Question

I need to sort an array of pairs by second element. How do I pass comparator for my pairs to the quickSort function?
I’m using the following ugly approach now:

type AccResult = (AccUnit, Long) // pair
class Comparator(a:AccResult) extends Ordered[AccResult] {
        def compare(that:AccResult) = lessCompare(a, that)
        def lessCompare(a:AccResult, that:AccResult) = if (a._2 == that._2) 0 else if (a._2 < that._2) -1 else 1
}
scala.util.Sorting.quickSort(data)(d => new Comparator(d))

Why is quickSort designed to have an ordered view instead of usual comparator argument?

Scala 2.7 solutions are preferred.

Answer 1

Ok, I’m not sure exactly what you are unhappy about what you are currently doing, but perhaps all you are looking for is this?

implicit def toComparator(a: AccResult) = new Comparator(a)
scala.util.Sorting.quickSort(data)

If, on the other hand, the problem is that the tuple is Ordered and you want a different ordering, well, that’s why it changed on Scala 2.8.

* EDIT *

Ouch! Sorry, I only now realize you said you preferred Scala 2.7 solutions. I have editted this answer soon to put the solution for 2.7 above. What follows is a 2.8 solution.

Scala 2.8 expects an Ordering, not an Ordered, which is a context bound, not a view bound. You’d write your code in 2.8 like this:

type AccResult = (AccUnit, Long) // pair 
implicit object AccResultOrdering extends Ordering[AccResult] { 
        def compare(x: AccResult, y: AccResult) = if (x._2 == y._2) 0 else if (x._2 < y._2) -1 else 1 
}

Or maybe just:

type AccResult = (AccUnit, Long) // pair 
implicit val AccResultOrdering = Ordering by ((_: AccResult)._2)

And use it like:

scala.util.Sorting.quickSort(data)

On the other hand, the usual way to do sort in Scala 2.8 is just to call one of the sorting methods on it, such as:

data.sortBy((_: AccResult)._2)