Scala函数式编程

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

    val ints = List(1, 2, 3, 4)

    // foreach(循环)
    ints.foreach(e => {
      println(e)
    })
    // 简化写法(当函数参数，只在函数体中出现一次，而且函数体没有嵌套调用时，可以使用下划线来简化函数定义)
    ints.foreach(println(_))

    // map(映射)
    val newMapList = ints.map(e => e + 1)

    // flatMap(将多个集合合并成一个)
    val flatMap = List("hadoop hive spark flink flume", "kudu hbase sqoop storm")
    val newFlatMapList = flatMap.flatMap(e => e.split(" "))
    // 简化写法
    flatMap.flatMap(_.split(" "))


    // filter(过滤)
    ints.filter(e => e % 2 == 0)
    ints.filter(_ % 2 == 0)

    // sorted(排序)
    ints.sorted
    // 按照某个字段排序
    val sortByField = List("01 hadoop", "02 flume", "03 hive", "04 spark")
    sortByField.sortBy(e => e.split(" ")(0))
    sortByField.sortBy(_.split(" ")(0))
    // 自定义排序
    val sortWithList = List(2,3,1,6,4,5)
    sortWithList.sortWith((x, y) => { x < y })
    // 简化写法
    sortWithList.sortWith(_ < _)

    // groupBy(分组)
    val groupByList = List("张三"->"男", "李四"->"女", "王五"->"男")
    groupByList.groupBy(e => e._2)
    groupByList.groupBy(_._2)

    // reduce(聚合)
    val reduceList = List(1,2,3,4,5,6,7,8,9,10)
    // 求和
    reduceList.reduce((x, y) => x + y)
    reduceList.reduce(_ + _)
    reduceList.sum

    // fold(设置初始值的reduce)
    val foldList = List(1,2,3,4,5,6,7,8,9,10)
    foldList.fold(10)(_ + _)


    println("=====================")
    // wordCount
    val strList = List("hadoop java hadoop hive hive hive scala scala")
    val wordList = strList.flatMap(_.split(" "))
    val wordListGroup = wordList.groupBy(word => word)
    val countMap = wordListGroup.map(e => {
      (e._1, e._2.length)
    })
    val result = countMap.toList.sortBy(_._2).reverse.take(3)
    println(result)

    // 简化
    strList
      .flatMap(_.split(" "))
      .groupBy(word => word)
      .map(e => (e._1, e._2.length))
      .toList.sortBy(_._2)
      .reverse
      .take(3)

  }
}