Scala Cookbook

11.21. Filtering a Map

Problem

You want to filter the elements contained in a map, either by directly modifying a mutable map, or by applying a filtering algorithm on an immutable map to create a new map.

Solution

Use the retain method to define the elements to retain when using a mutable map, and use filterKeys or filter to filter the elements in a mutable or immutable map, remembering to assign the result to a new variable.

Mutable maps

You can filter the elements in a mutable map using the retain method to specify which elements should be retained:

scala> var x = collection.mutable.Map(1 -> "a", 2 -> "b", 3 -> "c")
x: scala.collection.mutable.Map[Int,String] = Map(2 -> b, 1 -> a, 3 -> c)

scala> x.retain((k,v) => k > 1)
res0: scala.collection.mutable.Map[Int,String] = Map(2 -> b, 3 -> c)

scala> x
res1: scala.collection.mutable.Map[Int,String] = Map(2 -> b, 3 -> c)

As shown, retain modifies a mutable map in place. As implied by the anonymous function signature used in that example:

(k,v) => ...

your algorithm can test both the key and value of each element to decide which elements to retain in the map.

In a related note, the transform method doesn’t filter a map, but it lets you transform the elements in a mutable map:

scala> x.transform((k,v) => v.toUpperCase)
res0: scala.collection.mutable.Map[Int,String] = Map(2 -> B, 3 -> C)

scala> x
res1: scala.collection.mutable.Map[Int,String] = Map(2 -> B, 3 -> C)

Depending on your definition of “filter,” you can also remove elements from a map using ...