在Scala中获取嵌套地图的价值 [英] Get value of nested maps in Scala

问题描述

我有一个具有以下结构的地图:

I have a map with the following structure:

Map[String, Map[String, String]]

是否有一种优雅的方法来获取内部地图的价值?

Is there an elegant way of getting the value of the inner map?

推荐答案

只需按常规方法进行两次即可.

Just do it the normal way... twice.

val m = Map("a" -> Map("b" -> "c"))
m("a")("b")  // c

第一个操作m("a")返回内部的Map[String,String].第二个操作that("b")返回返回的Map内的String.

The first operation m("a") returns the inner Map[String,String]. The second operation that("b") returns the String inside of that returned Map.

与以下相同:

val m = Map("a" -> Map("b" -> "c"))
val m2 = m("a")  // Map(b -> c)
m2("b")          // c

另一方面，如果您认为它们的键可能不存在，请执行以下操作:

On the other hand, if you think that they keys may not be there, then do this:

for {
  x <- m.get("a")   // x = Map(b -> c)
  y <- x.get("b")   // y = c
} yield y
// Some(c)

for {
  x <- m.get("a")   // x = Map(b -> c)
  y <- x.get("d")   // fails
} yield y
// None

for {
  x <- m.get("c")   // fails
  y <- x.get("d")   // doesn't run
} yield y
// None

对于您的示例，key2是Option，就像m.get(key1)一样，因此您可以用相同的方式处理它:

For your example, key2 is an Option, just like m.get(key1), so you can handle it the same way:

val key1: String = "a"
val key2: Option[String] = Some("b")
for { 
  value1 <- m.get(key1)
  k2 <- key2
  value2 <- value1.get(k2) 
} yield value2
// Some(c)

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