Maps

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:

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`.
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

}

Synchronized sets and maps

Maps

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