Scala Cookbook

11.14. Adding, Updating, and Removing Elements with a Mutable Map

Problem

You want to add, remove, or update elements in a mutable map.

Solution

Add elements to a mutable map by simply assigning them, or with the += method. Remove elements with -= or --=. Update elements by reassigning them.

Given a new, mutable Map:

scala> var states = scala.collection.mutable.Map[String, String]()
states: scala.collection.mutable.Map[String,String] = Map()

You can add an element to a map by assigning a key to a value:

scala> states("AK") = "Alaska"

You can also add elements with the += method:

scala> states += ("AL" -> "Alabama")
res0: scala.collection.mutable.Map[String,String] =
  Map(AL -> Alabama, AK -> Alaska)

Add multiple elements at one time with +=:

scala> states += ("AR" -> "Arkansas", "AZ" -> "Arizona")
res1: scala.collection.mutable.Map[String,String] =
  Map(AL -> Alabama, AR -> Arkansas, AK -> Alaska, AZ -> Arizona)

Add multiple elements from another collection using ++=:

scala> states ++= List("CA" -> "California", "CO" -> "Colorado")
res2: scala.collection.mutable.Map[String,String] = Map(CO -> Colorado,
  AZ -> Arizona, AL -> Alabama, CA -> California, AR -> Arkansas,
  AK -> Alaska)

Remove a single element from a map by specifying its key with the -= method:

scala> states -= "AR"
res3: scala.collection.mutable.Map[String,String] =
  Map(AL -> Alabama, AK -> Alaska, AZ -> Arizona)

Remove multiple elements by key with the -= or --= methods:

scala> states -= ("AL", "AZ") res4: scala.collection.mutable.Map[String,String] ...