How to add elements to a Set in Scala (operators, methods)

This is an excerpt from the 1st Edition of the Scala Cookbook (partially modified for the internet). This is Recipe 11.26, “How to Add Elements to a Set in Scala”


You want to add elements to a mutable set, or create a new set by adding elements to an immutable set.


Mutable and immutable sets are handled differently, as demonstrated in the following examples.

Mutable set

Add elements to a mutable Set with the +=, ++=, and add methods:

// use var with mutable
scala> val set = scala.collection.mutable.Set[Int]()
set: scala.collection.mutable.Set[Int] = Set()

// add one element
scala> set += 1
res0: scala.collection.mutable.Set[Int] = Set(1)

// add multiple elements
scala> set += (2, 3)
res1: scala.collection.mutable.Set[Int] = Set(2, 1, 3)

// notice that there is no error when you add a duplicate element
scala> set += 2
res2: scala.collection.mutable.Set[Int] = Set(2, 6, 1, 4, 3, 5)

// add elements from any sequence (any TraversableOnce)
scala> set ++= Vector(4, 5)
res3: scala.collection.mutable.Set[Int] = Set(2, 1, 4, 3, 5)

scala> set.add(6)
res4: Boolean = true

scala> set.add(5)
res5: Boolean = false

The last two examples demonstrate a unique characteristic of the add method on a set: It returns true or false depending on whether or not the element was added to the set. The other methods silently fail if you attempt to add an element that’s already in the set.

You can test to see whether a set contains an element before adding it:


But as a practical matter, I use += and ++=, and ignore whether the element was already in the set.

Whereas the first example demonstrated how to create an empty set, you can also add elements to a mutable set when you declare it, just like other collections:

scala> val set = scala.collection.mutable.Set(1, 2, 3)
set: scala.collection.mutable.Set[Int] = Set(2, 1, 3)

Immutable Set in Scala

The following examples show how to create a new immutable set by adding elements to an existing immutable Scala Set.

First, create an immutable Set:

scala> val s1 = Set(1, 2)
s1: scala.collection.immutable.Set[Int] = Set(1, 2)

Create a new set by adding elements to a previous set with the + and ++ methods:

// add one element
scala> val s2 = s1 + 3
s2: scala.collection.immutable.Set[Int] = Set(1, 2, 3)

// add multiple elements (+ method has a varargs field)
scala> val s3 = s2 + (4, 5)
s3: scala.collection.immutable.Set[Int] = Set(5, 1, 2, 3, 4)

// add elements from another sequence
scala> val s4 = s3 ++ List(6, 7)
s4: scala.collection.immutable.Set[Int] = Set(5, 1, 6, 2, 7, 3, 4)

I showed these examples with immutable variables just to be clear about how the approach works. You can also declare your variable as a var, and reassign the resulting set back to the same variable:

scala> var set = Set(1, 2, 3)
set: scala.collection.immutable.Set[Int] = Set(1, 2, 3)

scala> set += 4

scala> set
res0: scala.collection.immutable.Set[Int] = Set(1, 2, 3, 4)

See Recipe 10.6, “Understanding Mutable Variables with Immutable Collections” for more information on the difference between mutable/immutable variables and mutable/immutable collections.