Idiomatic construction to check whether a collection is ordered

Question

With the intention of learning and further to this question, I've remained curious of the idiomatic alternatives to explicit recursion for an algorithm that checks whether a list (or collection) is ordered. (I'm keeping things simple here by using an operator to compare and Int as type; I'd like to look at the algorithm before delving into the generics of it)

The basic recursive version would be (by @Luigi Plinge):

def isOrdered(l:List[Int]): Boolean = l match {
  case Nil => true
  case x :: Nil => true
  case x :: xs => x <= xs.head && isOrdered(xs)
}

A poor performing idiomatic way would be:

def isOrdered(l: List[Int]) = l == l.sorted

An alternative algorithm using fold:

def isOrdered(l: List[Int]) =
  l.foldLeft((true, None:Option[Int]))((x,y) =>
    (x._1 && x._2.map(_ <= y).getOrElse(true), Some(y)))._1

It has the drawback that it will compare for all n elements of the list even if it could stop earlier after finding the first out-of-order element. Is there a way to "stop" fold and therefore making this a better solution?

Any other (elegant) alternatives?

Answer 1

This will exit after the first element that is out of order. It should thus perform well, but I haven't tested that. It's also a lot more elegant in my opinion. :)

def sorted(l:List[Int]) = l.view.zip(l.tail).forall(x => x._1 <= x._2)

Answer 2

By "idiomatic", I assume you're talking about McBride and Paterson's "Idioms" in their paper Applicative Programming With Effects. :o)

Here's how you would use their idioms to check if a collection is ordered:

import scalaz._
import Scalaz._

case class Lte[A](v: A, b: Boolean)

implicit def lteSemigroup[A:Order] = new Semigroup[Lte[A]] {
  def append(a1: Lte[A], a2: => Lte[A]) = {
    lazy val b = a1.v lte a2.v
    Lte(if (!a1.b || b) a1.v else a2.v, a1.b && b && a2.b)
  }
}

def isOrdered[T[_]:Traverse, A:Order](ta: T[A]) =
  ta.foldMapDefault(x => some(Lte(x, true))).fold(_.b, true)

Here's how this works:

Any data structure T[A] where there exists an implementation of Traverse[T], can be traversed with an Applicative functor, or "idiom", or "strong lax monoidal functor". It just so happens that every Monoid induces such an idiom for free (see section 4 of the paper).

A monoid is just an associative binary operation over some type, and an identity element for that operation. I'm defining a Semigroup[Lte[A]] (a semigroup is the same as a monoid, except without the identity element) whose associative operation tracks the lesser of two values and whether the left value is less than the right value. And of course Option[Lte[A]] is just the monoid generated freely by our semigroup.

Finally, foldMapDefault traverses the collection type T in the idiom induced by the monoid. The result b will contain true if each value was less than all the following ones (meaning the collection was ordered), or None if the T had no elements. Since an empty T is sorted by convention, we pass true as the second argument to the final fold of the Option.

As a bonus, this works for all traversable collections. A demo:

scala> val b = isOrdered(List(1,3,5,7,123))
b: Boolean = true

scala> val b = isOrdered(Seq(5,7,2,3,6))
b: Boolean = false

scala> val b = isOrdered(Map((2 -> 22, 33 -> 3)))
b: Boolean = true

scala> val b = isOrdered(some("hello"))
b: Boolean = true

A test:

import org.scalacheck._

scala> val p = forAll((xs: List[Int]) => (xs /== xs.sorted) ==> !isOrdered(xs))
p:org.scalacheck.Prop = Prop

scala> val q = forAll((xs: List[Int]) => isOrdered(xs.sorted))
q: org.scalacheck.Prop = Prop

scala> p && q check
+ OK, passed 100 tests.

And that's how you do idiomatic traversal to detect if a collection is ordered.

Answer 3

I'm going with this, which is pretty similar to Kim Stebel's, as a matter of fact.

def isOrdered(list: List[Int]): Boolean = (
  list 
  sliding 2 
  map { 
    case List(a, b) => () => a < b 
  } 
  forall (_())
)

Answer 4

In case you missed missingfaktor's elegant solution in the comments above:

Scala < 2.13.0

(l, l.tail).zipped.forall(_ <= _)

Scala 2.13.x+

l.lazyZip(l.tail).forall(_ <= _)

This solution is very readable and will exit on the first out-of-order element.

Answer 5

The recursive version is fine, but limited to List (with limited changes, it would work well on LinearSeq).

If it was implemented in the standard library (would make sense) it would probably be done in IterableLike and have a completely imperative implementation (see for instance method find)

You can interrupt the foldLeft with a return (in which case you need only the previous element and not boolean all along)

import Ordering.Implicits._
def isOrdered[A: Ordering](seq: Seq[A]): Boolean = {
  if (!seq.isEmpty)
    seq.tail.foldLeft(seq.head){(previous, current) => 
      if (previous > current) return false; current
    }
  true
}

but I don't see how it is any better or even idiomatic than an imperative implementation. I'm not sure I would not call it imperative actually.

Another solution could be

def isOrdered[A: Ordering](seq: Seq[A]): Boolean = 
  ! seq.sliding(2).exists{s => s.length == 2 && s(0) > s(1)}

Rather concise, and maybe that could be called idiomatic, I'm not sure. But I think it is not too clear. Moreover, all of those methods would probably perform much worse than the imperative or tail recursive version, and I do not think they have any added clarity that would buy that.

Also you should have a look at this question.

Answer 6

To stop iteration, you can use Iteratee:

import scalaz._
import Scalaz._
import IterV._
import math.Ordering
import Ordering.Implicits._

implicit val ListEnumerator = new Enumerator[List] {
  def apply[E, A](e: List[E], i: IterV[E, A]): IterV[E, A] = e match {
    case List() => i
    case x :: xs => i.fold(done = (_, _) => i,
                           cont = k => apply(xs, k(El(x))))
  }
}

def sorted[E: Ordering] : IterV[E, Boolean] = {
  def step(is: Boolean, e: E)(s: Input[E]): IterV[E, Boolean] = 
    s(el = e2 => if (is && e < e2)
                   Cont(step(is, e2))
                 else
                   Done(false, EOF[E]),
      empty = Cont(step(is, e)),
      eof = Done(is, EOF[E]))

  def first(s: Input[E]): IterV[E, Boolean] = 
    s(el = e1 => Cont(step(true, e1)),
      empty = Cont(first),
      eof = Done(true, EOF[E]))

  Cont(first)
}


scala> val s = sorted[Int]
s: scalaz.IterV[Int,Boolean] = scalaz.IterV$Cont$$anon$2@5e9132b3

scala> s(List(1,2,3)).run
res11: Boolean = true

scala> s(List(1,2,3,0)).run
res12: Boolean = false