Scala Cookbook

11.15. Adding, Updating, and Removing Elements with Immutable Maps

Problem

You want to add, update, or delete elements when working with an immutable map.

Solution

Use the correct operator for each purpose, remembering to assign the results to a new map.

To be clear about the approach, the following examples use an immutable map with a series of val variables. First, create an immutable map as a val:

scala> val a = Map("AL" -> "Alabama")
a: scala.collection.immutable.Map[String,String] =
   Map(AL -> Alabama)

Add one or more elements with the + method, assigning the result to a new Map variable during the process:

// add one element
scala> val b = a + ("AK" -> "Alaska")
b: scala.collection.immutable.Map[String,String] =
   Map(AL -> Alabama, AK -> Alaska)

// add multiple elements
scala> val c = b + ("AR" -> "Arkansas", "AZ" -> "Arizona")
c: scala.collection.immutable.Map[String,String] =
   Map(AL -> Alabama, AK -> Alaska, AR -> Arkansas, AZ -> Arizona)

To update a key/value pair with an immutable map, reassign the key and value while using the + method, and the new values replace the old:

scala> val d = c + ("AR" -> "banana")
d: scala.collection.immutable.Map[String,String] =
   Map(AL -> Alabama, AK -> Alaska, AR -> banana, AZ -> Arizona)

To remove one element, use the - method:

scala> val e = d - "AR"
e: scala.collection.immutable.Map[String,String] =
   Map(AL -> Alabama, AK -> Alaska, AZ -> Arizona)

To remove multiple elements, use the - or -- methods:

scala> val f = e - "AZ" - "AL" f: scala.collection.immutable.Map[String,String] ...