在Scala中,对集合调用
groupBy()
会返回一个值为集合的Map
,但我需要一个MultiMap
。进行转换的最简单方法是什么?是否可以避免创建新的MultiMap
并将所有内容复制过去?package blevins.example
object App extends Application {
implicit def multiMapable[B](c: Iterable[B]) = new {
def groupByMM[A](f: B => A) = {
import scala.collection.mutable._
val ret = new HashMap[A,Set[B]] with MultiMap[A,B]
for (e <- c) { ret.addBinding(f(e), e) }
ret
}
}
val c = List(1,2,3,4,5,6,7,8,9)
val mm = c.groupByMM { i => if (i < 5) "alpha" else "beta" }
mm.addBinding("alpha",12)
println(mm) // Map(beta -> Set(5, 7, 6, 9, 8), alpha -> Set(3, 1, 4, 2, 12))
}
补充说明
以下是将已有的Map[String,Set[Int]]转换成MultiMap的示例,而不需要复制其值:
object App extends Application {
import scala.collection.mutable._
val seed: Map[String,Set[Int]] = Map("even" -> Set(2,4,6), "odd" -> Set(1,3,5))
val multiMap = new MapProxy[String,Set[Int]] with MultiMap[String,Int] {
val self = seed
}
multiMap.addBinding("even", 8)
println(multiMap) // Map(odd -> Set(5, 3, 1), even -> Set(6, 8, 4, 2))
}