Iterating over Scala Maps (for and foreach loop examples)

Scala Map FAQ: How can I iterate/loop over a Scala Map?

There are several different ways to iterate over a Scala Map, and the method you choose depends on the problem you need to solve.

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A sample Map

To get started with our examples, let's create a simple Map we can work with:

scala> val m1 = Map("fname" -> "Al", "lname" -> "Alexander")
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Iterating over Scala maps

Once you have a Map, you can iterate over it using several different techniques. For me, this is by far the easiest technique:

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. Scala is beginning to remind me of the Perl slogan: "There's more than one way to do it", and this is good, because you can choose whichever approach makes the most sense for the problem at hand.

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Scala Map, keys, foreach, and tuples

To demonstrate a more "real world" example of looping over a Scala Map, while working through some programming examples in the book, Programming Collective Intelligence, I decided to code them up in Scala, and I wanted to share the approaches I prefer using the Scala foreach and for loops.

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.

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Iterating over a Scala Map - Summary

In summary, I hope these examples of iterating a Scala Map have been helpful. As you can see, there are many ways to loop over a Map, using for, foreach, tuples, and key/value approaches. The technique you use can be selected based on the problem you're trying to solve.

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There’s just one person behind this website; if this article was helpful (or interesting), I’d appreciate it if you’d share it. Thanks, Al.

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