将元素添加到 scala.collection.mutable.Map 的语法是什么？

What is the syntax for adding an element to a scala.collection.mutable.Map?

提问by Koala3

What is the syntax for adding an element to a scala.collection.mutable.Map ?

将元素添加到 scala.collection.mutable.Map 的语法是什么？

Here are some failed attempts:

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")

回答by Eastsun

The point is that the first line of your codes is not what you expected.

关键是你的代码的第一行不是你所期望的。

You should use:

你应该使用：

val map = scala.collection.mutable.Map[A,B]()

You then have multiple equivalent alternatives to add items:

然后，您有多个等效的替代方法来添加项目：

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))

And starting Scala 2.13:

并开始Scala 2.13：

scala> map.addOne("k4" -> "v4")
res5: map.type = HashMap(k1 -> v1, k2 -> v2, k3 -> v3, k4 -> v4)

回答by Kevin Wright

As always, you should question whether you truly need a mutable map.

与往常一样，您应该质疑是否真的需要可变映射。

Immutable maps are trivial to build:

不可变映射构建起来很简单：

val map = Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

Mutable maps are no different when first being built:

可变映射在第一次构建时没有什么不同：

val map = collection.mutable.Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

map += "nextkey" -> "nextval"

In both of these cases, inference will be used to determine the correct type parameters for the Map instance.

在这两种情况下，推理将用于确定 Map 实例的正确类型参数。

You can also hold an immutable map in a var, the variable will then be updated with a new immutable map instance every time you perform an "update"

您还可以在 a 中保存一个不可变的映射，var然后每次执行“更新”时该变量将使用新的不可变映射实例进行更新

var map = Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

map += "nextkey" -> "nextval"

If you don't have any initial values, you can use Map.empty:

如果您没有任何初始值，则可以使用 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

回答by Synesso

When you say

当你说

val map = scala.collection.mutable.Map

you are not creating a map instance, but instead aliasing the Map type.

您不是在创建地图实例，而是为地图类型设置别名。

map: collection.mutable.Map.type = scala.collection.mutable.Map$@fae93e

Try instead the following:

请尝试以下操作：

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))

回答by 0nIce

Create a new immutable map:

创建一个新的不可变映射：

scala> val m1 = Map("k0" -> "v0")   
m1: scala.collection.immutable.Map[String,String] = Map(k0 -> v0)

Add a new key/value pair to the above map (and create a new map, since they're both immutable):

向上面的映射添加一个新的键/值对（并创建一个新映射，因为它们都是不可变的）：

scala> val m2 = m1 + ("k1" -> "v1")             
m2: scala.collection.immutable.Map[String,String] = Map(k0 -> v0, k1 -> v1)

回答by Andy Dong

Create a mutable map without initial value:

创建一个没有初始值的可变映射：

scala> var d= collection.mutable.Map[Any, Any]()
d: scala.collection.mutable.Map[Any,Any] = Map()

Create a mutable map with initial values:

创建一个具有初始值的可变映射：

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)

Update existing key-value:

更新现有的键值：

scala> d("a")= "ABC"

Add new key-value:

添加新的键值：

scala> d(100)= "new element"

Check the updated map:

检查更新的地图：

scala> d
res123: scala.collection.mutable.Map[Any,Any] = Map(2 -> test, 100 -> new element, a -> ABC, 1 -> 234)

回答by kkress

var test = scala.collection.mutable.Map.empty[String, String]
test("myKey") = "myValue"