Simplest way to get the top n elements of a Scala Iterable

Question

Is there a simple and efficient solution to determine the top n elements of a Scala Iterable? I mean something like

iter.toList.sortBy(_.myAttr).take(2)
         


        

Answer 1

You don't need to sort the entire collection in order to determine the top N elements. However, I don't believe that this functionality is supplied by the raw library, so you would have to roll you own, possibly using the pimp-my-library pattern.

For example, you can get the nth element of a collection as follows:

  class Pimp[A, Repr <% TraversableLike[A, Repr]](self : Repr) {

    def nth(n : Int)(implicit ord : Ordering[A]) : A = {
      val trav : TraversableLike[A, Repr] = self
      var ltp : List[A] = Nil
      var etp : List[A] = Nil
      var mtp : List[A] = Nil
      trav.headOption match {
        case None      => error("Cannot get " + n + " element of empty collection")
        case Some(piv) =>
          trav.foreach { a =>
            val cf = ord.compare(piv, a)
            if (cf == 0) etp ::= a
            else if (cf > 0) ltp ::= a
            else mtp ::= a
          }
          if (n < ltp.length)
            new Pimp[A, List[A]](ltp.reverse).nth(n)(ord)
          else if (n < (ltp.length + etp.length))
            piv
          else
            new Pimp[A, List[A]](mtp.reverse).nth(n - ltp.length - etp.length)(ord)
      }
    }
  }

(This is not very functional; sorry)

It's then trivial to get the top n elements:

def topN(n : Int)(implicit ord : Ordering[A], bf : CanBuildFrom[Repr, A, Repr]) ={
  val b = bf()
  val elem = new Pimp[A, Repr](self).nth(n)(ord)
  import util.control.Breaks._
  breakable {
    var soFar = 0
    self.foreach { tt =>
      if (ord.compare(tt, elem) < 0) {
         b += tt
         soFar += 1
      }
    }
    assert (soFar <= n)
    if (soFar < n) {
      self.foreach { tt =>
        if (ord.compare(tt, elem) == 0) {
          b += tt
          soFar += 1
        }
        if (soFar == n) break
      }
    }

  }
  b.result()
}

Unfortunately I'm having trouble getting this pimp to be discovered via this implicit:

implicit def t2n[A, Repr <% TraversableLike[A, Repr]](t : Repr) : Pimp[A, Repr] 
  = new Pimp[A, Repr](t)

I get this:

scala> List(4, 3, 6, 7, 1, 2, 8, 5).topN(4)
:9: error: could not find implicit value for evidence parameter of type (List[Int]) => scala.collection.TraversableLike[A,List[Int]]
   List(4, 3, 6, 7, 1, 2, 8, 5).topN(4)
       ^

However, the code actually works OK:

scala> new Pimp(List(4, 3, 6, 7, 1, 2, 8, 5)).topN(4)
res3: List[Int] = List(3, 1, 2, 4)

And

scala> new Pimp("ioanusdhpisjdmpsdsvfgewqw").topN(6)
res2: java.lang.String = affffdfe