11.6 扫描(scan)

扫描，即对某个集合的所有元素做fold操作，但是会把产生的所有中间结果放置于一个集合中保存

package com.atguigu.day11.operate

object ScanDemo1 {
  def main(args: Array[String]): Unit = {
    var list1 = List(10, 1, 3, 5, 8, 9, 22, 89)
    //         返回的是一个保存中间结果的集合 100,  90, 89, 86,...
    val list2: List[Int] = list1.scan(100)(minus)
    println(list2)
    val list3: List[Int] = list1.scanLeft(100)(minus)
    println(list3)
    val list4: List[Int] = list1.scanRight(100)(minus)
    println(list4)
  }

  def minus(a: Int, b: Int) = {
    a - b
  }
}

练习1

val sentence = "AAAAAAAAAABBBBBBBBCCCCCDDDDDDD" 将sentence 中各个字符，通过foldLeft存放到一个ArrayBuffer中

package com.atguigu.day11.operate

import scala.collection.mutable.ArrayBuffer

object Test1 {
  def main(args: Array[String]): Unit = {
    val sentence = "AAAAAAAAAABBBBBBBBCCCCCDDDDDDD"
    var arr:ArrayBuffer[Char] = new ArrayBuffer[Char]

    sentence.foldLeft(arr)(putArray)
    println(arr)
  }

  def putArray(arr: ArrayBuffer[Char], c: Char) : ArrayBuffer[Char]= {
    arr.append(c)
    arr
  }
}

练习2:

val sentence = "AAAAAAAAAABBBBBBBBCCCCCDDDDDDD" 使用映射集合，统计一句话中，各个字母出现的次数

package com.atguigu.day11.operate

import scala.collection.mutable

object Test2 {
  def main(args: Array[String]): Unit = {
    // 统计每个字符出现的次数  A 3, B 10
    val sentence = "AAAAAAAAAABBBBBBBBCCCCCDDDDDDD"
    val map: mutable.Map[Char, Int] = mutable.Map[Char, Int]()
    sentence.foldLeft(map)(charCount)
    println(map)
  }

  def charCount(map: mutable.Map[Char, Int], c: Char) = {
    map(c) = map.getOrElse(c, 0) + 1
    map
  }
}