Scala中Array和List的区别说明

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Scala中Array和List的区别说明

Scala中Array和List的区别说明

目录Scala Array和List的区别Scala快排List和Array数组效率实测

Scala Array和List的区别

Difference between Array and List in scala

Q:什么时候用Array(Buffer)和List(Buffer)?

A:Scala中的List是不可变的递归数据(immutable recursive data),是Scala中的一种基础结构,你应该多用List而不是Array(Array实际上是mutable,不可变(immutable)的Array是IndexedSeq)

Mutable Structures

ListBuffer提供一个常数时间的转换到List。

一个Scala的Array应该是由java array生成的,因此一个Array[Int]也许比List[Int]更有效率。

但是,我认为Scala中数组尽量少用,因为它感觉是你真的需要知道底层发生了什么,来决定是否Array将所需的基本数据类型进行备份,或者可能boxed as a wrapper type.

Performance differences

Array

List

Access the ith element

O(1)

O(i)

Discard the ith element

O(n)

O(i)

Insert an element at i

O(n)

O(i)

Reverse

O(n)

O(n)

Concatenate (length m,n)

O(n+m)

O(n)

Calculate the length

O(1)

O(n)

memory differences

Array

List

Get the first i elements

O(i)

O(i)

Drop the first i elements

O(n-i)

O(1)

Insert an element at i

O(n)

O(i)

Reverse

O(n)

O(n)

Concatenate (length m,n)

O(n+m)

O(n)

所以,除非你需要快速随机访问或需要count batches of elements,否则,列表比数组更好。

Scala快排List和Array数组效率实测

代码

package com.tingfeng.scala.test

import scala.annotation.tailrec

import scala.util.{Random, Sorting}

/**

* 快速排序测试

*/

object SortTest {

/**

* 初始化一个数组,产生随机数字填充

* @param size

* @return

*/

def initRandomList(size :Int):List[Int]={

val random = new Random()

def initList(size :Int,random: Random):List[Int] = size match {

case 0 => Nil

case 1 => List(random.nextInt())

case s:Int =>

val value = s / 2

if( s % 2 == 0) {

initList(value,random) ++ initList(value,random)

}else{

initList(value,random) ++ initList(value + 1,random)

}

}

initList(size,random)

}

/**

* 打印出使用的时间

* @param call

*/

def printTime(call : => Unit,tag: String = ""){

val startTime = System.currentTimeMillis()

println(tag)

call

println

println(s"use time : ${System.currentTimeMillis() - startTime}\n")

}

/**

* 交换数组中两个位置的值,经过测试这种按位与的方式比普通建立变量交换的效率更高

* @param array

* @param x

* @param y

*/

def swap(array: Array[Int],x:Int,y:Int):Unit ={

val t = array(x) ^ array(y)

array(x) = t ^ array(x)

array(y) = t ^ array(y)

}

/**

* 将传入的值直接返回,并且执行逻辑

* @param call

* @param any

* @tparam A

*/

def doThing[A<:any a=""> Unit):A = {

call(any)

any

}

/**

* 打印列表

*/

def printList[A<%Seq[Any]](seq:A,size :Int = 10):Unit={

seq.splitAt(size)._1.foreach(it => print(s"$it,"))

}

def shuffleIntSeq(seq: Array[Int],size: Int):Unit={

val random = new Random()

val maxSize = size/2

for(i <- 0 to maxSize){

swap(seq,i,maxSize + random.nextInt(maxSize))

}

}

def main(args: Array[String]): Unit = {

val size = 5000000

val printSize = 10

val list = initRandomList(size)

//打印出钱100个,和List快速排序的时间花费

printTime(printList[List[Int]](qSortList(list),Math.min(10,size)),"qSortList")

val array = list.toArray

printTime(printList[Array[Int]](doThing[Array[Int]](array,Sorting.quickSort),Math.min(printSize,size)),"Sorting.quickSort")

shuffleIntSeq(array,size)

printTime(printList[Array[Int]](doThing[Array[Int]](array,qSortArray1),Math.min(printSize,size)),"qSortArray1")

shuffleIntSeq(array,size)

printTime(printList[Array[Int]](doThing[Array[Int]](array,qSortArray2),Math.min(printSize,size)),"qSortArray2")

shuffleIntSeq(array,size)

printTime(printList[Array[Int]](doThing[Array[Int]](array,qSortArray3),Math.min(printSize,size)),"qSortArray3")

shuffleIntSeq(array,size)

printTime(printList[Array[Int]](doThing[Array[Int]](array,qSortArray4),Math.min(printSize,size)),"qSortArray4")

}

/**

* 对List快速排序

* @param list

* @return

*/

def qSortList(list: List[Int]):List[Int] = list match {

case Nil => Nil

case head :: other =>

val (left, right) = other.partition(_ < head)

(qSortList(left) :+ head) ++ qSortList(right)

}

/**

* 通过每次比较数组‘head'值与其余值的方式直接实现

* 比‘head'小的值移动到其前,比‘head'大的移动到其之后

* @param array

*/

def qSortArray1(array: Array[Int]):Unit = {

def sort(ay : Array[Int],start: Int,end: Int):Unit={

if(start >= end) {

return

}

val head = ay(start)

var spliteIndex = start

for (i <- start + 1 to end){

if(ay(i) < head){

swap(array,spliteIndex,i)

spliteIndex += 1

}

}

if(start != spliteIndex){

sort(ay, start, spliteIndex)

}

if(start == spliteIndex){

spliteIndex += 1

}

if(spliteIndex != end){

sort(ay, spliteIndex, end)

}

}

sort(array,0,array.size - 1)

}

/**

* 将数据以中线拆分左右两部分,交换值,使得右边值比左边大,

* 再以左或者右边交换的界限分为两部分做递归

* @param array

*/

def qSortArray2(array: Array[Int]) {

def sort(l: Int, r: Int) {

val pivot = array((l + r) / 2)

var lv = l; var rv = r

while (lv <= rv) {

while (array(lv) < pivot) lv += 1

while (array(rv) > pivot) rv -= 1

if (lv <= rv) {

swap(array,lv, rv)

lv += 1

rv -= 1

}

}

if (l < rv) sort(l, rv)

if (rv < r) sort(lv, r)

}

sort(0, array.length - 1)

}

/**

* 系统自带的过滤函数,无法排序成功,因为filter返回的是引用

* @param xs

* @return

*/

def qSortArray3(xs: Array[Int]): Array[Int] ={

if (xs.length <= 1){

xs

}else {

val pivot = xs(xs.length / 2)

val left = xs filter (pivot > _)

val cu = xs filter (pivot == _ )

val right = xs filter (pivot < _ )

Array.concat(

qSortArray3(left),cu,qSortArray3(right))

}

}

/**

* 系统自带的分割函数,无法排序成功,因为partition返回的是引用,数据量大的时候会栈溢出失败

* @param xs

* @return

*/

def qSortArray4(array: Array[Int]): Array[Int] ={

if (array.length <= 1){

array

}else {

val head = array(0)

val (left,right) = array.tail partition (_ < head )

Array.concat(qSortArray4(left),Array(head),qSortArray4(right))

}

}

}

测试结果

qSortList

-2147483293,-2147483096,-2147481318,-2147480959,-2147479572,-2147479284,-2147478285,-2147477579,-2147476191,-2147475936,

use time : 28808

Sorting.quickSort

-2147483293,-2147483096,-2147481318,-2147480959,-2147479572,-2147479284,-2147478285,-2147477579,-2147476191,-2147475936,

use time : 773

qSortArray1

-2147483293,-2147483096,-2147481318,-2147480959,-2147479572,-2147479284,-2147478285,-2147477579,-2147476191,-2147475936,

use time : 1335

qSortArray2

-2147483293,-2147483096,-2147481318,-2147480959,-2147479572,-2147479284,-2147478285,-2147477579,-2147476191,-2147475936,

use time : 629

qSortArray3

508128328,554399267,876118465,968407914,1274954088,1550124974,296879812,2125832312,1874291320,965362519,

use time : 10617

qSortArray4

865409973,-645195021,-735017922,-1893119148,1838343395,1038029591,-560471115,-182627393,-228613831,220531987,

use time : 6904

Process finished with exit code 0

环境:版本Scala2.12.6 , win10 ,ryzen5 1600 , 8G

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