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Tabs for Scala 2/3 in collections-2.13/maps.
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‎_overviews/collections-2.13/maps.md

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@@ -24,68 +24,87 @@ The fundamental operations on maps are similar to those on sets. They are summar
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### Operations in Class Map ###
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| WHAT IT IS | WHAT IT DOES |
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| ------ | ------ |
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| **Lookups:** | |
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| `ms get k` |The value associated with key `k` in map `ms` as an option, `None` if not found.|
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| `ms(k)` |(or, written out, `ms apply k`) The value associated with key `k` in map `ms`, or exception if not found.|
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| `ms getOrElse (k, d)` |The value associated with key `k` in map `ms`, or the default value `d` if not found.|
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| `ms contains k` |Tests whether `ms` contains a mapping for key `k`.|
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| `ms isDefinedAt k` |Same as `contains`. |
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| **Subcollections:** | |
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| WHAT IT IS | WHAT IT DOES |
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| ------ | ------ |
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| **Lookups:** | |
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| `ms.get(k)` |The value associated with key `k` in map `ms` as an option, `None` if not found.|
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| `ms(k)` |(or, written out, `ms.apply(k)`) The value associated with key `k` in map `ms`, or exception if not found.|
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| `ms.getOrElse(k, d)` |The value associated with key `k` in map `ms`, or the default value `d` if not found.|
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| `ms.contains(k)` |Tests whether `ms` contains a mapping for key `k`.|
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| `ms.isDefinedAt(k)` |Same as `contains`. |
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| **Subcollections:** | |
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| `ms.keys` |An iterable containing each key in `ms`. |
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| `ms.keySet` |A set containing each key in `ms`. |
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| `ms.keysIterator` |An iterator yielding each key in `ms`. |
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| `ms.values` |An iterable containing each value associated with a key in `ms`.|
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| `ms.valuesIterator` |An iterator yielding each value associated with a key in `ms`.|
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| **Transformation:** | |
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| `ms.view filterKeys p` |A map view containing only those mappings in `ms` where the key satisfies predicate `p`.|
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| `ms.view mapValues f` |A map view resulting from applying function `f` to each value associated with a key in `ms`.|
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| **Transformation:** | |
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| `ms.view.filterKeys(p)` |A map view containing only those mappings in `ms` where the key satisfies predicate `p`.|
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| `ms.view.mapValues(f)` |A map view resulting from applying function `f` to each value associated with a key in `ms`.|
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Immutable maps support in addition operations to add and remove mappings by returning new `Map`s, as summarized in the following table.
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### Operations in Class immutable.Map ###
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| WHAT IT IS | WHAT IT DOES |
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| ------ | ------ |
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| **Additions and Updates:**| |
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| WHAT IT IS | WHAT IT DOES |
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| ------ | ------ |
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| **Additions and Updates:**| |
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| `ms.updated(k, v)`<br>or `ms + (k -> v)` |The map containing all mappings of `ms` as well as the mapping `k -> v` from key `k` to value `v`.|
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| **Removals:** | |
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| `ms remove k`<br>or `ms - k` |The map containing all mappings of `ms` except for any mapping of key `k`.|
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| `ms removeAll ks`<br>or `ms -- ks` |The map containing all mappings of `ms` except for any mapping with a key in `ks`.|
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| **Removals:** | |
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| `ms.remove(k)`<br>or `ms - k` |The map containing all mappings of `ms` except for any mapping of key `k`.|
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| `ms.removeAll(ks)`<br>or `ms -- ks` |The map containing all mappings of `ms` except for any mapping with a key in `ks`.|
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Mutable maps support in addition the operations summarized in the following table.
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### Operations in Class mutable.Map ###
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| WHAT IT IS | WHAT IT DOES |
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| ------ | ------ |
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| **Additions and Updates:**| |
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| `ms(k) = v` |(Or, written out, `ms.update(k, v)`). Adds mapping from key `k` to value `v` to map ms as a side effect, overwriting any previous mapping of `k`.|
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| `ms.addOne(k -> v)`<br>or `ms += (k -> v)` |Adds mapping from key `k` to value `v` to map `ms` as a side effect and returns `ms` itself.|
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| WHAT IT IS | WHAT IT DOES |
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| ------ | ------ |
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| **Additions and Updates:** | |
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| `ms(k) = v` |(Or, written out, `ms.update(k, v)`). Adds mapping from key `k` to value `v` to map ms as a side effect, overwriting any previous mapping of `k`.|
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| `ms.addOne(k -> v)`<br>or `ms += (k -> v)` |Adds mapping from key `k` to value `v` to map `ms` as a side effect and returns `ms` itself.|
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| `ms addAll xvs`<br>or `ms ++= kvs` |Adds all mappings in `kvs` to `ms` as a side effect and returns `ms` itself.|
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| `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.|
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| `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`.|
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| **Removals:**| |
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| `ms subtractOne k`<br>or `ms -= k` |Removes mapping with key `k` from ms as a side effect and returns `ms` itself.|
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| `ms subtractAll ks`<br>or `ms --= ks` |Removes all keys in `ks` from `ms` as a side effect and returns `ms` itself.|
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| `ms remove k` |Removes any mapping with key `k` from `ms` and returns any value previously associated with `k` as an option.|
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| `ms filterInPlace p` |Keeps only those mappings in `ms` that have a key satisfying predicate `p`.|
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| `ms.clear()` |Removes all mappings from `ms`. |
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| **Transformation:** | |
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| `ms mapValuesInPlace f` |Transforms all associated values in map `ms` with function `f`.|
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| **Cloning:** | |
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| `ms.clone` |Returns a new mutable map with the same mappings as `ms`.|
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| `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.|
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| `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`.|
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| **Removals:** | |
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| `ms.subtractOne(k)`<br>or `ms -= k` |Removes mapping with key `k` from ms as a side effect and returns `ms` itself.|
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| `ms.subtractAll(ks)`<br>or `ms --= ks` |Removes all keys in `ks` from `ms` as a side effect and returns `ms` itself.|
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| `ms.remove(k)` |Removes any mapping with key `k` from `ms` and returns any value previously associated with `k` as an option.|
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| `ms.filterInPlace(p)` |Keeps only those mappings in `ms` that have a key satisfying predicate `p`.|
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| `ms.clear()` |Removes all mappings from `ms`. |
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| **Transformation:** | |
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| `ms.mapValuesInPlace(f)` |Transforms all associated values in map `ms` with function `f`.|
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| **Cloning:** | |
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| `ms.clone` |Returns a new mutable map with the same mappings as `ms`.|
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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.
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The `getOrElseUpdate` is useful for accessing maps that act as caches. Say you have an expensive computation triggered by invoking a function `f`:
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scala> def f(x: String) = {
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println("taking my time."); sleep(100)
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x.reverse }
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f: (x: String)String
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{% tabs expensive-computation-reverse class=tabs-scala-version %}
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{% tab 'Scala 2' for=expensive-computation-reverse %}
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```scala
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scala> def f(x: String): String = {
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println("taking my time."); Thread.sleep(100)
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x.reverse
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}
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f: (x: String)String
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```
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{% endtab %}
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{% tab 'Scala 3' for=expensive-computation-reverse %}
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```scala
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scala> def f(x: String): String =
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println("taking my time."); Thread.sleep(100)
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x.reverse
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def f(x: String): String
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```
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{% endtab %}
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{% endtabs %}
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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`.
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You can now create a more efficient caching version of the `f` function:
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scala> def cachedF(s: String) = cache.getOrElseUpdate(s, f(s))
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scala> def cachedF(s: String): String = cache.getOrElseUpdate(s, f(s))
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cachedF: (s: String)String
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scala> cachedF("abc")
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taking my time.
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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:
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def cachedF(arg: String) = cache get arg match {
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case Some(result) => result
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case None =>
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val result = f(x)
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cache(arg) = result
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result
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}
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{% tabs cacheF class=tabs-scala-version %}
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{% tab 'Scala 2' for=cacheF %}
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```scala
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def cachedF(arg: String): String = cache.get(arg) match {
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case Some(result) => result
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case None =>
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val result = f(x)
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cache(arg) = result
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result
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}
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```
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{% endtab %}
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{% tab 'Scala 3' for=cacheF %}
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```scala
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def cachedF(arg: String): String = cache.get(arg) match
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case Some(result) => result
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case None =>
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val result = f(x)
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cache(arg) = result
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result
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```
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{% endtab %}
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{% endtabs %}

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