Collections

Maps

Language

A Map is an Iterable consisting of pairs of keys and values (also named mappings or associations). Scala’s Predef object offers an implicit conversion that lets you write key -> value as an alternate syntax for the pair (key, value). For instance Map("x" -> 24, "y" -> 25, "z" -> 26) means exactly the same as Map(("x", 24), ("y", 25), ("z", 26)), but reads better.

The fundamental operations on maps are similar to those on sets. They are summarized in the following table and fall into the following categories:

  • Lookup operations apply, get, getOrElse, contains, and isDefinedAt. These turn maps into partial functions from keys to values. The fundamental lookup method for a map is: def get(key): Option[Value]. The operation “m get key” tests whether the map contains an association for the given key. If so, it returns the associated value in a Some. If no key is defined in the map, get returns None. Maps also define an apply method that returns the value associated with a given key directly, without wrapping it in an Option. If the key is not defined in the map, an exception is raised.
  • Additions and updates +, ++, updated, which let you add new bindings to a map or change existing bindings.
  • Removals -, --, which remove bindings from a map.
  • Subcollection producers keys, keySet, keysIterator, values, valuesIterator, which return a map’s keys and values separately in various forms.
  • Transformations filterKeys and mapValues, which produce a new map by filtering and transforming bindings of an existing map.

Operations in Class Map

WHAT IT IS WHAT IT DOES
Lookups:  
ms get k The value associated with key k in map ms as an option, None if not found.
ms(k) (or, written out, ms apply k) The value associated with key k in map ms, or exception if not found.
ms getOrElse (k, d) The value associated with key k in map ms, or the default value d if not found.
ms contains k Tests whether ms contains a mapping for key k.
ms isDefinedAt k Same as contains.
Subcollections:  
ms.keys An iterable containing each key in ms.
ms.keySet A set containing each key in ms.
ms.keysIterator An iterator yielding each key in ms.
ms.values An iterable containing each value associated with a key in ms.
ms.valuesIterator An iterator yielding each value associated with a key in ms.
Transformation:  
ms.view filterKeys p A map view containing only those mappings in ms where the key satisfies predicate p.
ms.view mapValues f A map view resulting from applying function f to each value associated with a key in ms.

Immutable maps support in addition operations to add and remove mappings by returning new Maps, as summarized in the following table.

Operations in Class immutable.Map

WHAT IT IS WHAT IT DOES
Additions and Updates:  
ms.updated(k, v)
or ms + (k -> v)
The map containing all mappings of ms as well as the mapping k -> v from key k to value v.
Removals:  
ms remove k
or ms - k
The map containing all mappings of ms except for any mapping of key k.
ms removeAll ks
or ms -- ks
The map containing all mappings of ms except for any mapping with a key in ks.

Mutable maps support in addition the operations summarized in the following table.

Operations in Class mutable.Map

WHAT IT IS WHAT IT DOES
Additions and Updates:  
ms(k) = v (Or, written out, ms.update(x, v)). Adds mapping from key k to value v to map ms as a side effect, overwriting any previous mapping of k.
ms.addOne(k -> v)
or ms += (k -> v)
Adds mapping from key k to value v to map ms as a side effect and returns ms itself.
ms addAll xvs
or ms ++= kvs
Adds all mappings in kvs to ms as a side effect and returns ms itself.
ms.put(k, v) Adds mapping from key k to value v to ms and returns any value previously associated with k as an option.
ms getOrElseUpdate (k, d) If key k is defined in map ms, return its associated value. Otherwise, update ms with the mapping k -> d and return d.
Removals:  
ms subtractOne k
or ms -= k
Removes mapping with key k from ms as a side effect and returns ms itself.
ms subtractAll ks
or ms --= ks
Removes all keys in ks from ms as a side effect and returns ms itself.
ms remove k Removes any mapping with key k from ms and returns any value previously associated with k as an option.
ms filterInPlace p Keeps only those mappings in ms that have a key satisfying predicate p.
ms.clear() Removes all mappings from ms.
Transformation:  
ms mapValuesInPlace f Transforms all associated values in map ms with function f.
Cloning:  
ms.clone Returns a new mutable map with the same mappings as ms.

The addition and removal operations for maps mirror those for sets. A mutable map m is usually updated “in place”, using the two variants m(key) = value or m += (key -> value). There is also the variant m.put(key, value), which returns an Option value that contains the value previously associated with key, or None if the key did not exist in the map before.

The getOrElseUpdate is useful for accessing maps that act as caches. Say you have an expensive computation triggered by invoking a function f:

scala> def f(x: String) = {
       println("taking my time."); sleep(100)
       x.reverse }
f: (x: String)String

Assume further that f has no side-effects, so invoking it again with the same argument will always yield the same result. In that case you could save time by storing previously computed bindings of argument and results of f in a map and only computing the result of f if a result of an argument was not found there. One could say the map is a cache for the computations of the function f.

scala> val cache = collection.mutable.Map[String, String]()
cache: scala.collection.mutable.Map[String,String] = Map()

You can now create a more efficient caching version of the f function:

scala> def cachedF(s: String) = cache.getOrElseUpdate(s, f(s))
cachedF: (s: String)String
scala> cachedF("abc")
taking my time.
res3: String = cba
scala> cachedF("abc")
res4: String = cba

Note that the second argument to getOrElseUpdate is “by-name”, so the computation of f("abc") above is only performed if getOrElseUpdate requires the value of its second argument, which is precisely if its first argument is not found in the cache map. You could also have implemented cachedF directly, using just basic map operations, but it would take more code to do so:

def cachedF(arg: String) = cache get arg match {
  case Some(result) => result
  case None =>
    val result = f(x)
    cache(arg) = result
    result
}

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