Scala入门到精通——第三节 Array、List

本节主要内容

1. 数组操作实战
2. 列表List操作实战

数组操作实战

1 定长数组

//定义一个长度为10的数值数组scala> val numberArray=new Array[Int](10)numberArray: Array[Int] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0)//定义一个长度为10的String类型数组scala> val strArray=new Array[String](10)strArray: Array[String] = Array(null, null, null, null, null, null, null, null,null, null)//可以看出：复杂对象类型在数组定义时被初始化为null，数值型被初始化为0//数组元素赋值scala> strArray(0)="First Element"//需要注意的是，val strArray=new Array[String](10)//这意味着strArray不能被改变，但数组内容是可以改变的scala> strArrayres62: Array[String] = Array(First Element, null, null, null, null, null, null,null, null, null)//另一种定长数组定义方式//这种调用方式其实是调用其apply方法进行数组创建操作scala> val strArray2=Array("First","Second")strArray2: Array[String] = Array(First, Second)Scala中的Array以Java中的Array方式实现

2 变长数组 ArrayBuffer

//要使用ArrayBuffer，先要引入scala.collection.mutable.ArrayBufferscala> import scala.collection.mutable.ArrayBufferimport scala.collection.mutable.ArrayBuffer//创建String类型ArrayBuffer数组缓冲scala> val strArrayVar=ArrayBuffer[String]()strArrayVar: scala.collection.mutable.ArrayBuffer[String] = ArrayBuffer()//+=意思是在尾部添加元素scala>     strArrayVar+="Hello"res63: strArrayVar.type = ArrayBuffer(Hello)//+=后面还可以跟多个元素的集合//注意操作后的返回值scala> strArrayVar+=("World","Programmer")res64: strArrayVar.type = ArrayBuffer(Hello, World, Programmer)//显示完整数组内容scala> strArrayVarres65: scala.collection.mutable.ArrayBuffer[String] = ArrayBuffer(Hello, World,Programmer)//++=用于向数组中追加内容，++=右侧可以是任何集合//追加Array数组scala> strArrayVar++=Array("Wllcome","To","XueTuWuYou")res66: strArrayVar.type = ArrayBuffer(Hello, World, Programmer, Wllcome, To, XueTuWuYou)//追加Listscala> strArrayVar++=List("Wellcome","To","XueTuWuYou")res67: strArrayVar.type = ArrayBuffer(Hello, World, Programmer, Wllcome, To, XueTuWuYou, Wellcome, To, XueTuWuYou)//删除末尾n个元素scala> strArrayVar.trimEnd(3)scala> strArrayVarres69: scala.collection.mutable.ArrayBuffer[String] = ArrayBuffer(Hello, World,Programmer, Wllcome, To, XueTuWuYou)

//创建整型数组缓冲scala> var intArrayVar=ArrayBuffer(1,1,2)intArrayVar: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 1, 2)//在数组索引为0的位置插入元素6scala> intArrayVar.insert(0,6)scala> intArrayVarres72: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(6, 1, 1, 2)//在数组索引为0的位置插入元素7,8,9scala> intArrayVar.insert(0,7,8,9)scala> intArrayVarres74: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(7, 8, 9, 6, 1, 1,2)//从索引0开始，删除4个元素scala> intArrayVar.remove(0,4)scala> intArrayVarres77: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 1, 2)//转成定长数组scala> intArrayVar.toArrayres78: Array[Int] = Array(1, 1, 2)//将定长数组转成ArrayBufferscala> res78.toBufferres80: scala.collection.mutable.Buffer[Int] = ArrayBuffer(1, 1, 2)

3 数组的遍历

//toscala> for(i <- 0 to intArrayVar.length-1) println("Array Element: " +intArrayVar(i))Array Element: 1Array Element: 1Array Element: 2//untilscala> for(i <- 0 until intArrayVar.length) println("Array Element: " +intArrayVar(i))Array Element: 1Array Element: 1Array Element: 2//数组方式（推荐使用）scala> for(i <- intArrayVar) println("Array Element: " + i)Array Element: 1Array Element: 1Array Element: 2//步长为2scala>  for(i <- 0 until (intArrayVar.length,2)) println("Array Element: " +intArrayVar(i))Array Element: 1Array Element: 2//倒序输出scala> for( i<- (0 until intArrayVar.length).reverse) println("Array Element: "+ intArrayVar(i))Array Element: 2Array Element: 1Array Element: 1

4 数组转换

//生成新的数组，原数组不变//缓冲数据转换后产生的仍然是缓冲数组scala> var intArrayVar2=for(i <- intArrayVar) yield i*2intArrayVar2: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(2, 2, 4)//定长数组转转后产生的仍然是定长数组，原数组不变scala> var intArrayNoBuffer=Array(1,2,3)intArrayNoBuffer: Array[Int] = Array(1, 2, 3)scala> var intArrayNoBuffer2=for(i <- intArrayNoBuffer) yield i*2intArrayNoBuffer2: Array[Int] = Array(2, 4, 6)//加入过滤条件scala>  var intArrayNoBuffer2=for(i <- intArrayNoBuffer if i>=2) yield i*2intArrayNoBuffer2: Array[Int] = Array(4, 6)

5 数组操作中的常用算法

//定义一个整型数组scala> val intArr=Array(1,2,3,4,5,6,7,8,9,10)intArr: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)//求和scala> intArr.sumres87: Int = 55//求最大值scala> intArr.maxres88: Int = 10scala> ArrayBuffer("Hello","Hell","Hey","Happy").maxres90: String = Hey//求最小值scala> intArr.minres89: Int = 1//toString()方法scala> intArr.toString()res94: String = [I@141aba8//mkString()方法scala> intArr.mkString(",")res96: String = 1,2,3,4,5,6,7,8,9,10scala> intArr.mkString("<",",",">")res97: String = <1,2,3,4,5,6,7,8,9,10>

6 ArrayBuffer Scaladoc解析

初学者在查看sacaladoc时常常会感到困惑，不用担心，随着学习的深入，api文档中的内容将逐渐清晰
下面给出两个示例：
++=方法传入的参数类型是TraversableOnce Trait的子类，它返回的是更新好的ArrayBuffer

dropWhile传入的是一个函数，该函数返回值是布尔类型，dropWhile反回的是操作后的ArrayBuffer

7 多维数组

通过数组的数组实现多维数组的定义：

//定义2行3列数组scala> var multiDimArr=Array(Array(1,2,3),Array(2,3,4))multiDimArr: Array[Array[Int]] = Array(Array(1, 2, 3), Array(2, 3, 4))//获取第一行第三列元素scala> multiDimArr(0)(2)res99: Int = 3//多维数组的遍历scala> for(i <- multiDimArr) println( i.mkString(","))1,2,32,3,4

列表List操作实战

1 List类型定义及List的特点

//字符串类型Listscala> val fruit=List("Apple","Banana","Orange")fruit: List[String] = List(Apple, Banana, Orange)//前一个语句与下面语句等同scala> val fruit=List.apply("Apple","Banana","Orange")fruit: List[String] = List(Apple, Banana, Orange)//数值类型Listscala> val nums=List(1,2,3,4,5)nums: List[Int] = List(1, 2, 3, 4, 5)//多重List，List的子元素为Listscala> val diagMatrix=List(List(1,0,0),List(0,1,0),List(0,0,1))diagMatrix: List[List[Int]] = List(List(1, 0, 0), List(0, 1, 0), List(0, 0, 1))//遍历Listscala> for (i <- nums) println("List Element: "+i)List Element: 1List Element: 2List Element: 3List Element: 4List Element: 5

不难看出，List与Array有着诸多的相似之处，但它们有两个明显的区别：
1 List一但创建，其值不能被改变
如前面的nums，改变其值的话，编译器会报错

scala> nums(3)=4<console>:10: error: value update is not a member of List[Int]              nums(3)=4              ^

2 List具有递归结构（Recursive Structure),例如链表结构

List类型和其它类型集合一样，它具有协变性（Covariant)，即对于类型S和T，如果S是T的子类型，则List[S]也是List[T]的子类型

例如

scala> var listStr:List[Object]=List("This","Is","Covariant","Example")listStr: List[Object] = List(This, Is, Covariant, Example)//空的List，其类型为Nothing,Nothing在Scala的继承层次中的最低层//，即Nothing是任何Scala其它类型如String,Object等的子类scala> var listStr=List()listStr: List[Nothing] = List()scala> var listStr:List[String]=List()listStr: List[String] = List()

2 List常用构造方法

//采用::及Nil进行列表构建scala> val nums = 1 :: (2 :: (3 :: (4 :: Nil)))nums: List[Int] = List(1, 2, 3, 4)//由于::操作符的优先级是从右往左的，因此上一条语句等同于下面这条语句scala> val nums=1::2::3::4::Nilnums: List[Int] = List(1, 2, 3, 4)

3 List常用操作

//判断是否为空scala> nums.isEmptyres108: Boolean = false//取第一个无素scala> nums.headres109: Int = 1//取除第一个元素外剩余的元素，返回的是列表scala> nums.tailres114: List[Int] = List(2, 3, 4)//取列表第二个元素scala> nums.tail.headres115: Int = 2//插入排序算法实现def isort(xs: List[Int]): List[Int] =if (xs.isEmpty) Nilelse insert(xs.head, isort(xs.tail))def insert(x: Int, xs: List[Int]): List[Int] =if (xs.isEmpty || x <= xs.head) x :: xselse xs.head :: insert(x, xs.tail)//List连接操作scala> List(1,2,3):::List(4,5,6)res116: List[Int] = List(1, 2, 3, 4, 5, 6)//取除最后一个元素外的元素，返回的是列表scala> nums.initres117: List[Int] = List(1, 2, 3)//取列表最后一个元素scala> nums.lastres118: Int = 4//列表元素倒置scala> nums.reverseres119: List[Int] = List(4, 3, 2, 1)//一些好玩的方法调用scala> nums.reverse.reverse==numsres120: Boolean = truescala> nums.reverse.initres121: List[Int] = List(4, 3, 2)scala> nums.tail.reverseres122: List[Int] = List(4, 3, 2)//丢弃前n个元素scala> nums drop 3res123: List[Int] = List(4)scala> nums drop 1res124: List[Int] = List(2, 3, 4)//获取前n个元素scala> nums take 1res125: List[Int] = List(1)scala> nums.take(3)res126: List[Int] = List(1, 2, 3)//将列表进行分割scala> nums.splitAt(2)res127: (List[Int], List[Int]) = (List(1, 2),List(3, 4))//前一个操作与下列语句等同scala> (nums.take(2),nums.drop(2))res128: (List[Int], List[Int]) = (List(1, 2),List(3, 4))//Zip操作scala> val nums=List(1,2,3,4)nums: List[Int] = List(1, 2, 3, 4)scala> val chars=List('1','2','3','4')chars: List[Char] = List(1, 2, 3, 4)//返回的是List类型的元组(Tuple）scala> nums zip charsres130: List[(Int, Char)] = List((1,1), (2,2), (3,3), (4,4))//List toString方法scala> nums.toStringres131: String = List(1, 2, 3, 4)//List mkString方法scala> nums.mkStringres132: String = 1234//转换成数组scala> nums.toArrayres134: Array[Int] = Array(1, 2, 3, 4)

1. List伴生对象方法

//apply方法scala>  List.apply(1, 2, 3)res139: List[Int] = List(1, 2, 3)//range方法，构建某一值范围内的Listscala>  List.range(2, 6)res140: List[Int] = List(2, 3, 4, 5)//步长为2scala>  List.range(2, 6,2)res141: List[Int] = List(2, 4)//步长为-1scala>  List.range(2, 6,-1)res142: List[Int] = List()scala>  List.range(6,2 ,-1)res143: List[Int] = List(6, 5, 4, 3)//构建相同元素的Listscala> List.make(5, "hey")res144: List[String] = List(hey, hey, hey, hey, hey)//unzip方法scala> List.unzip(res145)res146: (List[Int], List[Char]) = (List(1, 2, 3, 4),List(1, 2, 3, 4))//list.flatten，将列表平滑成第一个无素scala> val xss =     | List(List('a', 'b'), List('c'), List('d', 'e'))xss: List[List[Char]] = List(List(a, b), List(c), List(d, e))scala> xss.flattenres147: List[Char] = List(a, b, c, d, e)//列表连接scala> List.concat(List('a', 'b'), List('c'))res148: List[Char] = List(a, b, c)

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