Example练习-集合部分

• List

  val systemUsers: MutableList<Int> = mutableListOf(1,2,3)
  val sudoers: List<Int> = systemUsers
  fun addSystemUser(newUser: Int) {
  systemUsers.add(newUser)
  }
  fun getSysSudoers():List<Int> {
    return sudoers
  }
  fun main() {
    addSystemUser(4)
    println("Tot sudoers: ${getSysSudoers().size}")
    getSysSudoers().forEach {
       i -> println("Some useful info on user $i")
    }
    // getSysSudoers().add(5) <- Error!
  }

• Set

  val openIssues:MutableSet<String> = mutableSetOf("uniqueDescr1", "uniqueDescr2", "uniqueDescr3")
  fun addIssue(uniqueDesc: String): Boolean {
    return openIssues.add(uniqueDesc)
  }
  fun getStatusLog(isAdded: Boolean): String {
    return if (isAdded) "registered correctly." else "marked as duplicate and rejected."   
  }
  fun main() {
    val aNewIssue: String = "uniqueDescr4"
    val anIssueAlreadyIn: String = "uniqueDescr2"
    println("Issue $aNewIssue ${getStatusLog(addIssue(aNewIssue))}")
    println("Issue &anIssueAlreadyIn ${getStatusLog(addIssue(anIssueAlreadyIn))}")
  }
  

• Map ```kotlin const val POINTS_X_PASS: Int = 15 val EZPassAccounts:MutableMap = mutableMapOf(1 to 100, 2 to 100, 3 to 100) val EZPassReport: Map = EZPassAccounts

fun updataPointCredit(id:Int) { if(EZPassAccounts.containsKey(id)){ EZPassAccounts[id] = EZPassAccounts.getValue(id) + POINTS_X_PASS } else { println(“Error: Trying to update a non-existing account”) } }

fun accountReport() { EZPassReport.forEach { k,v -> println(“ID $k: credit $v”) } } fun main() { accountsReport() // 6 updatePointsCredit(1) // 7 updatePointsCredit(1)
updatePointsCredit(5) // 8 accountsReport() // 9 }

- ** 有关操作**
   - filter
```kotlin
val numbers = listOf(1,2,3)
numbers.filter { it < 0 }
numbers.filter { x -> x>0 }

• map

  val numbers = listOf(1, -2, 3, -4, 5, -6)  
  val doubled = numbers.map { x -> x*2 }
  val triple = numbers.map { it*3 }
  

• any,all,none

  val numbers = listOf(1, -2, 3, -4, 5, -6)          
  val anyNegative = numbers.any { it < 0 } 
  val allEven = numbers.all { it % 2 == 0 }
  

• find,findLast

  val words = listOf("Lets", "find", "something", "in", "collection", "somehow")
  val first = words.find {it.startsWith("some")}
  val last = words.findLast{it.startsWith("some")}
  val nothing = words.find {it.contains("nothing")}
  

• first,last,firstOrNull

  val first = numbers.first()
  val first = numbers.last { it > 0 }
  val first = numbers.firstOrNull()
  

• count

  val numbers = val numbers = listOf(1, -2, 3, -4, 5, -6) 
  val totalCount = numbers.count()
  val evenCount = numbers.count {it%2==0}
  

• associateBy, groupBy

  fun main() {
  data class Person(val name: String, val city: String, val phone: String) 
  val people = listOf(                                                     
     Person("John", "Boston", "+1"),
     Person("Sarah", "Munich", "+4"),
     Person("Svyatoslav", "Saint-Petersburg", "+7"),
     Person("Vasilisa", "Saint-Petersburg", "+8"))
  val phoneBook = people.associateBy { it.phone }                          
  val cityBook = people.associateBy(Person::phone, Person::city)           
  val peopleCities = people.groupBy(Person::city, Person::name)            
  val lastPersonCity = people.associateBy(Person::city, Person::name)      
  println("People: $people")
  println("Phone book: $phoneBook")
  println("City book: $cityBook")
  println("People living in each city: $peopleCities")
  println("Last person living in each city: $lastPersonCity")
  }
  

• partition

  fun main() {
  val numbers = listOf(1, -2, 3, -4, 5, -6)
  val evenOdd = numbers.partition {it%2==0}
  val (p,n) = numbers.partition {it>0}
  println("${evenOdd.first}")
  println("$p")
  }
  

• flatMap

  val fruitsBag = listOf("apple","orange","banana","grapes")  // 1
  val clothesBag = listOf("shirts","pants","jeans")  
  val list = listOf(fruitsBag, clothesBag)
  val map = list.map {it}
  val flatMapBag = list.flatMap { it }
  

• sorted

  val shuffled = listOf(5, 4, 2, 1, 3, -10)   \
  println(shuffled.sorted())
  println(shuffled.sortedBy{-it})
  println(shuffled.sortedDescending())
  println(shuffled.sortedByDescending(abs(it)))
  

• 取元素

  val map = mapOf("key" to 42)
  val value1 = map["key"]
  val value2 = map.getValue("key")
  val mapWithDefault = map.withDefault {k -> k.length}
  val value3 = mapWithDefault.getValue("key2")
  try {
  map.getValue("anotherKey")
  } catch (e: NoSuchElementException) {
  println("Message: $e")
  }
  

• zip

  val A = listOf("a", "b", "c")                  // 1
  val B = listOf(1, 2, 3, 4)  
  val resultPairs = A zip B
  val resultReduce = B.zip(A) {a,b -> "$a$b"}
  

• getOrElse

  val list = listOf(0,10,20)
  println(list.getOrElse(1) {42})
  println(list.getOrElse(20) {42})
  //应用到map
  val map = mutableMapOf<String, Int?>()
  println(map.getOrElse("x") {1})
  map["x"] =3
  println(map.getOrElse("x") {1})
  map["x"] = null
  println(map.getOrElse("x") {1})