Scala in Impatient 习题解答3 数组相关A1

1. 编写一段代码，将a设置为一个n个随机整数的数组，要求随机数介于0（含）和n（不含 ）之间

    scala> val n = 10
    n: Int = 10
   
    scala> val a = (for (i <- 0 until n) yield scala.util.Random.nextInt(n)).toArray
    a: Array[Int] = Array(6, 7, 1, 7, 2, 0, 7, 9, 0, 6)
   

2. 交换相邻元素

    scala> val a = Array(1,2,3,4,5)
    a: Array[Int] = Array(1, 2, 3, 4, 5)
   
    for (i <- 0 until (a.length, 2)) {
        if (i + 1 < a.length) {
            var tmp = a(i)
            a(i) = a(i+1)
            a(i+1) = tmp
        }
    }
   
    scala> a
    res3: Array[Int] = Array(2, 1, 4, 3, 5)
   

3. 用yield生成新数组

    scala> val a = Array(1,2,3,4,5)
    a: Array[Int] = Array(1, 2, 3, 4, 5)
   
    for (i <- 0 until a.length) yield {
        if (i % 2 == 0) {
            if (i + 1 < a.length) a(i+1) else a(i)
        } else {
            if (i - 1 >= 0) a(i-1) else a(i)
        }
    }
   
    res2: scala.collection.immutable.IndexedSeq[Int] = Vector(2, 1, 4, 3, 5)
   

4. 给定一个数组，产出新的数组，正值在前;0和负数在后，两者都保持原顺序不变。

    scala> val a = Array(0,1,-2,-3,4,5,-6)
    a: Array[Int] = Array(0, 1, -2, -3, 4, 5, -6)
   
    scala> a.filter(_ > 0) ++ a.filter(_ <= 0)
    res5: Array[Int] = Array(1, 4, 5, 0, -2, -3, -6)
   

(好吧，有点投机去巧。)

1. 求平均值

    scala> val b = Array(0,1.44,-2.0,-3.1,4,5.99,-6)
    b: Array[Double] = Array(0.0, 1.44, -2.0, -3.1, 4.0, 5.99, -6.0)
   
    scala> b.sum / b.length
    res7: Double = 0.04714285714285715
   

2. Array[Int]和ArrayBuffer[Int]都可以用reverse方法反序排列。此题的用意是神马？难道Scala 2.8版本有什么特别吗？

3. 数组排重用ArrayOps.distinct

    scala> a
    res17: Array[Int] = Array(1, 2, 4, 3, 4, 5)
   
    scala> a.distinct
    res18: Array[Int] = Array(1, 2, 4, 3, 5)
   

4. 收集负值下标，反序，去掉最后一个。

    val indexes = for (i <- 0 until a.length if a(i) < 0) yield i
    for (i <- indexes.reverse.init) a.remove(i) 
   

与3.4的第1和第2两个代码比,remove是开销最大的，而本历两次循环，又没有避免使用 remove，连第1种都不如。效率由高到低是2>1>本例。

1. scala> import java.util.TimeZone
   import java.util.TimeZone
   
   val a = TimeZone.getAvailableIDs()
   a.filter(_.startsWith("America/")).map(_.drop("America/".length))
   

2. scala> import java.awt.datatransfer._
   import java.awt.datatransfer._
   
   scala> val flavors =
   SystemFlavorMap.getDefaultFlavorMap().asInstanceOf[SystemFlavorMap]
   flavors: java.awt.datatransfer.SystemFlavorMap =
   java.awt.datatransfer.SystemFlavorMap@1365301
   
   scala> import scala.collection.mutable.Buffer
   import scala.collection.mutable.Buffer
   
   scala> import scala.collection.JavaConversions.asScalaBuffer
   import scala.collection.JavaConversions.asScalaBuffer
   
   scala> val b: Buffer[java.lang.String] = flavors.getNativesForFlavor(DataFlavor.imageFlavor)
   b: scala.collection.mutable.Buffer[java.lang.String] = Buffer(image/jpeg,
   image/png, image/x-png, image/gif, PNG, JFIF)
   

其中import scala.collection.JavaConversions.asScalaBuffer的声明要写，否则会报 type mismatch 的错误。