How do I convert a Map[Int, Any] to a SortedMap in Scala? Or a TreeMap?

I would like to convert a Map[Int, Any] to a SortedMap or a TreeMap. Is ther开发者_如何学Goe an easy way to do it?

---

An alternative to using :_* as described by sblundy is to append the existing map to an empty SortedMap

import scala.collection.immutable.SortedMap
val m = Map(1 -> ("one":Any))
val sorted = SortedMap[Int, Any]() ++ m

---

Assuming you're using immutable maps

val m = Map(1 -> "one")
val t = scala.collection.immutable.TreeMap(m.toArray:_*)

The TreeMap companion object's apply method takes repeated map entry parameters (which are instances of Tuple2[_, _] of the appropriate parameter types). toArray produces an Array[Tuple2[Int, String]] (in this particular case). The : _* tells the compiler that the array's contents are to be treated as repeated parameters.

---

Here's a general way to convert between various Scala collections.

import collection.generic.CanBuildFrom
import collection.immutable.TreeMap

object test {
  class TraversableW[A](t: Traversable[A]) {
    def as[CC[X] <: Traversable[X]](implicit cbf: CanBuildFrom[Nothing, A, CC[A]]): CC[A] = t.map(identity)(collection.breakOut)
    def to[Result](implicit cbf: CanBuildFrom[Nothing, A, Result]): Result = t.map(identity)(collection.breakOut)
  }

  implicit def ToTraverseableW[A](t: Traversable[A]): TraversableW[A] = new TraversableW[A](t)

  List(1, 2, 3).as[Vector]
  List(1, 2, 3).to[Vector[Int]]
  List((1, 1), (2, 4), (3, 4)).to[Map[Int, Int]]
  List((1, 1), (2, 4), (3, 4)).to[TreeMap[Int, Int]]
  val tm: TreeMap[Int, Int] = List((1, 1), (2, 4), (3, 4)).to
  ("foo": Seq[Char]).as[Vector]
}

test

See also this question describing collection.breakOut: Scala 2.8 breakOut

CHALLENGE

Is it possible to adjust the implicits such that this works? Or would this only be possible if as were added to Traversable?

"foo".as[Vector]

---

Starting Scala 2.13, via factory builders applied with .to(factory):

Map(1 -> "a", 2 -> "b").to(collection.immutable.SortedMap)
// collection.immutable.SortedMap[Int,String] = TreeMap(1 -> "a", 2 -> "b")

Map(1 -> "a", 2 -> "b").to(collection.immutable.TreeMap)
// collection.immutable.TreeMap[Int,String] = TreeMap(1 -> "a", 2 -> "b")

---

Here's a way you can do it with a Scala implicit class:

implicit class ToSortedMap[A,B](tuples: TraversableOnce[(A, B)])
                               (implicit ordering: Ordering[A]) {
  def toSortedMap =
    SortedMap(tuples.toSeq: _*)
}

Since Map[A,B] has an implicit path to a TraversableOnce[Tuple2[A, B]], the following works:

scala> Map("b" -> 3, "c" -> 3, "a" -> 5).toSortedMap
res6: scala.collection.immutable.SortedMap[String,Int] = Map(a -> 5, b -> 3, c -> 3)

It will even work on a list of Tuple2s, similar to toMap:

scala> List(("c", 1), ("b", 3),("a", 6)).toSortedMap
res7: scala.collection.immutable.SortedMap[String,Int] = Map(a -> 6, b -> 3, c -> 1)

---

Since internal data structures in implementations are completely different, you'll have to add elements one-by-one anyway. So, do it explicitly:

val m = Map(1 -> "one")

var t = scala.collection.immutable.TreeMap[Int,String]()
t ++= m