如何向 scala.collection.mutable.Map
添加元素?
以下是一些失败的尝试:
val map = scala.collection.mutable.Map
map("mykey") = "myval"
map += "mykey" -> "myval"
map.put("mykey","myval")
如何向 scala.collection.mutable.Map
添加元素?
以下是一些失败的尝试:
val map = scala.collection.mutable.Map
map("mykey") = "myval"
map += "mykey" -> "myval"
map.put("mykey","myval")
问题在于你代码的第一行不是你期望的。
你应该使用:
val map = scala.collection.mutable.Map[A,B]()
您随后有多个等效的选择来添加项目:
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))
自 Scala 2.13
开始:
scala> map.addOne("k4" -> "v4")
res5: map.type = HashMap(k1 -> v1, k2 -> v2, k3 -> v3, k4 -> v4)
new
时空括号是不必要的。 - Daniel C. Sobral像往常一样,你应该质疑自己是否真的需要一个可变映射。
不可变映射很容易构建:
val map = Map(
"mykey" -> "myval",
"myotherkey" -> "otherval"
)
可变映射在刚开始创建时并没有什么不同:
val map = collection.mutable.Map(
"mykey" -> "myval",
"myotherkey" -> "otherval"
)
map += "nextkey" -> "nextval"
在这两种情况下,将使用推断来确定Map实例的正确类型参数。var
中持有一个不可变的map,每次执行“update”操作时,该变量将更新为新的不可变map实例。var map = Map(
"mykey" -> "myval",
"myotherkey" -> "otherval"
)
map += "nextkey" -> "nextval"
如果您没有任何初始值,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
map ++ Seq("another1" -> "anotherVal1", "another2" -> "anotherVal2")
- Brent Faust当你说
val map = scala.collection.mutable.Map
你并没有创建一个地图实例,而是为 Map 类型设置了一个别名。
map: collection.mutable.Map.type = scala.collection.mutable.Map$@fae93e
尝试使用以下代码:
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))
scala> val m1 = Map("k0" -> "v0")
m1: scala.collection.immutable.Map[String,String] = Map(k0 -> v0)
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"
scala> var d= collection.mutable.Map[Any, Any]()
d: scala.collection.mutable.Map[Any,Any] = Map()
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)
更新现有的键值对:
scala> d("a")= "ABC"
添加新的键值对:
scala> d(100)= "new element"
请查看更新后的地图:
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")
println(map1)
变量map:Map[String, String] = Map()
变量map1 = map + ("red" -> "#FF0000")
打印(map1)