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[英]what is equivalent function of Map.compute in scala.collection.mutable.Map
[英]What is the syntax for adding an element to a scala.collection.mutable.Map?
將元素添加到 scala.collection.mutable.Map 的語法是什么?
以下是一些失敗的嘗試:
val map = scala.collection.mutable.Map
map("mykey") = "myval"
map += "mykey" -> "myval"
map.put("mykey","myval")
關鍵是你的代碼的第一行不是你所期望的。
你應該使用:
val map = scala.collection.mutable.Map[A,B]()
然后,您有多個等效的替代方法來添加項目:
scala> val map = scala.collection.mutable.Map[String,String]()
map: scala.collection.mutable.Map[String,String] = Map()
scala> map("k1") = "v1"
scala> map
res1: scala.collection.mutable.Map[String,String] = Map((k1,v1))
scala> map += "k2" -> "v2"
res2: map.type = Map((k1,v1), (k2,v2))
scala> map.put("k3", "v3")
res3: Option[String] = None
scala> map
res4: scala.collection.mutable.Map[String,String] = Map((k3,v3), (k1,v1), (k2,v2))
從Scala 2.13
開始:
scala> map.addOne("k4" -> "v4")
res5: map.type = HashMap(k1 -> v1, k2 -> v2, k3 -> v3, k4 -> v4)
與往常一樣,您應該質疑是否真的需要可變映射。
不可變映射構建起來很簡單:
val map = Map(
"mykey" -> "myval",
"myotherkey" -> "otherval"
)
可變映射在第一次構建時沒有什么不同:
val map = collection.mutable.Map(
"mykey" -> "myval",
"myotherkey" -> "otherval"
)
map += "nextkey" -> "nextval"
在這兩種情況下,推理將用於確定 Map 實例的正確類型參數。
您還可以在var
保存不可變映射,然后每次執行“更新”時,該變量將使用新的不可變映射實例進行更新
var map = Map(
"mykey" -> "myval",
"myotherkey" -> "otherval"
)
map += "nextkey" -> "nextval"
如果您沒有任何初始值,則可以使用 Map.empty:
val map : Map[String, String] = Map.empty //immutable
val map = Map.empty[String,String] //immutable
val map = collection.mutable.Map.empty[String,String] //mutable
當你說
val map = scala.collection.mutable.Map
您不是在創建地圖實例,而是為地圖類型設置別名。
map: collection.mutable.Map.type = scala.collection.mutable.Map$@fae93e
請嘗試以下操作:
scala> val map = scala.collection.mutable.Map[String, Int]()
map: scala.collection.mutable.Map[String,Int] = Map()
scala> map("asdf") = 9
scala> map
res6: scala.collection.mutable.Map[String,Int] = Map((asdf,9))
創建一個新的不可變映射:
scala> val m1 = Map("k0" -> "v0")
m1: scala.collection.immutable.Map[String,String] = Map(k0 -> v0)
向上面的映射添加一個新的鍵/值對(並創建一個新映射,因為它們都是不可變的):
scala> val m2 = m1 + ("k1" -> "v1")
m2: scala.collection.immutable.Map[String,String] = Map(k0 -> v0, k1 -> v1)
var test = scala.collection.mutable.Map.empty[String, String]
test("myKey") = "myValue"
創建一個沒有初始值的可變映射:
scala> var d= collection.mutable.Map[Any, Any]()
d: scala.collection.mutable.Map[Any,Any] = Map()
創建一個具有初始值的可變映射:
scala> var d= collection.mutable.Map[Any, Any]("a"->3,1->234,2->"test")
d: scala.collection.mutable.Map[Any,Any] = Map(2 -> test, a -> 3, 1 -> 234)
更新現有的鍵值:
scala> d("a")= "ABC"
添加新的鍵值:
scala> d(100)= "new element"
檢查更新的地圖:
scala> d
res123: scala.collection.mutable.Map[Any,Any] = Map(2 -> test, 100 -> new element, a -> ABC, 1 -> 234)
var map:Map[String, String] = Map()
var map1 = map + ("red" -> "#FF0000")
打印(地圖1)
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