mutable.Map 中 withDefaultValue 的行为 [英] Behaviour of withDefaultValue in mutable.Map

问题描述

谁能解释一下可变映射中的默认值是如何工作的?

Can anyone explain how a default value in mutable map works?

scala> val mmap = mutable.Map[String, mutable.Set[String]]().withDefaultValue{mutable.Set[String]()}
mmap: scala.collection.mutable.Map[String,scala.collection.mutable.Set[String]] = Map()

scala> mmap("a") += "b"
res1: scala.collection.mutable.Set[String] = Set(b)

地图为空，没有键.

scala> mmap
res2: scala.collection.mutable.Map[String,scala.collection.mutable.Set[String]] = Map()

但我刚刚尝试编辑的关键是显示数据.

But the key I just tried to edit is showing data.

scala> mmap("a")
res3: scala.collection.mutable.Set[String] = Set(b)

为什么 res2 是空映射但 mmap("a") 有值?

Why is res2 an empty map but mmap("a") have a value?

推荐答案

通过修改地图中不存在的键，您基本上改变了默认值，而不是添加新键.

By modifying key that does not exist in the map you basically changing the default value, not adding a new key.

假设您想添加一些东西以在地图中使用 a 键进行设置.

Let's say you want to add some stuff to set with key a in your map.

val mmap = 
    mutable.Map[ String, mutable.Set[ String ] ]()
    .withDefaultValue( mutable.Set[ String ]() )

// Changind default value
mmap( "a" ) += "b"
mmap( "a" ) += "c"

现在 mmap 的默认值有 2 个元素但仍然没有键.现在您更改"了另一个不存在的密钥:

Now the default value of mmap has 2 elements but still no keys. Now you 'change' the other nonexisting key:

// Still changind default value
mmap( "c" ) += "c1"

实际上您仍在更改默认值

In reality you are still changing the default value

// Printing default value
println( mmap("a") )
// Result => Set(c, c1, b)

为了创建一个真正的键使用分配

In order to create a real key use assignment

// Creating a new key
mmap("b") = mutable.Set[ String ]( "b1", "b2" )

mmap.foreach( println )
// Result => (b,Set(b1, b2))

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