union, difference & intersection) can also be used with arrays.


Superset & Subset

A superset is a set that contains all the elements of another set.

You can check if one set is a superset of another.

Like this: >=

The range 10..40 contains 20..30 within it.

A subset is a set that is made from parts of another set: <=

How to Use Sorted Sets

If you want a set that always stays sorted you can use the SortedSet class.

There are some requirements to using this class:

  1. The objects you are adding to the set must implement the <=> method.
  2. The objects must be comparable to each other (comparing integer to integer, or string to string)

Here’s an example:

sorted_numbers =

sorted_numbers << 5
sorted_numbers << 2
sorted_numbers << 1

# SortedSet: {1, 2, 5}

Sets Video Tutorial

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You learned how to use sets in Ruby for better performance & easier coding. You also learned about the differences between arrays & sets.

Please share this article if you found it useful so more people can find it 🙂

Thanks for reading!