Question

我正在尝试编写通用方法来合并集合的映射集合。这是一个满口的，但假设你有几个groupBy的结果，你想用键连接。

以下是我要写的内容：

def mergeMaps[A, B <: Iterable[_]](maps: Seq[Map[A, B]]): Map[A, B] = maps.reduce { (leftMap, rightMap) =>
  (leftMap.toList ++ rightMap.toList).groupBy(_._1).map { case (k, v) =>
    k -> v.map(_._2).reduce(_ ++ _)
  }
}

我目前收到此错误：

<console>:9: error: Cannot construct a collection of type That with elements of type Any based on a collection of type Repr.
             k -> v.map(_._2).reduce(_ ++ _)
                                       ^

我认为这与CanBuildFrom魔法没有用reduce或其他什么东西相关。我怎样才能做到这一点？

Answer 1

减少不是这样的：

scala> val m1 = List("a"->1,"b"->2).groupBy(_._1)
m1: scala.collection.immutable.Map[String,List[(String, Int)]] = Map(b -> List((b,2)), a -> List((a,1)))

scala> val m2 = List("a"->3,"b"->4).groupBy(_._1)
m2: scala.collection.immutable.Map[String,List[(String, Int)]] = Map(b -> List((b,4)), a -> List((a,3)))

scala> m1 map { case (k,v) => (k, v ++ m2(k)) }
res0: scala.collection.immutable.Map[String,List[(String, Int)]] = Map(b -> List((b,2), (b,4)), a -> List((a,1), (a,3)))

或可能!(m2 contains k)：

scala> m1 map { case (k,v) => (k, (m2 get k) map (v ++ _) getOrElse v) }
res1: scala.collection.immutable.Map[String,List[(String, Int)]] = Map(b -> List((b,2), (b,4)), a -> List((a,1), (a,3)))

反之亦然：

scala> val m2 = List("a"->3,"b"->4,"c"->5).groupBy(_._1)
m2: scala.collection.immutable.Map[String,List[(String, Int)]] = Map(b -> List((b,4)), a -> List((a,3)), c -> List((c,5)))

scala> type M = Map[String,List[(String, Int)]]
defined type alias M

scala> def r(m1: M, m2: M) = (m1.keySet ++ m2.keySet).toList map (k => (k, m1.getOrElse(k,Nil) ++ m2.getOrElse(k,Nil))) toMap
warning: there was one feature warning; re-run with -feature for details
r: (m1: M, m2: M)scala.collection.immutable.Map[String,List[(String, Int)]]

scala> r(m1,m2)
res4: scala.collection.immutable.Map[String,List[(String, Int)]] = Map(b -> List((b,2), (b,4)), a -> List((a,1), (a,3)), c -> List((c,5)))

Answer 2

你不需要存在类型：

def mergeMaps[A, B](maps: Seq[Map[A, Iterable[B]]]) = 
  maps.map(_.toList).reduce(_ ++ _).groupBy(_._1).mapValues(_.map(_._2).flatten)


scala> mergeMaps(Seq(Map("a" -> List(1,2), "b" -> List(1)), Map("a" -> List(3))))
res20: scala.collection.immutable.Map[String,List[Int]] = 
       Map(b -> List(1), a -> List(1, 2, 3))

合并集合映射集合的通用方法

2 个答案: