Scala Cookbook

11.22. Sorting an Existing Map by Key or Value

Problem

You have an unsorted map and want to sort the elements in the map by the key or value.

Solution

Given a basic, immutable Map:

scala> val grades = Map("Kim" -> 90,
     |   "Al" -> 85,
     |   "Melissa" -> 95,
     |   "Emily" -> 91,
     |   "Hannah" -> 92
     | )
grades: scala.collection.immutable.Map[String,Int] =
  Map(Hannah -> 92, Melissa -> 95, Kim -> 90, Emily -> 91, Al -> 85)

You can sort the map by key, from low to high, using sortBy:

scala> import scala.collection.immutable.ListMap
import scala.collection.immutable.ListMap

scala> ListMap(grades.toSeq.sortBy(_._1):_*)
res0: scala.collection.immutable.ListMap[String,Int] =
  Map(Al -> 85, Emily -> 91, Hannah -> 92, Kim -> 90, Melissa -> 95)

You can also sort the keys in ascending or descending order using sortWith:

// low to high
scala> ListMap(grades.toSeq.sortWith(_._1 < _._1):_*)
res0: scala.collection.immutable.ListMap[String,Int] =
  Map(Al -> 85, Emily -> 91, Hannah -> 92, Kim -> 90, Melissa -> 95)

// high to low
scala> ListMap(grades.toSeq.sortWith(_._1 > _._1):_*)
res1: scala.collection.immutable.ListMap[String,Int] =
  Map(Melissa -> 95, Kim -> 90, Hannah -> 92, Emily -> 91, Al -> 85)

You can sort the map by value using sortBy:

scala> ListMap(grades.toSeq.sortBy(_._2):_*)
res0: scala.collection.immutable.ListMap[String,Int] =
  Map(Al -> 85, Kim -> 90, Emily -> 91, Hannah -> 92, Melissa -> 95)

You can also sort by value in ascending or descending order using sortWith:

// low to high
scala> ListMap(grades.toSeq.sortWith(_._2 ...