Access to nested map's values

Question

I have a Map

val m = Map(1->13, 2->Map(3->444, 4-> List("aaa", "bbb")))

I want to get its nested values:

// these all lead to an error
m.get(2)(3)
m.get(2).get(3)
m.get(2).get.get(3)

How do I do that?

Your problem comes from the fact that inside your map you're mixing values of heterogenous types (some are maps, others aren't) so your Map is interpreted as Map[Int,Any] (because Any is the common root in the type hierarchy for Int and Map.) Since Any does not have a "get" method or an "apply" method it all ends up with an error. — Jerome ROBERT, Jun 05 '13 at 06:19

score 5 · Accepted Answer · edited May 23 '17 at 12:15

5

You could actually do what you want, but it's not typesafe.

m.get(2).flatMap{ case m2: Map[Int, _] => m2.get(3) }

Since you have lost type information you have to cast explicitly, so if you want to get list's element you should do something like this:

m.get(2).flatMap{ case m2: Map[Int, _] => m2.get(4) }.map{ case l: List[_] => l(1) }

You should try to save type information. At least you could use Either.

edited May 23 '17 at 12:15

Community

answered Jun 05 '13 at 05:58

senia

At last one can use adhoc class – om-nom-nom Jun 05 '13 at 06:03
what I want to get List's 2nd element, how do I do that? – Alan Coromano Jun 05 '13 at 06:08
@om-nom-nom: Type parameters are not the biggest problem here. The whole idea of storing `Int` and `Map` in the same container is evil. – senia Jun 05 '13 at 06:10
@senia I understand :-) but if one badly needs such scheme, this might be an option. – om-nom-nom Jun 05 '13 at 06:12
1

@MariusKavansky: I've updated my answer, but i think you really should try save type information somehow. Scala is a typesafe language, you should not abuse it like this. – senia Jun 05 '13 at 06:17

score 1 · Answer 2 · answered Jun 05 '13 at 06:25

You have a map which has inconsistent types of key-value pairs. Hence there cannot be one generalized answer.

Firstly m.get(2) returns an Option[Any]. Doing m.get(2)(3) is basically trying to do:

val option = m.get(2) //option is of type Option[Any]
option(3) //error

Hence you need to do:

m.get(2) match {
case Some(i) => i match {
     case j:Map[Any,Any] => j(3)
     }
 }

Something of this sort.

2 Answers2