Scala List详解-优快云博客

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def main(args: Array[String]){
val bigData = List("Hadoop" , "Spark")
val data = List(1 , 2 , 3)
val bigData_Core = "Hadoop" :: ("Spark" :: Nil) //跟List("Hadoop" , "Spark")定义方式结果一样
val data_Int = 1 :: 2 :: 3 :: Nil
println(data.isEmpty)
println(data.head)
println(data.tail.head)//第二个元素
val List(a,b) = bigData
println("a : " + a + " === " + " b: " + b)
val x :: y :: rest = data //rest是剩下的元素所组成的一个子List
println("x : " + x + " === " + " y: " + y + " === " + rest )
val shuffledData = List(6,3,5,6,2,9,1)
println(sortList(shuffledData))
def sortList(list : List[Int]): List[Int] = list match{
case List() => List()
case head :: tail => compute (head, sortList(tail))
}
//递归排序步骤：
//sortList递归到最底层时，head是1，tail是Nil，compute(1, Nil)时返回List<1>
//往上，compute(9, List<1>)，结果：List<1,9>
//往上，compute(2, List<1,9>),结果：List<1,2,9>
//...
def compute(data : Int , dataSet : List[Int]) : List[Int] = dataSet match{
case List() => List(data)
case head :: tail => if (data <= head) data :: dataSet
else head :: compute(data, tail)
}
}

List的一阶函数操作

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def main(args: Array[String]) {
println(List (1,2,3,4) ::: List (4,5,6,7,8) ::: List (10,11)) //将多个List连接在一起（两个冒号是元素与元素、元素与列表的连接）
println(List (1,2,3,4) ::: (List (4,5,6,7,8) ::: List (10,11)))
println(List (1,2,3,4).length) //该方法比较慢，尽量不用
val bigData = List("Hadoop" , "Spark" , "Kafka")
println(bigData.last) //kafka
println(bigData.init) //除了最后一个元素之外的所有元素集合，这里是List("Hadoop", "Spark")
println(bigData.reverse)//产生新的List，结果是List("Kafka", "Spark", "Hadoop")
println(bigData) //还是原来的List("Hadoop", "Spark", "Kafka")
println(bigData take 2) //取前两个元素，List("Hadoop", "Spark")
println(bigData drop 2) //去掉前两个元素，List("kafka")
println(bigData splitAt 2) //将前两个元素一组，其余元素一组进行划分，(List("Hadoop", "Spark"), List("Kafka"))
println(bigData apply 2) //访问index=2的元素，跟下面的结果一样，不常用
println(bigData(2))
val data=List('a' ,'b', 'c', 'd', 'e', 'f')
println(data.indices) //返回所有元素的索引，Range(0,1,2,3,4,5)
println(data.indices zip data) //将两组元素配对，Vector((0,a),(1,b),(2,c),(3,d),(4,e),(5,f))
println(data.zipWithIndex)//将元素与索引配对更简捷写法，List((0,a),(1,b),(2,c),(3,d),(4,e),(5,f))
println(data.toString)//List('a' ,'b', 'c', 'd', 'e', 'f')
println(data.mkString ("[", ",,", "]"))//将集合按照一定样式输出，[a,,b,,c,,d,,e,,f]
println(data.mkString ("******"))//一个参数指分隔符，a******b******c******d******e******f
println(data mkString)//输出集合元素，abcdef
val buffer = new StringBuilder
data addString (buffer, "(", ";;", ")")//跟mkString结果一样，只不过是将结果放到buffer中，(a;;b;;c;;d;;e;;f)
println(buffer)
println(data) //List(a, b, c, d, e, f)
val array = data.toArray //将List转化为数组
println(array.toList) //将数组转化为List，List(a, b, c, d, e, f)
val new_Array = new Array[Char](10)
data.copyToArray(new_Array,3) //将List拷贝至数组，并将数组的前三个位置空出来
new_Array.foreach(print)
println
val iterator = data.toIterator //返回List的迭代器
println(iterator.next) //a
println(iterator.next) //b
}

归并排序：

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object MergedSort {
def main(args: Array[String]){
def mergedsort[T] (less: (T, T) => Boolean) (input: List[T]): List[T] = {
/**
* @param xList 要合并的有序列表
* @param yList 要合并的有序列表
* @return 合并后的列表
*/
def merge(xList: List[T], yList: List[T]): List[T] =
(xList, yList) match {
case (Nil, _) => yList
case (_, Nil) => xList
case (x :: xtail, y :: ytail) => //比较两个List的head，谁小谁放前面，剩下两个List再递归merge
if (less(x, y)) x :: merge(xtail, yList)
else y :: merge(xList, ytail)
}
val n = input.length / 2
if (n == 0) input
else {
val (x, y) = input splitAt n //把要排序的列表input平均分成两个列表
merge(mergedsort(less)(x), mergedsort(less)(y)) //先对分后的两个列表归并排序，再对排好的有序表进行归并
}
}
println(mergedsort((x: Int, y: Int) => x < y) (List (3, 7, 9, 5)))
val reversed_mergedsort=mergedsort((x: Int, y: Int) => x > y) _
println(reversed_mergedsort(List(3, 7, 9, 5)))
}
}

List的Map、flatMap、froeach、filter操作

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def main(args: Array[String]) {
println(List(1, 2, 3, 4, 6) map (_ + 1)) //map函数作用于List中的每一个元素，结果：List(2, 3, 4, 5, 7)
val data = List("Scala", "Hadoop", "Spark")
println(data map (_.length)) //结果：List(5, 6, 5)
println(data map (_.toList.reverse.mkString)) //结果：List(alacS, poodaH, krapS)
println(data.map(_.toList)) //结果：List(List(S, c, a, l, a), List(H, a, d, o, o, p), List(S, p, a, r, k))
println(data.flatMap(_.toList)) //将map操作产生的结果合并，产生新的集合，结果：List(S, c, a, l, a, H, a, d, o, o, p, S, p, a, r, k)
println(List.range(1, 10) flatMap (i => List.range(1, i) map (j => (i, j))))
var sum = 0
List(1, 2, 3, 4, 5) foreach (sum += _) //对每个元素进行没有结果的函数操作
println("sum : " + sum)
println(List(1, 2, 3, 4, 6, 7, 8, 9, 10) filter (_ % 2 ==0))
println(data filter (_.length == 5)) //过滤出符合要求的元素集合
println(List(1, 2, 3, 4, 5) partition (_ % 2 ==0)) //把元素按照某种条件分裂，结果：(List(2, 4),List(1, 3, 5))
println(List(1, 2, 3, 4, 5) find (_ % 2 ==0)) //找出集合中第一个满足条件的元素，结果：Some(2)，因为有可能找不到，所以是Optional的子类Some
println(List(1, 2, 3, 4, 5) find (_ <=0)) //没有结果，所以是None
println(List(1, 2, 3, 4, 5) takeWhile (_ < 4))//获取符合条件的元素，结果：List(1, 2, 3)
println(List(1, 2, 3, 4, 5) dropWhile (_ < 4)) //把符合条件的元素去掉，结果：List(4, 5)
println(List(1, 2, 3, 4, 5) span (_ < 4)) //把元素按条件分裂成不同部分，结果：(List(1, 2, 3),List(4, 5))
def hastotallyZeroRow(m: List[List[Int]]) = m exists (row => row forall (_ == 0))//forall，只有集合中所有元素都满足条件才返回true；exists，只要有一个元素满足条件就返回true
val m= List(List(1,0,0), List(0,0,0), List(0,0,1))
println(hastotallyZeroRow(m))
}

foldLeft、foldRight、sortWith

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def main(args: Array[String]){
println((1 to 5).foldLeft(0)(_+_) )//List从左往右，依次相加，0位初始值，0+1+2+3+4+5
println((0 /: (1 to 5))(_+_)) //也可以这么写：\:
println((1 to 5).foldRight(1)(_-_)) //初始值为1，从右往左，依次相减5-1 = 4，然后4-4=0,3-0=3,2-3=-1,1-（-1）=2
println(((1 to 5):\1)(_-_))
println(List(1, -3, 4, 2, 6) sortWith (_ < _))
println(List(1, -3, 4, 2, 6) sortWith (_ > _))
}

List伴生对象

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def main(args: Array[String]) {
println(List.apply(1, 2, 3)) //返回一个List(1,2,3)，底层调用toList方法
println(List.make(3, 5)) //构造一个具有重复元素的List，第一个参数是元素重复的次数，第二个参数是元素的内容，结果：List(5,5,5)
println(List.range(1, 5)) //生成一个List，左闭右开，List(1,2,3,4)
println(List.range(9, 1, -3))//指定步长，List(9,6,3)
val zipped = "abcde".toList zip List(1, 2, 3, 4, 5)
println(zipped) //将两个List组合起来，List((a,1), (b,2), (c,3), (d,4), (e,5))
println(zipped.unzip) //和zip操作相反，结果：(List(a, b, c, d, e),List(1, 2, 3, 4, 5))
println(List(List('a', 'b'), List('c'), List('d', 'e')).flatten)//将所有子List构成一个大的List，结果：List(a, b, c, d, e)
println(List.concat(List(), List('b'), List('c'))) //也是构成一个大的List，结果：List(b, c)
println(List.map2(List(10, 20), List(10, 10)) (_ * _)) //将两个List进行函数操作，结果：List(100, 200)
}

List继承体系分析以及方法操作源码解读

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sealed abstract class List[+A] extends AbstractSeq[A] //sealed常用来进行模式匹配
with LinearSeq[A]
with Product
with GenericTraversableTemplate[A, List]
with LinearSeqOptimized[A, List[A]] {
override def companion: GenericCompanion[List] = List
import scala.collection.{Iterable, Traversable, Seq, IndexedSeq}
def isEmpty: Boolean
def head: A
def tail: List[A]
...
}

* This class comes with two implementing case classes `scala.Nil`
* and `scala.::` that implement the abstract members `isEmpty`,
* `head` and `tail`.
List有两个实现类，Nil与::
子对象Nil的定义：

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case object Nil extends List[Nothing] { //空对象，全局唯一
override def isEmpty = true
override def head: Nothing =
throw new NoSuchElementException("head of empty list")
override def tail: List[Nothing] =
throw new UnsupportedOperationException("tail of empty list")
// Removal of equals method here might lead to an infinite recursion similar to IntMap.equals.
override def equals(that: Any) = that match {
case that1: scala.collection.GenSeq[_] => that1.isEmpty
case _ => false
}
}

子类::的定义：

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final case class ::[B](private var hd: B, private[scala] var tl: List[B]) extends List[B] {
override def head : B = hd
override def tail : List[B] = tl
override def isEmpty: Boolean = false
private def readObject(in: ObjectInputStream) {
val firstObject = in.readObject()
hd = firstObject.asInstanceOf[B]
assert(hd != ListSerializeEnd)
var current: ::[B] = this
while (true) in.readObject match {
case ListSerializeEnd =>
current.tl = Nil
return
case a =>
val list : ::[B] = new ::(a.asInstanceOf[B], Nil)
current.tl = list
current = list
}
}
private def writeObject(out: ObjectOutputStream) {
var xs: List[B] = this
while (!xs.isEmpty) { out.writeObject(xs.head); xs = xs.tail }
out.writeObject(ListSerializeEnd)
}
}

List的构造是的类型约束逆变、协变、下界

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abstract class Big_Data
class Hadoop extends Big_Data
class Spark extends Big_Data
object List_Constructor_Internals {
def main(args: Array[String]){
val hadoop = new Hadoop :: Nil //等价于Nil.::(new Hadoop)，结果：hadoop：List[Hadoop] = List(com.dt.scala.list.Hadoop@2666e815)，new Hadoop为List的head，Nil为List 的tail
val big_Data = new Spark :: hadoop //等价于hadoop.::(new Spark)，结果：big_Data：List[Big_data] = List(com.dt.scala.list.Spark@4e106082, com.dt.scala.list.Hadoop@2666e815)
}
//::源码
def ::[B >: A] (x: B): List[B] =
new scala.collection.immutable.::(x, this)
//看了::源码之后就会明白，往List[hadoop]里面.::(new Spark)怎么会返回List[Big_data]了
}