Scala Book

The Map Class

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The 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 we’ll show how to use the mutable 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
)

a nice 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.