Scala 的可变 Map 更新 [map(key) = newValue] 语法如何工作? [英] How does Scala's mutable Map update [map(key) = newValue] syntax work?

问题描述

我正在研究 Cay Horstmann 的 Scala 以编写不耐烦的书，在那里我遇到了这种更新可变地图的方式.

I'm working through Cay Horstmann's Scala for the Impatient book where I came across this way of updating a mutable map.

scala> val scores = scala.collection.mutable.Map("Alice" -> 10, "Bob" -> 3, "Cindy" -> 8)
scores: scala.collection.mutable.Map[String,Int] = Map(Bob -> 3, Alice -> 10, Cindy -> 8)

scala> scores("Alice") // retrieve the value of type Int
res2: Int = 10

scala> scores("Alice") = 5 // Update the Alice value to 5

scala> scores("Alice")
res4: Int = 5

看起来 scores("Alice") 在 MapLike.scala 中点击了 apply.但这只是返回值，而不是可以更新的东西.

It looks like scores("Alice") hits apply in MapLike.scala. But this only returns the value, not something that can be updated.

出于好奇，我在不可变映射上尝试了相同的语法，但出现以下错误，

Out of curiosity I tried the same syntax on an immutable map and was presented with the following error,

scala> val immutableScores = Map("Alice" -> 10, "Bob" -> 3, "Cindy" -> 8)
immutableScores: scala.collection.immutable.Map[String,Int] = Map(Alice -> 10, Bob -> 3, Cindy -> 8)

scala> immutableScores("Alice") = 5
<console>:9: error: value update is not a member of scala.collection.immutable.Map[String,Int]
              immutableScores("Alice") = 5
          ^

基于此，我假设 scores("Alice") = 5 被转换为 scores update ("Alice", 5) 但我没有了解它是如何工作的，或者它是如何实现的.

Based on that, I'm assuming that scores("Alice") = 5 is transformed into scores update ("Alice", 5) but I have no idea how it works, or how it is even possible.

它是如何工作的?

推荐答案

这是apply、update 语法的示例.

当您调用 map("Something") 时，它会调用 map.apply("Something")，后者又调用 get.

When you call map("Something") this calls map.apply("Something") which in turn calls get.

当您调用 map("Something") = "SomethingElse" 时，这会调用 map.update("Something", "SomethingElse")，后者又调用 放置.

When you call map("Something") = "SomethingElse" this calls map.update("Something", "SomethingElse") which in turn calls put.

看看这个更全面的解释.

这篇关于Scala 的可变 Map 更新 [map(key) = newValue] 语法如何工作?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！