CODEWITHSUNDEEP
scalahashmap
Alan Coromano
Alan Coromano

Reputation: 26048

Working with immutable HashMap in Scala

I have an immutable HashMap and want to add/remove values from it. The Scala api docs say that I have to use += and -= methods, but they don't work and I get the following error:

error: value += is not a member of scala.collection.immutable.HashMap

How do I add or remove values from a HashMap in Scala?

Upvotes: 4

Views: 12618

Answers (3)

4lex1v
4lex1v

Reputation: 21567

You are watching api for mutable HashMap, to add pair to immutable HashMap use + 

hashMap + ("key", "value")

or if you want to remove use -

hashMap - "key"

But you should remember that it would create a new structure

As for += method, i think that this design is not good, because in such case you have to use var instead of val, and that's not a functional way

Upvotes: 5

senia
senia

Reputation: 38045

There is no method += in immutable collections, but compiler rewrites constructions like a <opname>= b to a = a <opname> b if there is no method <opname>= in a.

var myMap = Map[Int, String]()
myMap += (1, "a")

Last line actually means:

myMap = myMap + (1, "a")

Upvotes: 3

om-nom-nom
om-nom-nom

Reputation: 62855

It does not work because immutable map yields new instance instead of modifying existing one (thus it is immutable). So using val with immutable map is not a legal: 

scala> val foo = Map.empty[Int, Int]
foo: scala.collection.immutable.Map[Int,Int] = Map()

scala> foo += 1 -> 2
<console>:9: error: value += is not a member of scala.collection.immutable.Map[Int,Int]
              foo += 1 -> 2
                  ^

scala> var bar = Map.empty[Int, Int]
bar: scala.collection.immutable.Map[Int,Int] = Map()

scala> bar += 2 -> 2

scala> bar
res2: scala.collection.immutable.Map[Int,Int] = Map(2 -> 2)

If you against using vars, opt to mutable maps.

Upvotes: 1

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