How can I convert immutable.Map to mutable.Map in Scala?

Question

How can I convert immutable.Map to mutable.Map in Scala so I can update the values in Map?

Answer 1

The cleanest way would be to use the mutable.Map varargs factory. Unlike the ++ approach, this uses the CanBuildFrom mechanism, and so has the potential to be more efficient if library code was written to take advantage of this:

val m = collection.immutable.Map(1->"one",2->"Two")
val n = collection.mutable.Map(m.toSeq: _*) 

This works because a Map can also be viewed as a sequence of Pairs.

Answer 2

val myImmutableMap = collection.immutable.Map(1->"one",2->"two")
val myMutableMap = collection.mutable.Map() ++ myImmutableMap

Answer 3

How about using collection.breakOut?

import collection.{mutable, immutable, breakOut}
val myImmutableMap = immutable.Map(1->"one",2->"two")
val myMutableMap: mutable.Map[Int, String] = myImmutableMap.map(identity)(breakOut)

Answer 4

There is a variant to create an empty mutable Map that has default values taken from the immutable Map. You may store a value and override the default at any time:

scala> import collection.immutable.{Map => IMap}
//import collection.immutable.{Map=>IMap}

scala> import collection.mutable.HashMap
//import collection.mutable.HashMap

scala> val iMap = IMap(1 -> "one", 2 -> "two")
//iMap: scala.collection.immutable.Map[Int,java.lang.String] = Map((1,one), (2,two))

scala> val mMap = new HashMap[Int,String] {      
     | override def default(key: Int): String = iMap(key)
     | }
//mMap: scala.collection.mutable.HashMap[Int,String] = Map()

scala> mMap(1)
//res0: String = one

scala> mMap(2)
//res1: String = two

scala> mMap(3)
//java.util.NoSuchElementException: key not found: 3
//  at scala.collection.MapLike$class.default(MapLike.scala:223)
//  at scala.collection.immutable.Map$Map2.default(Map.scala:110)
//  at scala.collection.MapLike$class.apply(MapLike.scala:134)
//  at scala.collection.immutable.Map$Map2.apply(Map.scala:110)
//  at $anon$1.default(<console>:9)
//  at $anon$1.default(<console>:8)
//  at scala.collection.MapLike$class.apply(MapLike.scala:134)....

scala> mMap(2) = "three"

scala> mMap(2)          
//res4: String = three

Caveat (see the comment by Rex Kerr): You will not be able to remove the elements coming from the immutable map:

scala> mMap.remove(1)
//res5: Option[String] = None

scala> mMap(1)
//res6: String = one

Answer 5

Starting Scala 2.13, via factory builders applied with .to(factory):

Map(1 -> "a", 2 -> "b").to(collection.mutable.Map)
// collection.mutable.Map[Int,String] = HashMap(1 -> "a", 2 -> "b")