Data Structures in Clojure and Elixir: Sets

My last post talked about Lists, Tuples and Vectors comparing Elixir and Clojure … now lets cover a related topic.. sets.

When you think of set, think of Math Sets.


There are two ways to create a Set in Elixir

Using the Pipe Operator and put:

set = |> MapSet.put("apple") |> MapSet.put("apple") |> MapSet.put("banana")
#MapSet<["apple", "banana"]>

Or use a List and pass that to

iex(1)> set =["apple","orange","grape", "grape"])
#MapSet<["apple", "grape", "orange"]>

I purposely put duplicates when I created it to see what would happen. It quietly dismissed the duplicate value and made a collection of unique values.

Passing a list to new would be a good way to filter out duplicates in a collection as well as build a set more easily.

iex(3)> unique_values =["apple","orange","grape", "grape"]) |> MapSet.to_list()
["apple", "grape", "orange"]

Some more functions for sets

iex(4)> fruit =["apple","orange","grape"])
#MapSet<["apple", "grape", "orange"]>

iex(5)> MapSet.member?(fruit, "apple") true

and subset

iex(4)> fruit =["apple","orange","grape"])
#MapSet<["apple", "grape", "orange"]>

iex(6)> other_fruit =["apple", "grape"]) #MapSet<["apple", "grape"]>

iex(8)> MapSet.subset?(other_fruit, fruit) true

Read more on MapSet


You have two ways to make a set in Clojure, using the set function and the #{} literal.

user=> (set [:apple :grape :orange :orange])
#{:orange :apple :grape}

user=> (set '(:apple :grape :orange :orange) #{:orange :apple :grape}

We are converting a vector or a list to a set.

Also testing putting duplicates and as expected, the result is all unique values.

user=> (def taco-restaurants #{"torchys" "maudies" "torchys" "taco bell"})

IllegalArgumentException Duplicate key: torchys clojure.lang.PersistentHashSet.createWithCheck ( RuntimeException Unmatched delimiter: ) clojure.lang.Util.runtimeException (

No, the error is not calling taco bell a restaurant, it is putting torchys twice. YIKES… the literal syntax does not like duplicates. If you think your values might have duplicates, put in vector first and pass to set.

Edit: Alex Miller pointed out, the reason we have an error it because it is an invalid set. Now I think it should error and since Elixir doesn't have a literal syntax it is not exactly the same thing to compare. Thanks for pointing that out why it errors Alex :)

Some useful functions for sets:

user=> (def valid-actions #{:get :post})

user=> (contains? valid-actions :post) true

user=> (contains? valid-actions :head) false

Use a set when you need to see if a certain value is one of X. It is surprisingly not easy to do the same with checking membership in a list or vector :) Use a set, you don't need duplicates anyways so it makes sense.

There is a Set namespace with functions for sets:

user=> (require '[clojure.set :as set])

user=> (set/subset? #{:post} valid-actions) true

user=> (set/subset? #{:head} valid-actions) false

Next, we'll cover Maps!