12. 内置高阶函数

1. 获取最大值和最小值

1.1 处理基础类型 min/max

minOrNull和maxOrNull

    val array = arrayOf(4, 3, 65, 34, 1)
    array.minOrNull()
    array.minOrNull()
    val list = listOf(5, 321, 4, 2, 5, 123, 4)
    val minOrNull = list.minOrNull()
    val maxOrNull = list.maxOrNull()
    println(minOrNull)
    println(maxOrNull)

1.2 处理所有类型最大值，最小值(minBy,maxBy)

根据对象的属性，找出对应的集合中的元素

数据类

data class People(val name: String = "测试", val age: Double)

    val data = listOf(
        People("赵", 5),
        People("钱", 3),
        People("孙", 51),
        People("李", 54)
    )
    val minByOrNull = data.minByOrNull { it.name }
    val maxByOrNull = data.maxByOrNull { it.name }
    println(minByOrNull)
    println(maxByOrNull)

2. 求和(sumof)

  val data = listOf(
        People("赵", 5.0),
        People("钱", 3.0),
        People("孙", 51.0),
        People("李", 54.0)
    )
    val minByOrNull = data.sumOf { it.age }

3. 转换集合(map)

根据lambda表达式，返回对应的集合

    val data = listOf(
        People("赵", 5.0),
        People("钱", 3.0),
        People("孙", 51.0),
        People("李", 54.0)
    )
    val minByOrNull:List<Double> = data.map { it.age }

4. fliter

根据表达式条件转换集合

   val data = listOf(
        People("赵", 5.0),
        People("钱", 3.0),
        People("孙", 51.0),
        People("李", 54.0)
    )
    val minByOrNull:List<Double> = data.map { it.age }.filter { it<10 }
    minByOrNull.forEach { println(it) }

5. 分组(groupBy)

根据元素中的属性的值分组

    val data = listOf(
        People("赵", 5.0),
        People("钱", 5.0),
        People("孙", 51.0),
        People("李", 54.0)
    )
    val minByOrNull:Map<Double,List<People>> = data.groupBy { it.age }
    minByOrNull.forEach { println(it) }

6. fold

指定一个初始值，让集合中的每个元素对它执行操作。

操作的参数有两个，累计值和集合元素。

可以做递增，递减，字符串连接，初始值减去元素的总和等。

6.1 递增

val data = listOf(
        People("赵", 5),
        People("钱", 5),
        People("孙", 51),
        People("李", 54)
    )
    val fold = data.fold(0) { run,item  ->
        println(run+item.age)
        run+item.age
      }
    println(fold)

可以看出，每次输入的值都在依次改变

6.2 字符串连接-依次执行

fold

val data = listOf(
        People("赵", 5),
        People("钱", 5),
        People("孙", 51),
        People("李", 54)
    )
    val fold = data.fold("") { run,item  ->
        run+"姓氏："+item.name+" 年龄："+item.age+"\n"
      }
    println(fold)

从右向左就是foldRight

reduce

reduce用法和fold类似，reduce第一步将第一个元素和第二个元素作为第一步的操作参数。

   val data = listOf(
        People("赵", 5),
        People("钱", 5),
        People("孙", 51),
        People("李", 54)
    )
    val fold = data.reduce {sum, element ->
        println(sum.age+element.age)
        element
      }
    println(fold)

输出

10
56
105
People(name=李, age=54)

reduce只能返回集合的元素的类型