Scala Book

The Set Class

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    The Scala Set class is an iterable collection with no duplicate elements.

    Scala has both mutable and immutable Set classes. In this lesson we’ll show how to use the mutable class.

    Adding elements to a Set

    To use a mutable Set, first import it:

    val set = scala.collection.mutable.Set[Int]()

    You add elements to a mutable Set with the +=, ++=, and add methods. Here are a few examples:

    set += 1
set += 2 += 3
set ++= Vector(4, 5)

    The REPL shows how these examples work:

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

scala> set += 1
val res0: scala.collection.mutable.Set[Int] = Set(1)

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

scala> set ++= Vector(4, 5)
val res2: scala.collection.mutable.Set[Int] = Set(1, 5, 2, 3, 4)

    Notice that if you try to add a value to a set that’s already in it, the attempt is quietly ignored:

    scala> set += 2
val res3: scala.collection.mutable.Set[Int] = Set(1, 5, 2, 3, 4)

    Set also has an add method that returns true if an element is added to a set, and false if it wasn’t added. The REPL shows how it works:

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

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

    Deleting elements from a Set

    You remove elements from a set using the -= and --= methods, as shown in the following examples:

    scala> val 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 varargs 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()

    There are more methods for working with sets, including clear and remove, as shown in these examples:

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

// clear
scala> set.clear()

scala> set
res0: scala.collection.mutable.Set[Int] = Set()

// remove
scala> val 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)
res1: Boolean = true

scala> set
res2: scala.collection.mutable.Set[Int] = Set(1, 4, 3, 5)

scala> set.remove(40)
res3: Boolean = false

    More Sets

    Scala has several more Set classes, including SortedSet, LinkedHashSet, and more. Please see the Set class documentation for more details on those classes.

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