google kotlin 训练营-函数

原文

main 函数

fun main(args: Array<String>) {
println("Hello, world!")
}

一切都有值

下面的输出 kotlin.Unit(println 本身没有返回值,所以返回 kotlin.Unit)

val isUnit = println("This is an expression")
println(isUnit)

创建函数

import java.util.*    // required import

fun randomDay() : String {
    val week = arrayOf ("Monday", "Tuesday", "Wednesday", "Thursday",
        "Friday", "Saturday", "Sunday")
    return week[Random().nextInt(week.size)]
}

fun fishFood (day : String) : String {
    return when (day) {
        "Monday" -> "flakes"
        "Wednesday" -> "redworms"
        "Thursday" -> "granules"
        "Friday" -> "mosquitoes"
        "Sunday" -> "plankton"
        else -> "nothing"
    }
}

fun feedTheFish() {
    val day = randomDay()
    val food = fishFood(day)
    println ("Today is $day and the fish eat $food")
}

fun main(args: Array<String>) {
    feedTheFish()
}

给函数参数添加默认值(有默认值的参数可以不传,没有的必传),

fun swim(speed: String = "fast") {
   println("swimming $speed")
}

函数参数默认值不一定需要是值,也可以是函数调用

fun shouldChangeWater (day: String, temperature: Int = 22, dirty: Int = getDirtySensorReading()): Boolean {
}

单表达式的函数可以简写成

fun isTooHot(temperature: Int) = temperature > 30

过滤器

it 代表正在遍历元素
不满足条件的被过滤掉

val decorations = listOf ("rock", "pagoda", "plastic plant", "alligator", "flowerpot")

fun main() {
    println( decorations.filter {it[0] == 'p'})
}

惰性过滤器

val filtered = decorations.asSequence().filter { it[0] == 'p' }
println("filtered: $filtered")

输出

filtered: kotlin.sequences.FilteringSequence@386cc1c4

val newList = filtered.toList()
println("new list: $newList")

才输出值

new list: [pagoda, plastic plant]

转换

val lazyMap = decorations.asSequence().map {
        println("access: $it")
        it
}

过滤+转换

val lazyMap2 = decorations.asSequence().filter {it[0] == 'p'}.map {
    println("access: $it")
    it
}
println("-----")
println("filtered: ${lazyMap2.toList()}")

lambda

var dirtyLevel = 20
val waterFilter = { dirty : Int -> dirty / 2}
println(waterFilter(dirtyLevel))

高阶函数

函数的参数是另一个函数

fun updateDirty(dirty: Int, operation: (Int) -> Int): Int {
   return operation(dirty)
}

调用

val waterFilter: (Int) -> Int = { dirty -> dirty / 2 }
println(updateDirty(30, waterFilter))

传命名函数时

fun increaseDirty( start: Int ) = start + 1

println(updateDirty(15, ::increaseDirty))

如果最后一个参数是函数的话,有一个特殊写法

var dirtyLevel = 19;
dirtyLevel = updateDirty(dirtyLevel) { dirtyLevel -> dirtyLevel + 23}
println(dirtyLevel)