前言

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1. map() 将集合内元素按照规则进行转换

  （1）为集合中元素，添加前缀

    val list = List(1, 2, 4, 9, 3)
    val list1 = list.map((num: Int) => {
       "00000"+num
    })
    println(list1.mkString(","))
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  （2）flatMap()扁平化

    val source = Source.fromFile("data/word.txt")
    val list = source.getLines().toList
    source.close()
    val words = list.flatMap(line => line.split(" "))
    println(words)
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2. filter() 根据指定的规则对集合内元素进行筛选过滤

    val list = List(1, 2, 4, 9, 3)
    val list2 = list.filter(num => num % 2 == 1)
    println(list2.mkString(","))
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3. groupBy() 按照指定的规则对数据进行分组

    val list = List(1, 2, 4, 9, 3)
    val list3 = list.groupBy(num => num % 3)
    println(list3.mkString(","))
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4. sortBy() 按照指定的规则对集合的数据进行排序

  （1）默认规则为升序，对于字符串采用字典序排序

    val list5 = List(1, 4, 3, 2)
    val list6 = list5.sortBy(num => num)
    println(list6)
    val list7 = List("1", "111", "2", "3", "22")
    val list8 = list7.sortBy(str => str)
    println(list8)
    val list9 = list7.sortBy(str => str.toInt)
    println(list9)
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  （2）需要使用降序，可传递第二个参数列表Ordering.Int.reverse

    val list5 = List(1, 4, 3, 2)
    val list10 = list5.sortBy(num => num)(Ordering.Int.reverse)
    println(list10)
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  （3）对象排序
scala的元组，默认按照第一个排序，第一个相同时，按照第二个排序，依次类推

  先按照年龄排序(从小到大)，再按照薪水排序（从大到小）

package com.cz.part7

object T23 {
  def main(args: Array[String]): Unit = {
    val user1 = new User
    user1.name = "cz"
    user1.age = 18
    user1.sal = 8000

    val user2 = new User
    user2.name = "fd"
    user2.age = 39
    user2.sal = 10000

    val user3 = new User
    user3.name = "fe"
    user3.age = 5
    user3.sal = 20000

    val user4 = new User
    user4.name = "iop"
    user4.age = 18
    user4.sal = 7000

    val users: List[User] = List(user1, user2, user3, user4)
    val users1 = users.sortBy((user: User) => (user.age, user.sal))(Ordering.Tuple2(Ordering.Int, Ordering.Int.reverse))
    println(users1)
  }

  class User {
    var age: Int = _
    var name: String = _
    var sal: Int = _

    override def toString: String = s"age:$age name:$name sal:$sal"
  }

}

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  （4）sortWith()自定义排序

    val users2 = users.sortWith(
      (left, right) => {
        if (left.age < right.age) {
          true
        } else if (left.age == right.age) {
          left.sal > right.sal
        } else {
          false
        }
      }
    )
    println(users2)
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