Scala: How to sort an array within a specified range of indices? - scala

Question

And I have a comparison function "compr" already in the code to compare two values.
I want something like this:
Sorting.stableSort(arr[i,j] , compr)
where arr[i,j] is a range of element in array.

Answer 1

Take the slice as a view, sort and copy it back (or take a slice as a working buffer).
scala> val vs = Array(3,2,8,5,4,9,1,10,6,7)
vs: Array[Int] = Array(3, 2, 8, 5, 4, 9, 1, 10, 6, 7)
scala> vs.view(2,5).toSeq.sorted.copyToArray(vs,2)
scala> vs
res31: Array[Int] = Array(3, 2, 4, 5, 8, 9, 1, 10, 6, 7)
Outside the REPL, the extra .toSeq isn't needed:
vs.view(2,5).sorted.copyToArray(vs,2)
Updated:
scala 2.13.8> val vs = Array(3, 2, 8, 5, 4, 9, 1, 10, 6, 7)
val vs: Array[Int] = Array(3, 2, 8, 5, 4, 9, 1, 10, 6, 7)
scala 2.13.8> vs.view.slice(2,5).sorted.copyToArray(vs,2)
val res0: Int = 3
scala 2.13.8> vs
val res1: Array[Int] = Array(3, 2, 4, 5, 8, 9, 1, 10, 6, 7)

Answer 2

Split array into three parts, sort middle part and then concat them, not the most efficient way, but this is FP who cares about performance =)
val sorted =
for {
first <- l.take(FROM)
sortingPart <- l.slice(FROM, UNTIL)
lastPart <- l.takeRight(UNTIL)
} yield (first ++ Sorter.sort(sortingPart) ++ lastPart)

Answer 3

Something like that:
def stableSort[T](x: Seq[T], i: Int, j: Int, comp: (T,T) => Boolean ):Seq[T] = {
x.take(i) ++ x.slice(i,j).sortWith(comp) ++ x.drop(i+j-1)
}
def comp: (Int,Int) => Boolean = { case (x1,x2) => x1 < x2 }
val x = Array(1,9,5,6,3)
stableSort(x,1,4, comp)
// > res0: Seq[Int] = ArrayBuffer(1, 5, 6, 9, 3)
If your class implements Ordering it would be less cumbersome.

Answer 4

This should be as good as you can get without reimplementing the sort. Creates just one extra array with the size of the slice to be sorted.
def stableSort[K:reflect.ClassTag](xs:Array[K], from:Int, to:Int, comp:(K,K) => Boolean) : Unit = {
val tmp = xs.slice(from,to)
scala.util.Sorting.stableSort(tmp, comp)
tmp.copyToArray(xs, from)
}