BigData-Notes/notes/Scala集合类型.md

集合

一、集合简介
二、集合结构
        3.1 scala.collection
        3.2 scala.collection.mutable
        3.2 scala.collection.immutable
三、Trait Traversable
四、Trait Iterable
五、修改集合

一、集合简介

Scala中拥有多种集合类型，主要分为可变的和不可变的集合两大类：

• 可变集合： 可以被修改。即可以更改，添加，删除集合中的元素；

• 不可变集合类：不能被修改。虽然可以对不可变集合执行添加，移除或更新操作，但是这些操作都会返回一个新的集合，而不是修改原来的集合。

二、集合结构

scala所有的集合类主要位于scala.collection 、scala.collection.mutable、scala.collection.immutable、scala.collection.generic包中 ，大部分集合类存在三类变体，分别位于scala.collection, scala.collection.immutable, scala.collection.mutable包。

• scala.collection.immutable包是中的集合是不可变的；
• scala.collection.mutable包中的集合是可变的；
• scala.collection包中的集合，既可以是可变的，也可以是不可变的。

val sortSet = scala.collection.SortedSet(1, 2, 3, 4, 5)
val mutableSet = collection.mutable.SortedSet(1, 2, 3, 4, 5)
val immutableSet = collection.immutable.SortedSet(1, 2, 3, 4, 5)

如果你仅写了Set 而没有任何加前缀也没有进行任何import，则Scala默认采用不可变集合类。

scala> Set(1,2,3,4,5)
res0: scala.collection.immutable.Set[Int] = Set(5, 1, 2, 3, 4)

3.1 scala.collection

scala.collection包中所有集合如下图：

3.2 scala.collection.mutable

scala.collection.mutable包中所有集合如下图：

3.2 scala.collection.immutable

scala.collection.immutable包中所有集合如下图：

三、Trait Traversable

Scala中所有集合的顶层实现是Traversable 。它唯一的抽象方法是foreach：

def foreach[U](f: Elem => U)

实现Traversable的集合类只需要定义这个方法，所有其他方法都可以从Traversable继承。Traversable中定义了很多方法，如下:

下面表格来源于官方文档：https://docs.scala-lang.org/overviews/collections/trait-traversable.html

方法	描述
Abstract Method:
xs foreach f	Executes function f for every element of xs.
Addition:
xs ++ ys	A collection consisting of the elements of both xs and ys. ys is a TraversableOnce collection, i.e., either a Traversable or an Iterator.
Maps:
xs map f	The collection obtained from applying the function f to every element in xs.
xs flatMap f	The collection obtained from applying the collection-valued function f to every element in xs and concatenating the results.
xs collect f	The collection obtained from applying the partial function f to every element in xs for which it is defined and collecting the results.
Conversions:
xs.toArray	Converts the collection to an array.
xs.toList	Converts the collection to a list.
xs.toIterable	Converts the collection to an iterable.
xs.toSeq	Converts the collection to a sequence.
xs.toIndexedSeq	Converts the collection to an indexed sequence.
xs.toStream	Converts the collection to a lazily computed stream.
xs.toSet	Converts the collection to a set.
xs.toMap	Converts the collection of key/value pairs to a map. If the collection does not have pairs as elements, calling this operation results in a static type error.
Copying:
xs copyToBuffer buf	Copies all elements of the collection to buffer buf.
xs copyToArray(arr, s, n)	Copies at most n elements of the collection to array arr starting at index s. The last two arguments are optional.
Size info:
xs.isEmpty	Tests whether the collection is empty.
xs.nonEmpty	Tests whether the collection contains elements.
xs.size	The number of elements in the collection.
xs.hasDefiniteSize	True if xs is known to have finite size.
Element Retrieval:
xs.head	The first element of the collection (or, some element, if no order is defined).
xs.headOption	The first element of xs in an option value, or None if xs is empty.
xs.last	The last element of the collection (or, some element, if no order is defined).
xs.lastOption	The last element of xs in an option value, or None if xs is empty.
xs find p	An option containing the first element in xs that satisfies p, or None if no element qualifies.
Subcollections:
xs.tail	The rest of the collection except xs.head.
xs.init	The rest of the collection except xs.last.
xs slice (from, to)	A collection consisting of elements in some index range of xs (from from up to, and excluding to).
xs take n	A collection consisting of the first n elements of xs (or, some arbitrary n elements, if no order is defined).
xs drop n	The rest of the collection except xs take n.
xs takeWhile p	The longest prefix of elements in the collection that all satisfy p.
xs dropWhile p	The collection without the longest prefix of elements that all satisfy p.
xs filter p	The collection consisting of those elements of xs that satisfy the predicate p.
xs withFilter p	A non-strict filter of this collection. Subsequent calls to map, flatMap, foreach, and withFilter will only apply to those elements of xs for which the condition p is true.
xs filterNot p	The collection consisting of those elements of xs that do not satisfy the predicate p.
Subdivisions:
xs splitAt n	Split xs at a position, giving the pair of collections (xs take n, xs drop n).
xs span p	Split xs according to a predicate, giving the pair of collections (xs takeWhile p, xs.dropWhile p).
xs partition p	Split xs into a pair of collections; one with elements that satisfy the predicate p, the other with elements that do not, giving the pair of collections (xs filter p, xs.filterNot p)
xs groupBy f	Partition xs into a map of collections according to a discriminator function f.
Element Conditions:
xs forall p	A boolean indicating whether the predicate p holds for all elements of xs.
xs exists p	A boolean indicating whether the predicate p holds for some element in xs.
xs count p	The number of elements in xs that satisfy the predicate p.
Folds:
(z /: xs)(op)	Apply binary operation op between successive elements of xs, going left to right and starting with z.
(xs :\ z)(op)	Apply binary operation op between successive elements of xs, going right to left and starting with z.
xs.foldLeft(z)(op)	Same as (z /: xs)(op).
xs.foldRight(z)(op)	Same as (xs :\ z)(op).
xs reduceLeft op	Apply binary operation op between successive elements of non-empty collection xs, going left to right.
xs reduceRight op	Apply binary operation op between successive elements of non-empty collection xs, going right to left.
Specific Folds:
xs.sum	The sum of the numeric element values of collection xs.
xs.product	The product of the numeric element values of collection xs.
xs.min	The minimum of the ordered element values of collection xs.
xs.max	The maximum of the ordered element values of collection xs.
Strings:
xs addString (b, start, sep, end)	Adds a string to StringBuilder b that shows all elements of xs between separators sep enclosed in strings start and end. start, sep, end are all optional.
xs mkString (start, sep, end)	Converts the collection to a string that shows all elements of xs between separators sep enclosed in strings start and end. start, sep, end are all optional.
xs.stringPrefix	The collection name at the beginning of the string returned from xs.toString.
Views:
xs.view	Produces a view over xs.
xs view (from, to)	Produces a view that represents the elements in some index range of xs.

四、Trait Iterable

Iterable拓展了Traversable，并定义了一些额外的方法，Scala中所有的集合都直接或者间接实现了Iterable。

下面表格来源于官方文档：https://docs.scala-lang.org/overviews/collections/trait-iterable.html

方法	描述
Abstract Method:
xs.iterator	An iterator that yields every element in xs, in the same order as foreach traverses elements.
Other Iterators:
xs grouped size	An iterator that yields fixed-sized “chunks” of this collection.
xs sliding size	An iterator that yields a sliding fixed-sized window of elements in this collection.
Subcollections:
xs takeRight n	A collection consisting of the last n elements of xs (or, some arbitrary n elements, if no order is defined).
xs dropRight n	The rest of the collection except xs takeRight n.
Zippers:
xs zip ys	An iterable of pairs of corresponding elements from xs and ys.
xs zipAll (ys, x, y)	An iterable of pairs of corresponding elements from xs and ys, where the shorter sequence is extended to match the longer one by appending elements x or y.
xs.zipWithIndex	An iterable of pairs of elements from xs with their indices.
Comparison:
xs sameElements ys	A test whether xs and ys contain the same elements in the same order

五、修改集合

当你想对集合添加或者删除元素，需要根据不同的集合类型选择不同的操作符号。

操作符	描述	集合类型
coll(k) 即coll.apply(k)	获取指定位置的元素	Seq, Map
coll :+ elem elem +: coll	向集合末尾或者集合头增加元素	Seq
coll + elem coll + (e1, e2, ...)	追加元素	Seq, Map
coll - elem coll - (e1, e2, ...)	删除元素	Set, Map, ArrayBuffer
coll ++ coll2 coll2 ++: coll	合并集合	Iterable
coll -- coll2	移除coll中包含的coll2中的元素	Set, Map, ArrayBuffer
elem :: lst lst2 :: lst	把指定列表(lst2)或者元素(elem)添加到列表(lst)头部	List
list ::: list2	合并List	List
set | set2 set & set2 set &~ set2	并集、交集、差集	Set
coll += elem coll += (e1, e2, ...) coll ++= coll2 coll -= elem coll -= (e1, e2, ...) coll --= coll2	添加或者删除元素，并将修改后的结果赋值给集合本身	可变集合
elem +=: coll coll2 ++=: coll	在集合头部追加元素或集合	ArrayBuffer

参考资料

1. Martin Odersky . Scala编程(第3版)[M] . 电子工业出版社 . 2018-1-1
2. 凯.S.霍斯特曼 . 快学Scala(第2版)[M] . 电子工业出版社 . 2017-7
3. https://docs.scala-lang.org/overviews/collections/overview.html
4. https://docs.scala-lang.org/overviews/collections/trait-traversable.html
5. https://docs.scala-lang.org/overviews/collections/trait-iterable.html