map-class.md

type	layout	title	description	partof	overview-name	num	outof	previous-page	next-page	new-version
section	multipage-overview	The Map Class	This page provides examples of the Scala 'Map' class, including how to add and remove elements from a Map, and iterate over Map elements.	scala_book	Scala Book	32	54	vector-class	set-class	/scala3/book/collections-classes.html#maps

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:

valstates=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:

importscala.collection.mutable.Map

Then you can create a Map like this:

valstates= 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>valstates= 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, AReallyBigState)

Traversing a Map

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

valratings=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.

See also

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