scala - 在Scala中反转 map (字符串->列表)

Question

我有一个Map [String，List [String]]，我想将其反转。例如，如果我有类似的东西

  "1" -> List("a","b","c")
  "2" -> List("a","j","k")
  "3" -> List("a","c")

结果应该是

  "a" -> List("1","2","3")
  "b" -> List("1")
  "c" -> List("1","3")
  "j" -> List("2")
  "k" -> List("2")

我已经试过了:

  m.map(_.swap)

但是它返回一个Map [List [String]，String]:

  List("a","b","c") -> "1"
  List("a","j","k") -> "2" 
  List("a","c") -> "3"

Answer 1

map 反转有点复杂。

val m = Map("1" -> List("a","b","c")
           ,"2" -> List("a","j","k")
           ,"3" -> List("a","c"))

m flatten {case(k, vs) => vs.map((_, k))} groupBy (_._1) mapValues {_.map(_._2)}
//res0: Map[String,Iterable[String]] = Map(j -> List(2), a -> List(1, 2, 3), b -> List(1), c -> List(1, 3), k -> List(2))

将 Map展平为一个元组集合。 groupBy将使用旧值作为新键创建一个新的 Map。然后，通过删除键(以前的值)元素来取消元组的值。