How to use priority queues in Scala?

Question

I am trying to implement A* search in Scala (version 2.10), but I\'ve ran into a brick wall - I can\'t figure out how to use Scala\'s Priority Queue. It seems like a simple task

Answer 1

There is actually pre-defined lexicographical order for tuples -- but you need to import it:

import scala.math.Ordering.Implicits._

Moreover, you can define your own ordering. Suppose I want to arrange tuples, based on the difference between first and second members of the tuple:

scala> import scala.collection.mutable.PriorityQueue
//  import scala.collection.mutable.PriorityQueue

scala> def diff(t2: (Int,Int)) = math.abs(t2._1 - t2._2)
// diff: (t2: (Int, Int))Int

scala> val x = new PriorityQueue[(Int, Int)]()(Ordering.by(diff))
// x: scala.collection.mutable.PriorityQueue[(Int, Int)] = PriorityQueue()

scala> x.enqueue(1 -> 1)

scala> x.enqueue(1 -> 2)

scala> x.enqueue(1 -> 3)

scala> x.enqueue(1 -> 4)

scala> x.enqueue(1 -> 0)

scala> x
// res5: scala.collection.mutable.PriorityQueue[(Int, Int)] = PriorityQueue((1,4), (1,3), (1,2), (1,1), (1,0))

Answer 2

Indeed, there is no implicit ordering on pairs of integers (a, b). What would it be? Perhaps they are both positive and you can use (a - 1.0/b)? Or they are not, and you can use, what, (a + atan(b/pi))? If you have an ordering in mind, you can consider wrapping your pairs in a type that has your ordering.