The Scala Map Class

The Scala Map class documentation describes a Map as an iterable sequence that consists of pairs of keys and values. A simple Map looks like this:

val states = Map(
    "AK" -> "Alaska",
    "IL" -> "Illinois",
    "KY" -> "Kentucky"
)

Scala has both mutable and immutable Map classes. In this lesson I’ll show how to use the mutable class. (Please see the Scala Cookbook for examples of the immutable Map class.)

Creating a mutable Map

To use the mutable Map class, first import it:

import scala.collection.mutable.Map

Then you can create a Map like this:

val states = collection.mutable.Map("AK" -> "Alaska")

Adding elements to a Map

Now you can add a single element to the Map with +=, like this:

states += ("AL" -> "Alabama")

You also add multiple elements using +=:

states += ("AR" -> "Arkansas", "AZ" -> "Arizona")

You can add elements from another Map using ++=:

states ++= Map("CA" -> "California", "CO" -> "Colorado")

The REPL shows how these examples work:

scala> val states = collection.mutable.Map("AK" -> "Alaska")
states: scala.collection.mutable.Map[String,String] = Map(AK -> Alaska)

scala> states += ("AL" -> "Alabama")
res0: states.type = Map(AL -> Alabama, AK -> Alaska)

scala> states += ("AR" -> "Arkansas", "AZ" -> "Arizona")
res1: states.type = Map(AZ -> Arizona, AL -> Alabama, AR -> Arkansas, AK -> Alaska)

scala> states ++= Map("CA" -> "California", "CO" -> "Colorado")
res2: states.type = Map(CO -> Colorado, AZ -> Arizona, AL -> Alabama, CA -> California, AR -> Arkansas, AK -> Alaska)

Removing elements from a Map

You remove elements from a Map using -= and --= and specifying the key values, as shown in the following examples:

states -= "AR"
states -= ("AL", "AZ")
states --= List("AL", "AZ")

The REPL shows how these examples work:

scala> states -= "AR"
res3: states.type = Map(CO -> Colorado, AZ -> Arizona, AL -> Alabama, CA -> California, AK -> Alaska)

scala> states -= ("AL", "AZ")
res4: states.type = Map(CO -> Colorado, CA -> California, AK -> Alaska)

scala> states --= List("AL", "AZ")
res5: states.type = Map(CO -> Colorado, CA -> California, AK -> Alaska)

Updating Map elements

You update Map elements by reassigning their key to a new value:

states("AK") = "Alaska, A Really Big State"

The REPL shows the current Map state:

scala> states("AK") = "Alaska, A Really Big State"

scala> states
res6: scala.collection.mutable.Map[String,String] = Map(CO -> Colorado, CA -> California, AK -> Alaska, A Really Big State)

Traversing a Map

There are several different ways to iterate over the elements in a map. Given a sample map:

val ratings = Map(
    "Lady in the Water"-> 3.0, 
    "Snakes on a Plane"-> 4.0,
    "You, Me and Dupree"-> 3.5
)

my preferred way to loop over all of the map elements is with this for loop syntax:

for ((k,v) <- ratings) println(s"key: $k, value: $v")

Using a match expression with the foreach method is also very readable:

ratings.foreach {
    case(movie, rating) => println(s"key: $movie, value: $rating")
}

The ratings map data in this example comes from the old-but-good book, Programming Collective Intelligence.

See also

There are other ways to work with Scala Maps, and a nice collection of Map classes for different needs. See the Scala Cookbook for many more examples.