Scala基础用法(二)

流程控制

• if
  

      var a = 3;
      if(a > 0){
        println(s"$a > 0")
      }else{
        println(s"$a < 0")
      }
      
  3 > 0

• whilr

      var b = 3;
      while (b < 10){
        println(b)
        b += 1;
      }
      
  3
  4
  5
  6
  7
  8
  9

循环

• 构造一个循环

      var one :Range.Inclusive = 1 to 10
      println(one)
      结果: Range(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
      
      var one :Range.Inclusive = 1 to (10, 2)
      println(one)
      结果: Range(1, 3, 5, 7, 9)
      
      var seqs = 1 until (10, 3)
      println(seqs)
      结果: Range(1, 4, 7)

• 打印集合

      var seqs = 1 until 10
      for(i <- seqs){
        print(i)
      }
      结果： 123456789
      
      // 循环体没有break，有守卫
     var seqs = 1 until 10
      for(i <- seqs if(i % 2 == 0)){
        print(i)
      }
      结果：2468

• 打印九九乘法表

      for(i <- 1 to 9){
        for(j <- 1 to 9){
          if(j<=i) print(s"$i * $j = ${i * j}	")
          if(j == 1) println()
        }
      }
      
   简写:
      var num = 0;
       for(i <- 1 to 9; j <- 1 to 9){
           num += 1
          if(j<=i) print(s"$i * $j = ${i * j}	")
          if(j == 1) println()
      }
      println(num)
      num 打印81次
      
   性能优化:
      var num = 0;
      for(i <- 1 to 9; j <- 1 to 9 if(j<=i)){
        num += 1
           print(s"$i * $j = ${i * j}	")
          if(j == 1) println()
      }
      println(num)
      num 打印45次

• 循环结果收集

      val ints = for(i <- 1 to 10) yield i
      println(ints)
      结果：Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    
      val ints = for(i <- 1 to 10) yield {
        val num = 2;
        i * num;
      }
      println(ints)
      结果: Vector(2, 4, 6, 8, 10, 12, 14, 16, 18, 20)

函数

• 函数无返回值

      var y = fun01()
      def fun01(){
        println("fun001")
      }
      结果：fun001 函数要被调用才能打印
           ()  无结果返回

• 函数有返回值，需要1.有定义

     def fun002(): Int ={
       3
     }
      var x = fun002();
      println(x)

• 有参

      def fun03(a:Int): Unit ={
        println(a)
      }

• 递归

      def fun04(num:Int): Int ={
        if(num == 1){
          num
        }else{
           num * fun04(num - 1)
        }
      }
      val i = fun04(4)
      println(i)
      
      结果: 24 
  return 4 * fun04(4 - 1)
  解析：
  fun04(4 - 1) = 3 * 2 * 1
  2 * fun04(2-1)
  fun04(2-1) = 1

• 参数默认值

      def fun05 (a:Int = 10, b:String = "abc"): Unit ={
        println(s"$a	$b")
      }
      fun05()
      结果：10    abc
      
      def fun05 (a:Int = 10, b:String = "abc"): Unit ={
        println(s"$a	$b")
      }
      fun05(20)
      结果： 20 abc
      
      def fun05 (a:Int = 10, b:String = "abc"): Unit ={
        println(s"$a	$b")
      }
      fun05(b = "wang")
      结果： 10    wang

• 匿名函数

      var fun06 = (a:Int, b:Int) => {
        a + b
      }
      val str = fun06(3, 4)
      val str:Int = fun06(3, 4)
      println(str)
      结果：7
      
      var fun06:(Int, Int)=>Int   =   (a:Int, b:Int) =>{
        a + b
      }
      val str:Int = fun06(3, 4)
      println(str)
      结果：7
      
      (Int, Int) => Int (参数类型列表) => 返回值类型
      (a:Int, b:Int) => {a + b}     (参数实现列表) => 函数体

• 嵌套函数
            方法里面套娃
• 偏函数引用

      def fun07(date:Date, level:String, msg:String){
        println(s"date=$date	 tp=$level 	 msg=$msg")
      }
      fun07(new Date(), "info", "MESSAGE")
      结果：date=Sat Jan 30 19:02:31 CST 2021     tp=info      msg=MESSAGE
      
      将日志级别恒定为error级别
      var fun = fun07(_:Date, "error", _:String)
      fun(new Date(), "exception...")
      结果: date=Sat Jan 30 19:03:56 CST 2021     tp=error      msg=exception...

• 可变参数

      def fun08(a:Int*): Unit ={
        for(e <- a){
          println(e)
        }
      }
      fun08(5, 6 , 7)
      
      结果：
      5
      6
      7
      

• forEach

      def fun08(a:Int*): Unit ={
        a.foreach( (x:Int)=> {
          println(x)
        })
      }
      fun08(5, 6 , 7)
      
      当有一个参数时候
      def fun08(a:Int*): Unit ={
        a.foreach( println(_))
      }
      fun08(5, 6 , 7)
      
      当有一个参数时候
      def fun08(a:Int*): Unit ={
        a.foreach( println)
      }
      fun08(5, 6 , 7)

高阶函数

• 函数作为参数

      def computer(a:Int, b:Int, fun:(Int, Int) => Int): Unit ={
        var res: Int = fun(a , b)
        println(res)
      }
      computer(4, 5, (x:Int, y: Int)=>{ x * y})
      computer(4, 5, _+_)

• 函数作为返回值

      def fun09(i: String): (Int, Int) => Int = {
        def plus(x: Int, y: Int): Int = {
          x + y
        }
        if (i.equals("+")) {
          plus
        } else {
          (x: Int, y: Int) => {
            x * y
          }
        }
      }
  
      computer(4, 5, fun09("+"))
      def computer(a: Int, b: Int, fun: (Int, Int) => Int): Unit = {
        var res: Int = fun(a, b)
        println(res)
      }
      结果： 9

• 柯里化

      def fun10(a:Int)(b:Int)(c:String): Unit ={
        println(s"$a	$b	$c")
      }
      fun10(3)(8)("aa")
      结果:  3    8    aa
  
  
  应用: 一个可变参数想多个类型，可以拆成多个可变参数
      def fun11(a:Int*)(b:String*): Unit ={
        a.foreach(print)
        println()
        b.foreach(print)
      }
      fun11(1,2,3)("a","b","c")
      结果：
          123
          abc

• 函数赋值

  def fun12(): Unit ={
  println("我是context")
  }
  val context = fun12
  println(context)
  val func = fun12 _ 这样写只有函数调用的时候才打印
  func()

  结果：

  我是context
  ()
  我是context