10-2 集合之List

scala中优先采用不可变集，List是Nil、有head加上tail

scala> val list = List(1,2,3,4,5)
list: List[Int] = List(1, 2, 3, 4, 5)

scala> list.getClass
res3: Class[_ <: List[Int]] = class scala.collection.immutable.$colon$colon

head取出第一个元素

scala> list.head
res4: Int = 1

headOption当List是Nil时有用

scala> val list3 = Nil
list3: scala.collection.immutable.Nil.type = List()

scala> list3.head
java.util.NoSuchElementException: head of empty list
  at scala.collection.immutable.Nil$.head(List.scala:420)
  ... 32 elided

scala> list3.headOption
res26: Option[Nothing] = None

tail取出不含第一个元素的列表

scala> list.tail
res5: List[Int] = List(2, 3, 4, 5)

scala> list.tail.tail.tail.tail
res6: List[Int] = List(5)

scala> list.tail.tail.tail.tail.tail
res7: List[Int] = List()

scala> list.tail.tail.tail.tail.tail.tail
java.lang.UnsupportedOperationException: tail of empty list
  at scala.collection.immutable.Nil$.tail(List.scala:422)
  at scala.collection.immutable.Nil$.tail(List.scala:417)
  ... 32 elided

tials

scala> list.tails
res9: Iterator[List[Int]] = non-empty iterator

scala> list.tails.foreach(println)
List(1, 2, 3, 4, 5)
List(2, 3, 4, 5)
List(3, 4, 5)
List(4, 5)
List(5)
List()

reverse

scala> list.reverse
res28: List[Int] = List(5, 4, 3, 2, 1)

scala> list
res29: List[Int] = List(1, 2, 3, 4, 5)

indexOf

scala> list.indexOf
   def indexOf[B >: Int](elem: B,from: Int): Int   def indexOf[B >: Int](elem: B): Int

scala> list.indexOf(3)
res30: Int = 2

scala> list.indexOf(3,3)
res31: Int = -1

sortBy

scala> val list = List(3,2,5,1,4)
list: List[Int] = List(3, 2, 5, 1, 4)

scala> list.sortBy
   def sortBy[B](f: Int => B)(implicit ord: scala.math.Ordering[B]): List[Int]

scala> list.sortBy(+_)
res37: List[Int] = List(1, 2, 3, 4, 5)

scala> list.sortBy(-_)
res38: List[Int] = List(5, 4, 3, 2, 1)

scala> list.sortBy(_)
<console>:13: error: missing parameter type for expanded function ((x$1) => list.sortBy(x$1))
       list.sortBy(_)
                   ^

scala> val list2 = List("q","qweee","ee","eee")
list2: List[String] = List(q, qweee, ee, eee)

scala> list2.sortBy(_.length)
res40: List[String] = List(q, ee, eee, qweee)

scala> list2.sortBy(x=>x)
res43: List[String] = List(ee, eee, q, qweee)

sortWith

scala> list
res48: List[Int] = List(3, 2, 5, 1, 4)

scala> list.sortWith
   def sortWith(lt: (Int, Int) => Boolean): List[Int]

scala> list.sortWith((a,b) => a > b)　　　　//降序
res49: List[Int] = List(5, 4, 3, 2, 1)

scala> list.sortWith((a,b) => a < b)　　　　//升序
res50: List[Int] = List(1, 2, 3, 4, 5)

scala> list.sortWith((a,b) => a == b)　　　　//不变
res51: List[Int] = List(3, 2, 5, 1, 4)

sorted升序

scala> list.sorted
   def sorted[B >: Int](implicit ord: scala.math.Ordering[B]): List[Int]

scala> list.sorted
res52: List[Int] = List(1, 2, 3, 4, 5)

take

scala> list.take
   override def take(n: Int): List[Int]

scala> list.take(2)　　//取出前n个元素
res53: List[Int] = List(3, 2)

scala> list
res54: List[Int] = List(3, 2, 5, 1, 4)

slice

scala> list.slice
   override def slice(from: Int,until: Int): List[Int]

scala> list.sorted.slice(2,3)
res55: List[Int] = List(3)

scala> list.sorted.slice(2,5)
res56: List[Int] = List(3, 4, 5)

toArray

scala> list.toArray
   def toArray[B >: Int](implicit evidence$1: scala.reflect.ClassTag[B]): Array[B]

scala> list.toArray
res57: Array[Int] = Array(3, 2, 5, 1, 4)

groupBy

scala> val list = List(2,4,3,2,4,5,6,5,5)
list: List[Int] = List(2, 4, 3, 2, 4, 5, 6, 5, 5)

scala> list.groupBy(+_)
res73: scala.collection.immutable.Map[Int,List[Int]] = Map(5 -> List(5, 5, 5), 6 -> List(6), 2 -> List(2, 2), 3 -> List(3), 4 -> List(4, 4))

scala> list.groupBy(x => if(x%2==0)"even" else "odd")
res77: scala.collection.immutable.Map[String,List[Int]] = Map(odd -> List(3, 5, 5, 5), even -> List(2, 4, 2, 4, 6))

map和flapMap对比

scala> val lst = List("hello ni hao","hello en","en ne")
lst: List[String] = List(hello ni hao, hello en, en ne)

scala> lst.map(_.split(" +"))
res105: List[Array[String]] = List(Array(hello, ni, hao), Array(hello, en), Array(en, ne))

scala> lst.flatMap(_.split(" +"))
res106: List[String] = List(hello, ni, hao, hello, en, en, ne)

可变列表

scala> import scala.collection.mutable.MutableList
import scala.collection.mutable.MutableList

scala> val lst = MutableList(3,2,4)
lst: scala.collection.mutable.MutableList[Int] = MutableList(3, 2, 4)

scala> lst += 5
res65: lst.type = MutableList(3, 2, 4, 5)

scala> lst
res66: scala.collection.mutable.MutableList[Int] = MutableList(3, 2, 4, 5)

scala> lst ++= (1 to 2)
res67: lst.type = MutableList(3, 2, 4, 5, 1, 2)

scala> lst
res68: scala.collection.mutable.MutableList[Int] = MutableList(3, 2, 4, 5, 1, 2)