foldLeft

package spark2021

object foldLeft_test {
  def main(args: Array[String]): Unit = {




    //场景一：列表数值相加
    val list = List(1,2,3,4,5)
    val sum_list: Int = list.foldLeft(0)(_+_)
    println(sum_list)
    //map++更新数据 map+添加数据

    //合并两个Map
    //++:重复key值相加，不覆盖
    val map1 = Map("aa" -> 1, "bb" -> 2)
    val map2 = Map("aa" -> 2, "cc" -> 4)

    val map3: Map[String, Int] = map1++map2
    println(map3) //Map(aa -> 2.0, bb -> 2.0, cc -> 4.0)

    //foldLeft:map相同key的value没有被覆盖，而是相加合并了
    val map4: Map[String, Int] = map1.foldLeft(map2) {
      case (newMap, (name, score)) => {
        newMap + (name -> (score + map2.getOrElse(name, 0)))
      }
    }
    println(map4) //Map(aa -> 3, cc -> 4, bb -> 2)

    //统计不同长度单词的总词频
    val wordsMap = Map("apple" -> 20, "pear" -> 10, "pineapple" -> 25, "grape" -> 30)
    val map5= wordsMap.foldLeft(Map.empty[Int,Int]){
      case(newMap,(fruit,count))=>{
        newMap+(fruit.length->(count+newMap.getOrElse(fruit.length,0)))
      }
    }
    println(map5)

  }

}