Scala之高阶函数 | TUNANのBlog

目标

sorted
sortBy
sortWith
flatten
map
flatMap
filter
groupBy
flod
reduce
wc

sorted

排序，默认升序

val array = Array(1, 2, 5, 4, 3)
    array.sorted

sortBy

排序，指定排序规则

val arrMap = Array(('a', 1), ('b', 4), ('d', 3), ('c', 2))
    arrMap.sortBy(_._1)
    arrMap.sortBy(_._2)

sortWith

排序，多个字段联合排序

val arrMap2 = Array(("a", 2), ("a", 1), ("b", 2), ("d", 5), ("c", 4))
    arrMap2.sortWith((t1,t2)=>{
      if (!t1._1.equalsIgnoreCase(t2._1)){
        t1._1.compareTo(t2._1)<0
      }else{
        t1._2.compareTo(t2._2)<0
      }
    })

flatten

扁平化

val flatten = Array(Array(1, 2, 3), Array(4, 5, 6))
    flatten.flatten

map

映射

val map = Array("this is a demo,hello world")
    map.map(_.split(" "))

    map.map(_.split(" ")).flatten.map((_,1))

flatMap

flatMap ==> flatten + map

val flatMap = Array("this is a demo", "hello word")
    flatMap.flatMap(_.split(" "))
    flatMap.flatMap(_.split(" ")).map((_,1))

    flatMap.flatMap(line => (for (i<- line.split(" ")) yield (i,1)))

filter

过滤

val filter = Array("this is a demo", "hello word")
    filter.filter(_.contains("hello"))
    filter.filterNot(_.contains("hello"))

groupBy

分组

val groupBy = Array("good", "good", "study")
    groupBy.groupBy(x=>x)
    groupBy.groupBy(x=>x).map(t=>(t._1,t._2.size))

flod

聚合，需要指定默认值

val flod = Array(1, 2, 3, 1, 2, 4, 5, 6, 8, 4)
    flod.fold(10)((a,b)=>{
      println(a+"  :  "+b)
      a-b
    })

reduce

聚合

val reduce = Array(1, 2, 3, 1, 2, 4, 5, 6, 8, 4)
    reduce.reduce(_+_)
    reduce.reduce(_-_)
    reduce.reduceLeft(_+_)
    reduce.reduceLeft(_-_)
    reduce.reduceRight(_+_)
    reduce.reduceRight(_-_)

wc

方案1

val wc = Array("this is demo", "good good study", "day day up")
    wc.flatMap(_.split(" "))
      .groupBy(word=>word)
      .toList
      .map(t=>(t._1,t._2.size))
      .sortBy(_._1)
      .mkString(",")

方案2

wc.flatMap(_.split(" "))
     .map((_,1))
     .groupBy(_._1)
     .toList
     .map(t=>(t._1,t._2.size))
     .sortBy(_._1)
     .mkString("\t")