CODEWITHSUNDEEP
scalaindexingsetimmutabilitymutable
Enthusiast Coder
Enthusiast Coder

Reputation: 51

How to change elements of set in scala

In scala, you can have 2 types of set where the elements are immutable or mutable, But as you cannot index these sets, so how can you change the elements of the latter sets??

Upvotes: 0

Views: 1111

Answers (1)

Jörg W Mittag
Jörg W Mittag

Reputation: 369604

In scala, you can have 2 types of set where the elements are immutable or mutable

That is not what the distinction between scala.collection.immutable.Set and scala.collection.mutable.Set is. It is not about mutability or immutability of the elements, it is about mutability or immutability of the sets themselves.

But as you cannot index these sets, so how can you change the elements of the latter sets

It is not about changing the elements of mutable sets. It is about changing the mutable sets themselves:

import scala.collection.mutable.{  Set => MSet}
import scala.collection.immutable.{Set => ISet}


val mset = MSet(1, 2, 3)
val iset = ISet(1, 2, 3)

mset += 4
mset //=> HashSet(1, 2, 3, 4): scala.collection.mutable.Set[Int]

val iset2 = iset + 4
iset //=> Set(1, 2, 3): scala.collection.immutable.Set[Int]
iset2 //=> Set(1, 2, 3, 4): scala.collection.immutable.Set[Int]

iset += 4
// ERROR: value += is not a member of scala.collection.immutable.Set[Int]

[Scastie link]

The difference between the two is that you can't add an element to an immutable set. Instead, when you call the scala.collection.immutable.Set.+(elem: A): Set[A] method, it will return a new set (iset2 in this case) that has the same elements as the original set (iset) plus the element we wanted to add.

Whereas the scala.collection.mutable.Set.+=(elem: A): Set.this.type method returns the same set (and in fact, as you can see in my example above, I actually ignore the return value) and mutates it to contain the additional element.

In Scala, the idea of a set is much closer to the mathematical idea of sets instead of as a collection. It is not a collection that contains the elements, rather it is a predicate function that you can ask "is this element a member of the set"?

Upvotes: 3

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