Scala Array和List的區(qū)別
Difference between Array and List in scala
Q：什么時(shí)候用Array（Buffer）和List（Buffer）?
A：Scala中的List是不可變的遞歸數(shù)據(jù)（immutable recursive data），是Scala中的一種基礎(chǔ)結(jié)構(gòu)，你應(yīng)該多用List而不是Array（Array實(shí)際上是mutable，不可變（immutable）的Array是IndexedSeq）
Mutable Structures
ListBuffer提供一個(gè)常數(shù)時(shí)間的轉(zhuǎn)換到List。
一個(gè)Scala的Array應(yīng)該是由Java array生成的，因此一個(gè)Array[Int]也許比List[Int]更有效率。
但是，我認(rèn)為Scala中數(shù)組盡量少用，因?yàn)樗杏X是你真的需要知道底層發(fā)生了什么，來決定是否Array將所需的基本數(shù)據(jù)類型進(jìn)行備份，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，除非你需要快速隨機(jī)訪問或需要count batches of elements,否則，列表比數(shù)組更好。
Scala快排List和Array數(shù)組效率實(shí)測(cè)
代碼
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package com.tingfeng.scala.testimport scala.annotation.tailrecimport scala.util.{Random, Sorting}/** * 快速排序測(cè)試 */object SortTest { /** * 初始化一個(gè)數(shù)組，產(chǎn)生隨機(jī)數(shù)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) } /** * 打印出使用的時(shí)間 * @param call */ def printTime(call : => Unit,tag: String = ""){ val startTime = System.currentTimeMillis() println(tag) call println println(s"use time : ${System.currentTimeMillis() - startTime}\n") } /** * 交換數(shù)組中兩個(gè)位置的值，經(jīng)過測(cè)試這種按位與的方式比普通建立變量交換的效率更高 * @param array * @param x * @param y */ def swap(array: Array[Int],x:Int,y:Int):Unit ={ val t = array(x) ^ array(y) array(x) = t ^ array(x) array(y) = t ^ array(y) } /** * 將傳入的值直接返回,并且執(zhí)行邏輯 * @param call * @param any * @tparam A */ def doThing[A<:Any](any: A,call: A => Unit):A = { call(any) any } /** * 打印列表 */ def printList[A<%Seq[Any]](seq:A,size :Int = 10):Unit={ seq.splitAt(size)._1.foreach(it => print(s"$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) //打印出錢100個(gè)，和List快速排序的時(shí)間花費(fèi) 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") } /** * 對(duì)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) } /** * 通過每次比較數(shù)組‘head'值與其余值的方式直接實(shí)現(xiàn) * 比‘head'小的值移動(dòng)到其前，比‘head'大的移動(dòng)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) } /** * 將數(shù)據(jù)以中線拆分左右兩部分，交換值，使得右邊值比左邊大， * 再以左或者右邊交換的界限分為兩部分做遞歸 * @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) } /** * 系統(tǒng)自帶的過濾函數(shù),無法排序成功，因?yàn)閒ilter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)) } } /** * 系統(tǒng)自帶的分割函數(shù),無法排序成功，因?yàn)閜artition返回的是引用，數(shù)據(jù)量大的時(shí)候會(huì)棧溢出失敗 * @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)) } }} |
測(cè)試結(jié)果
qSortList
-2147483293,-2147483096,-2147481318,-2147480959,-2147479572,-2147479284,-2147478285,-2147477579,-2147476191,-2147475936,
use time : 28808Sorting.quickSort
-2147483293,-2147483096,-2147481318,-2147480959,-2147479572,-2147479284,-2147478285,-2147477579,-2147476191,-2147475936,
use time : 773qSortArray1
-2147483293,-2147483096,-2147481318,-2147480959,-2147479572,-2147479284,-2147478285,-2147477579,-2147476191,-2147475936,
use time : 1335qSortArray2
-2147483293,-2147483096,-2147481318,-2147480959,-2147479572,-2147479284,-2147478285,-2147477579,-2147476191,-2147475936,
use time : 629qSortArray3
508128328,554399267,876118465,968407914,1274954088,1550124974,296879812,2125832312,1874291320,965362519,
use time : 10617qSortArray4
865409973,-645195021,-735017922,-1893119148,1838343395,1038029591,-560471115,-182627393,-228613831,220531987,
use time : 6904
Process finished with exit code 0
環(huán)境：版本Scala2.12.6 ， win10 ，ryzen5 1600 ， 8G
以上為個(gè)人經(jīng)驗(yàn)，希望能給大家一個(gè)參考，也希望大家多多支持服務(wù)器之家。
原文鏈接：https://blog.csdn.net/power0405hf/article/details/50235541
