Scala Maps (Map class examples)

Here's a quick look at how to use the Scala Map class, with a colllection of Map class examples.

The immutable Map class is in scope by default, so you can create an immutable map without an import, like this:

val states = Map("AL" -> "Alabama", "AK" -> "Alaska")

To create a mutable Map, import it first:

var states = scala.collection.mutable.Map("AL" -> "Alabama")

Adding, removing, and updating mutable Map elements

The following examples show how to add, remove, and update elements in a mutable Scala Map:

// create an empty map
var states = scala.collection.mutable.Map[String, String]()

// create a map with initial elements
var states = scala.collection.mutable.Map("AL" -> "Alabama", "AK" -> "Alaska")

// add elements with +=
states += ("AZ" -> "Arizona")
states += ("CO" -> "Colorado", "KY" -> "Kentucky")

// remove elements with -=
states -= "KY"
states -= ("AZ", "CO")

// update elements by reassigning them
states("AK") = "Alaska, The Big State"

Iterating over Scala maps

Once you have a Map, you can iterate over it using several different techniques. I prefer using the for loop (or for comprehension):

scala> val m1 = Map("fname" -> "Al", "lname" -> "Alexander")

scala> for ((k,v) <- m1) printf("key: %s, value: %s\n", k, v)
key: fname, value: Al
key: lname, value: Alexander

This page has some other Map and for loop examples, which I've reproduced here:

// version 1 (tuples)
m1 foreach (x => println (x._1 + "-->" + x._2))

// version 2 (foreach and case)
m1 foreach {case (key, value) => println (key + "-->" + value)}

You can choose whatever format you prefer.

A few more ways to iterate over a Scala Map

To demonstrate a more "real world" example of looping over a Scala Map, while I was working through some programming examples in the book, Programming Collective Intelligence, I decided to code them up in Scala.

To begin with, I defined my Scala Map like this:

val p1Ratings = Map("Lady in the Water"-> 3.0, 
                    "Snakes on a Plane"-> 4.0,
                    "You, Me and Dupree"-> 3.5)

In my case, when I'm iterating over the Map I'm really just interested in the Map keys, so the cleanest way to loop over every Map element is like this:

p1Ratings.keys.foreach( (movie) => 
  if (p2Ratings.contains(movie)) similarItems += (movie -> true)
)

While I chose that looping method in my code, I could also use the "tuples" approach, where movie is a Tuple, and I only use the first element of the Tuple, which happens to be my keys:

p1Ratings foreach ( (movie) => 
  if (p2Ratings.contains(movie._1)) similarItems += (movie._1 -> true)
)

In that approach, I ignore the second element of each Tuple, because I don't need it. (Which is why I don't like this approach for this instance.)

In a similar approach, I loop over the Map as shown next, creating a field named rating1 which I again don't use because I don't need it:

for ((movie1, rating1) <- p1Ratings) {
  if (p2Ratings.contains(movie1)) similarItems += (movie1 -> true)
}

These last two approaches will work better, and look a little more logical, if you need to access the key and value for each map element, but in my case, since I don't need to values, I'm using the first approach shown above.