Scala sort stable?

Question

Scala collections have a sortBy method. Is this method stable?

 def sortList(source : List[Int]) : List[Int] = source.sortBy(_ % 2) 

Will this example always keep order?

Answer 1

Yes, it is stable. Link from scala source code:

https://github.com/scala/scala/blob/2.11.x/src/library/scala/collection/SeqLike.scala#L627

 def sortBy[B](f: A => B)(implicit ord: Ordering[B]): Repr = sorted(ord on f) /** Sorts this $coll according to an Ordering. * * The sort is stable. That is, elements that are equal (as determined by * `lt`) appear in the same order in the sorted sequence as in the original. * * @see [[scala.math.Ordering]] * * @param ord the ordering to be used to compare elements. * @return a $coll consisting of the elements of this $coll * sorted according to the ordering `ord`. */ def sorted[B >: A](implicit ord: Ordering[B]): Repr = { val len = this.length val b = newBuilder if (len == 1) b ++= this else if (len > 1) { b.sizeHint(len) val arr = new Array[AnyRef](len) // Previously used ArraySeq for more compact but slower code var i = 0 for (x <- this) { arr(i) = x.asInstanceOf[AnyRef] i += 1 } java.util.Arrays.sort(arr, ord.asInstanceOf[Ordering[Object]]) i = 0 while (i < arr.length) { b += arr(i).asInstanceOf[A] i += 1 } } b.result() }