Scala Cookbook

11.25. Deleting Elements from Sets

Problem

You want to remove elements from a mutable or immutable set.

Solution

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

Mutable set

When working with a mutable Set, remove elements from the set using the -= and --= methods, as shown in the following examples:

scala> var set = scala.collection.mutable.Set(1, 2, 3, 4, 5)
set: scala.collection.mutable.Set[Int] = Set(2, 1, 4, 3, 5)

// one element
scala> set -= 1
res0: scala.collection.mutable.Set[Int] = Set(2, 4, 3, 5)

// two or more elements (-= has a varags field)
scala> set -= (2, 3)
res1: scala.collection.mutable.Set[Int] = Set(4, 5)

// multiple elements defined in another sequence
scala> set --= Array(4,5)
res2: scala.collection.mutable.Set[Int] = Set()

You can also use other methods like retain, clear, and remove, depending on your needs:

// retain
scala> var set = scala.collection.mutable.Set(1, 2, 3, 4, 5)
set: scala.collection.mutable.Set[Int] = Set(2, 1, 4, 3, 5)

scala> set.retain(_ > 2)

scala> set
res0: scala.collection.mutable.Set[Int] = Set(4, 3, 5)

// clear
scala> var set = scala.collection.mutable.Set(1, 2, 3, 4, 5)
set: scala.collection.mutable.Set[Int] = Set(2, 1, 4, 3, 5)

scala> set.clear

scala> set
res1: scala.collection.mutable.Set[Int] = Set()

// remove
scala> var set = scala.collection.mutable.Set(1, 2, 3, 4, 5)
set: scala.collection.mutable.Set[Int] = Set(2, 1, 4, 3, 5)

scala> set.remove(2)
res2: Boolean = true

scala> set res3: scala.collection.mutable.Set[Int] ...