更干净的元组 groupBy

我有一系列键值对（String、Int），我想按键将它们分组为值序列（即Seq[(String, Int)]) => Map[String, Iterable[Int]])).

明显地，toMap在这里没有用，并且groupBy将值维护为元组。我想出的最好的办法是：

val seq: Seq[( String, Int )]
// ...
seq.groupBy( _._1 ).mapValues( _.map( _._2 ) )

有没有更干净的方法来做到这一点？

这是一个皮条客，添加了一个toMultiMap可遍历的方法。它能解决你的问题吗？

import collection._
import mutable.Builder
import generic.CanBuildFrom

class TraversableOnceExt[CC, A](coll: CC, asTraversable: CC => TraversableOnce[A]) {

  def toMultiMap[T, U, That](implicit ev: A <:< (T, U), cbf: CanBuildFrom[CC, U, That]): immutable.Map[T, That] =
    toMultiMapBy(ev)

  def toMultiMapBy[T, U, That](f: A => (T, U))(implicit cbf: CanBuildFrom[CC, U, That]): immutable.Map[T, That] = {
    val mutMap = mutable.Map.empty[T, mutable.Builder[U, That]]
    for (x <- asTraversable(coll)) {
      val (key, value) = f(x)
      val builder = mutMap.getOrElseUpdate(key, cbf(coll))
      builder += value
    }
    val mapBuilder = immutable.Map.newBuilder[T, That]
    for ((k, v) <- mutMap)
      mapBuilder += ((k, v.result))
    mapBuilder.result
  }
}

implicit def commomExtendTraversable[A, C[A] <: TraversableOnce[A]](coll: C[A]): TraversableOnceExt[C[A], A] =
  new TraversableOnceExt[C[A], A](coll, identity)

可以像这样使用：

val map = List(1 -> 'a', 1 -> 'à', 2 -> 'b').toMultiMap
println(map)  // Map(1 -> List(a, à), 2 -> List(b))

val byFirstLetter = Set("abc", "aeiou", "cdef").toMultiMapBy(elem => (elem.head, elem))
println(byFirstLetter) // Map(c -> Set(cdef), a -> Set(abc, aeiou))

如果添加以下隐式定义，它也适用于类似集合的对象，例如Strings and Arrays:

implicit def commomExtendStringTraversable(string: String): TraversableOnceExt[String, Char] =
  new TraversableOnceExt[String, Char](string, implicitly)

implicit def commomExtendArrayTraversable[A](array: Array[A]): TraversableOnceExt[Array[A], A] =
  new TraversableOnceExt[Array[A], A](array, implicitly)

Then:

val withArrays = Array(1 -> 'a', 1 -> 'à', 2 -> 'b').toMultiMap
println(withArrays) // Map(1 -> [C@377653ae, 2 -> [C@396fe0f4)

val byLowercaseCode = "Mama".toMultiMapBy(c => (c.toLower.toInt, c))
println(byLowercaseCode) // Map(97 -> aa, 109 -> Mm)