Scala Cookbook

11.17. Traversing a Map

Problem

You want to iterate over the elements in a map.

Solution

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

val ratings = Map("Lady in the Water"-> 3.0,
                  "Snakes on a Plane"-> 4.0,
                  "You, Me and Dupree"-> 3.5)

my preferred 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 following approach shows how to use the Tuple syntax to access the key and value fields:

ratings.foreach(x => println(s"key: ${x._1}, value: ${x._2}"))

If you just want to use the keys in the map, the keys method returns an Iterable you can use:

ratings.keys.foreach((movie) => println(movie))

For simple examples like this, that expression can be reduced as follows:

ratings.keys.foreach(println)

In the same way, use the values method to iterate over the values in the map:

ratings.values.foreach((rating) => println(rating))

Note: Those are not my movie ratings. They are taken from the book, Programming Collective Intelligence (O’Reilly), by Toby Segaran.

Operating on map values

If you want to traverse the map to perform an operation on its values, the mapValues method may be a better solution. It lets you perform a function on each map value, and returns the modified map:

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