《快学Scala》第三章习题解答

RT。

package com.scalalearn.scala.main
//java中的List转为scala buffer至关重要的引入
import scala.collection.JavaConversions.asScalaBuffer
import java.awt.datatransfer.{DataFlavor, SystemFlavorMap}
import scala.collection.mutable.Buffer
import scala.collection.mutable.ArrayBuffer
import scala.util.Random
/**
  * 快学scala 03习题
  */
object LearnScala03 {
  //1.编写一段代码，将a设置为n个随机整数的数组，要求随机数位于[0,n)之间
  def execise1(n:Int):Array[Int] = {
    val random:Random = new Random()
    val randomArr:Array[Int] = new Array[Int](n)
    for(i <- 0 until n) randomArr(i) = random.nextInt(n)
    randomArr
  }

  //2.编写一个程序，将整数数组中相邻的元素置换；例如，Array(1,2,3,4,5)置换后变为Array(2,1,4,3,5)
  def execise2(arr:Array[Int]):Array[Int] = {
    for(i <- Range(0,arr.length/2+1,2)){
      val temp = arr(i)
      arr(i) = arr(i+1)
      arr(i+1) = temp
    }
    arr
  }

  //3.重复前一个练习，不过这一次生成的新的值交换过的数组，用for/yeild
  def execise3(arr:Array[Int]):Array[Int] = {
    val newArr = for( i <- Range(0,arr.length)) yield {
      if (arr.length % 2 == 1 && i == arr.length - 1) arr(i) else if (i % 2 == 0) arr(i+1) else arr(i-1)
    }
    newArr.toArray
  }

  //4.给定一个整数数组，产生一个新的数组，包含原数组中的所有正值，按原有顺序排序
  //之后的元素是所有的零或者负值，按原有顺序排序
  def execise4(arr:Array[Int]):Array[Int] = {
    var newArr:Array[Int] = for (elem <- arr if elem > 0) yield elem
    val newArr1:Array[Int] = for (elem <- arr if elem <= 0) yield elem
    newArr ++= newArr1
    newArr
  }

  //5.如何计算Array[Double]的平均值
  def execise5(arr:Array[Double]):Double = {
    arr.sum/arr.length
  }

  //7.写一个程序，输出数组中的所有值，去掉重复项
  def execise7(arr:Array[Int]):Array[Int] = {
    arr.distinct
  }

  //9.创建一个java.util.TimeZone.getAvailableIDs返回的时区的集合，判断条件是它们在美洲；去掉"America/"前缀并排序
  def execise9():Unit = {
    var timezoneArr:Array[String] = java.util.TimeZone.getAvailableIDs()
    var filterArr = for(elem <- timezoneArr if !elem.startsWith("America/")) yield elem
    var sortedArr = filterArr.sorted
    for(elem <- sortedArr) println(elem)
  }

  //10.引入java.awt.datatransfer._并构建一个类型为SystemFlavorMap的对象
  //然后以DataFlavor.imageFlavor为参数调用getNativesForFlavor的方法，以Scala缓冲保留返回值
  //为什么要用这样一个晦涩难懂的类？因为java标准类库中很难得到试用java.util.List的代码
  def execise10():Unit = {
    val flavors = SystemFlavorMap.getDefaultFlavorMap.asInstanceOf[SystemFlavorMap]
    val buffer:Buffer[String] = flavors.getNativesForFlavor(DataFlavor.imageFlavor)
  }

  def main(args:Array[String]):Unit={
    println("=================execise1=====================")
    val arr1 = LearnScala03.execise1(4)
    for(n <- arr1) print(n+" ")
    println("\n")

    println("=================execise2=====================")
    val arr2 = LearnScala03.execise2(Array(1,2,3,4,5))
    for(n <- arr2) print(n+" ")
    println("\n")

    println("=================execise3=====================")
    val arr3 = LearnScala03.execise3(Array(1,2,3,4,5))
    for(n <- arr3) print(n+" ")
    println("\n")

    println("=================execise4=====================")
    val arr4 = LearnScala03.execise4(Array(3,1,-2,6,0,-1,7))
    for(n <- arr4) print(n+" ")
    println("\n")

    println("=================execise5=====================")
    println(LearnScala03.execise5(Array(1.0,2.0,3.0)))

    println("=================execise6=====================")
    //如何组织Array[Int]使他们反序排列，如果是ArrayBuffer呢？
    val arr61 = Array[Int](1,2,3)
    val arr62 = new Array[Int](arr61.length)
    for(i <- 0 until arr61.length){
      arr62(arr61.length - 1 - i) = arr61(i)
    }
    for(n <- arr62) print(n+" ")
    println("\n")

    val arr63 = ArrayBuffer(1,2,3)
    val arr64 = arr63.reverse
    for(n <- arr64) print(n+" ")
    println("\n")

    println("=================execise7=====================")
    val arr7 = LearnScala03.execise7(Array(1,3,1,2,4,2))
    for(n <- arr7) print(n+" ")
    println("\n")

    //第8题看不懂题意

    println("=================execise9=====================")
    LearnScala03.execise9()
  }
}