kotlin 学习记录 基本语法 （2）

https://huanglizhuo.gitbooks.io/kotlin-in-chinese/content/GettingStarted/Basic-Syntax.html

使用循环

fun main(args: Array<String>) {
  val items = listOf("apple", "banana", "kiwi")
  for (item in items) {
    println(item)
  }
}

或者

fun main(args: Array<String>) {
  val items = listOf("apple", "banana", "kiwi")
  for (index in items.indices) {
    println("item at $index is ${items[index]}")
  }
}
使用 while 循环
fun main(args: Array<String>) {
  val items = listOf("apple", "banana", "kiwi")
  var index = 0
  while (index < items.size) { println("item at $index is ${items[index]}") index++ } } 
参看while循环
使用 when 表达式
fun describe(obj: Any): String =
when (obj) {
  1          -> "One"
  "Hello"    -> "Greeting"
  is Long    -> "Long"
  !is String -> "Not a string"
  else       -> "Unknown"
}

fun main(args: Array<String>) {
  println(describe(1))
  println(describe("Hello"))
  println(describe(1000L))
  println(describe(2))
  println(describe("other"))
}
参看when表达式
  
  
  
  

   
   
   
   

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使用ranges
使用 in 操作符检查数值是否在某个范围内：
  
  
  
  

   
   
   
   

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fun main(args: Array<String>) {
  val x = 10
  val y = 9
  if (x in 1..y+1) {
      println("fits in range")
  }
}
检查数值是否在范围外：
  
  
  
  

   
   
   
   

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fun main(args: Array<String>) {
  val list = listOf("a", "b", "c")

  if (-1 !in 0..list.lastIndex) {
    println("-1 is out of range")
  }
  if (list.size !in list.indices) {
    println("list size is out of valid list indices range too")
  }
}
在范围内迭代：
  
  
  
  

   
   
   
   

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fun main(args: Array<String>) {
  for (x in 1..5) {
    print(x)
  }
}
或者使用步进：
  
  
  
  

   
   
   
   

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fun main(args: Array<String>) {
  for (x in 1..10 step 2) {
    print(x)
  }
  for (x in 9 downTo 0 step 3) {
    print(x)
  }
}
参看Ranges
  
  
  
  

   
   
   
   

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使用集合
对一个集合进行迭代：
  
  
  
  

   
   
   
   

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fun main(args: Array<String>) {
  val items = listOf("apple", "banana", "kiwi")
  for (item in items) {
    println(item)
  }
}
使用 in 操作符检查集合中是否包含某个对象
  
  
  
  

   
   
   
   

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fun main(args: Array<String>) {
  val items = setOf("apple", "banana", "kiwi")
  when {
    "orange" in items -> println("juicy")
    "apple" in items -> println("apple is fine too")
  }
}
使用lambda表达式过滤和映射集合：
  
  
  
  

   
   
   
   

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fun main(args: Array<String>) {
  val fruits = listOf("banana", "avocado", "apple", "kiwi")
  fruits
    .filter { it.startsWith("a") }
    .sortedBy { it }
    .map { it.toUpperCase() }
    .forEach { println(it) }
}