Scala Book

Common Map Methods

Language

In this lesson we’ll demonstrate some of the most commonly used Map methods. In these initial examples we’ll use an immutable Map, and Scala also has a mutable Map class that you can modify in place, and it’s demonstrated a little later in this lesson.

For these examples we won’t break the Map methods down into individual sections; we’ll just provide a brief comment before each method.

Given this immutable Map:

val m = Map(
    1 -> "a", 
    2 -> "b", 
    3 -> "c",
    4 -> "d"
)

Here are some examples of methods available to that Map:

// how to iterate over Map elements
scala> for ((k,v) <- m) printf("key: %s, value: %s\n", k, v)
key: 1, value: a
key: 2, value: b
key: 3, value: c
key: 4, value: d

// how to get the keys from a Map
scala> val keys = m.keys
keys: Iterable[Int] = Set(1, 2, 3, 4)

// how to get the values from a Map
scala> val values = m.values
val values: Iterable[String] = MapLike.DefaultValuesIterable(a, b, c, d)

// how to test if a Map contains a value
scala> val contains3 = m.contains(3)
contains3: Boolean = true

// how to transform Map values
scala> val ucMap = m.transform((k,v) => v.toUpperCase)
ucMap: scala.collection.immutable.Map[Int,String] = Map(1 -> A, 2 -> B, 3 -> C, 4 -> D)

// how to filter a Map by its keys
scala> val twoAndThree = m.view.filterKeys(Set(2,3)).toMap
twoAndThree: scala.collection.immutable.Map[Int,String] = Map(2 -> b, 3 -> c)

// how to take the first two elements from a Map
scala> val firstTwoElements = m.take(2)
firstTwoElements: scala.collection.immutable.Map[Int,String] = Map(1 -> a, 2 -> b)

Note that the last example probably only makes sense for a sorted Map.

Mutable Map examples

Here are a few examples of methods that are available on the mutable Map class. Given this initial mutable Map:

val states = scala.collection.mutable.Map(
    "AL" -> "Alabama", 
    "AK" -> "Alaska"
)

Here are some things you can do with a mutable Map:

// 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"

// retain elements by supplying a function that operates on
// the keys and/or values
states.retain((k,v) => k == "AK")

See also

There are many more things you can do with maps. See the Map class documentation for more details and examples.

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