Scala学习笔记（四）—— 数组

定长数组Array

定义定长数组用Array，有如下几种方法：

• 初始化一个长度为8的定长数组，其所有元素默认值均为0

   scala> new Array[Int](8)
   res0: Array[Int] = Array(0, 0, 0, 0, 0, 0, 0, 0)

• 调用静态方法，直接为数组赋值

   scala> val arr2 = Array[Int](8)
   arr2: Array[Int] = Array(8)
  
   scala> arr2.length
   res16: Int = 1
  
   scala> Array(1,"1")
   res17: Array[Any] = Array(1, 1)
  
   scala> val arr3 = Array(1,"string",3.14)
   arr3: Array[Any] = Array(1, string, 3.14)
  
   scala> val arr4 = Array(1,2,3)
   arr4: Array[Int] = Array(1, 2, 3)

变长数组ArrayBuffer

• 需要导入import scala.collection.mutable.ArrayBuffer包

   scala> import scala.collection.mutable.ArrayBuffer
   import scala.collection.mutable.ArrayBuffer

• 初始化有以下两种方法：

   scala> import scala.collection.mutable.ArrayBuffer
   import scala.collection.mutable.ArrayBuffer
  
   scala> val ab =new ArrayBuffer[Int]()
   ab: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer()
  
   scala> val ab1 = ArrayBuffer(1,2)
   ab1: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 2)

• 使用+=追加（多个）元素，使用++= 追加数组

   scala> ab += 1
   res0: ab.type = ArrayBuffer(1)
  
   scala> ab += 2
   res1: ab.type = ArrayBuffer(1, 2)
  
   scala> ab += (3,4,5)
   res2: ab.type = ArrayBuffer(1, 2, 3, 4, 5)
  
   scala> ab ++= Array(6,7)
   res4: ab.type = ArrayBuffer(1, 2, 3, 4, 5, 6, 7)
  
   scala> ab ++= ArrayBuffer(8,9)
   res5: ab.type = ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8, 9)

• 指定位置插入元素：第一个入参是指定要插入的下标，后面的元素则插入在该下标及其之后

    scala> ab
    res0: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 2, 3, 4, 5, 6,7, 8, 9)
  
    scala> ab.insert(0,0)
  
    scala> ab
    res2: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
  
    scala> ab.length
    res3: Int = 10
  
    scala> ab.insert(11,11)
    java.lang.IndexOutOfBoundsException: 11
      at scala.collection.mutable.ArrayBuffer.insertAll(ArrayBuffer.scala:139)
      at scala.collection.mutable.BufferLike$class.insert(BufferLike.scala:167)
      at scala.collection.mutable.AbstractBuffer.insert(Buffer.scala:49)
      ... 32 elided
  
    scala> ab.insert(10,10)
  
    scala> ab
    res7: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

  注意：插入的下标 <= 该数组的长度，若插入的下标为数组的长度则表示在末尾追加；若插入的下标大于该数组的长度，则报错数组越界 IndexOutOfBoundsException

• 删除指定位置的元素：第一个入参是指定要删除的下标；第二个入参是要删除的数据的长度，默认值为1

   scala> ab.length
   res9: Int = 11
  
   scala> ab.remove(11)
   java.lang.IndexOutOfBoundsException: 11
     at scala.collection.mutable.ResizableArray$class.apply(ResizableArray.scala:43
   )
     at scala.collection.mutable.ArrayBuffer.apply(ArrayBuffer.scala:48)
     at scala.collection.mutable.ArrayBuffer.remove(ArrayBuffer.scala:169)
     ... 32 elided
  
   scala> ab.remove(10)
   res11: Int = 10
  
   scala> ab
   res12: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
  
   scala> ab.remove(9,2)
   java.lang.IndexOutOfBoundsException: 9
     at scala.collection.mutable.ArrayBuffer.remove(ArrayBuffer.scala:158)
     ... 32 elided
  
   scala> ab.length
   res14: Int = 10
  
   scala> ab.remove(9,1)
  
   scala> ab
   res16: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8)

　　　　注意：要删除的下标 < 数组的长度， 否则报错数组越界 IndexOutOfBoundsException

• 遍历数组

　　　　请参考笔记（二）for循环

• 数组常用算法

  　用filter方法进行过滤

  scala> ab
  res22: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 6, 7, 8)
  
  scala> ab.filter(_ % 2 == 0)
  res23: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 2, 4, 6, 8)

  用map方法进行数组转换

   scala> ab
   res24: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8)
  
   scala> ab.map(_ * 10)
   res25: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 10, 20, 30, 40, 50, 60, 70, 80)

  例：将数组中偶数取出来乘以100

   scala> ab
   res26: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8)
  
   scala> ab.filter(_ % 2 == 0).map(_*100)
   res27: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 200, 400, 600, 800)

  数组求和、最小值、最大值、排序

  scala> ab
  res35: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 10, 50, 30, 5, 6, 7, 8)
  
  scala> ab.sum
  res36: Int = 126
  
  scala> ab.min
  res38: Int = 0
  
  scala> ab.max
  res39: Int = 50
  
  scala> ab.sorted
  res40: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 30, 50)