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Maps

A Map is an Iterable consisting of pairs of keys and values (also named mappings or associations). Scala's Predef class 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:

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.

Additions and Updates:

ms + (k -> v) The map containing all mappings of ms as well as the mapping k -> v from key k to value v.
ms + (k -> v, l -> w) The map containing all mappings of ms as well as the given key/value pairs.
ms ++ kvs The map containing all mappings of ms as well as all key/value pairs of kvs.
ms updated (k, v) Same as ms + (k -> v).

Removals:

ms - k The map containing all mappings of ms except for any mapping of key k.
ms - (k, l, m) The map containing all mappings of ms except for any mapping with the given keys.
ms -- ks The map containing all mappings of ms except for any mapping with a key in ks.

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 filterKeys p A map view containing only those mappings in ms where the key satisfies predicate p.
ms mapValues f A map view resulting from applying function f to each value associated with a key in ms.

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 += (k -> v) Adds mapping from key k to value v to map ms as a side effect and returns ms itself.
ms += (k -> v, l -> w) Adds the given mappings to ms as a side effect and returns ms itself.
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 -= k Removes mapping with key k from ms as a side effect and returns ms itself.
ms -= (k, l, m) Removes mappings with the given keys from ms as a side effect and returns ms itself.
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 retain p Keeps only those mappings in ms that have a key satisfying predicate p.
ms.clear() Removes all mappings from ms.

Transformation:

ms transform 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. As is the for sets, mutable maps also support the non-destructive addition operations +, -, and updated, but they are used less frequently because they involve a copying of the mutable map. Instead, a mutable map is m usually updated "in place", using the two variants m(key) = value or m += (key -> value). There are 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.

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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