The problem

P05 (*) Reverse a list.


scala> reverse(List(1, 1, 2, 3, 5, 8))
res0: List[Int] = List(8, 5, 3, 2, 1, 1)

Initial thoughts

Another classical problem of computer science and functional languages. The solution will be straightforward. As happened with the length() function developed for problem 04, this one may help solving problem 02 in a pure recursive way. Moreover this problem may be solved with folding like problem 04.

The procedural solution

List type provides a suitable built-in method reverse() to return a reversed copy of the list. Pay attention to the fact that this method does not reverse the list in place, but returns a new list.

def reverse[A](ls: List[A]) = ls.reverse

The recursive solution

def reverse[A](l: List[A]): List[A] = l match {
    case h :: tail => reverse(tail) ::: List(h)
    case Nil => Nil

This solution gives us the chance to dig into list concatenation operators in Scala. The double colon operator :: has been introduced in problem 01 as something that divides the head element of a list from the tail. As a matter of fact, that operator appends an element at the beginning of a list

scala> val l = List(2,3,4,5,6)
l: List[Int] = List(2, 3, 4, 5, 6)

scala> 1::l
res0: List[Int] = List(1, 2, 3, 4, 5, 6)

Pay attention that pattern matching does not only a "pattern comparison", but also an assignment. In Scala, when you write

scala> 5 match {
     |  case a => a + 1
     | }
res3: Int = 6

what you do is to first try to match 5 with a, and if it is successful, Scala assigns the value 5 to the variable (in this case a is a free variable, so the match is always satisfied).

So the list pattern matching we already used

scala> List(1,2,3,4) match {
     |  case h::tail => h
     |  case _ => Nil
     | }
res7: Any = 1

tries to match the incoming List(1,2,3,4) with something in the form h::tail, which means "a list with a single element head and a list tail". Using the :: operator ensures that h is considered a single element.

This, however, is just a mnemonic. The truth is that :: in Scala is also a class derived from List which is constructed passing a single element and a list. This means that the above example may be explicityl written as

scala> List(1,2,3,4) match {
     |  case ::(h,tail) => h
     |  case _ => Nil
     | }
res7: Any = 1

So, while the effect is the same, there is difference between :: as a method of List objects and :: as a class used in pattern matching. Also remember that in Scala methods which name ends with : are right-associative (see the language specification. This means that 0 :: List(1,2,3) is a shorthand form for List(1,2,3).::(0)

The triple colon operator :::, conversely, is just a method of List objects and there is no class with that name. It prepends a whole list to another list (joins the lists), and ending with : it is also a right-associative method.

scala> List(1,2,3) ::: List(4,5,6)
res18: List[Int] = List(1, 2, 3, 4, 5, 6)

scala> List(4,5,6).:::(List(1,2,3))
res0: List[Int] = List(1, 2, 3, 4, 5, 6)

Now the recursive solution becomes more clear: the first pattern matching case just exchanges the head of the list with the tail, but recursively calls the function on the tail itself.

As already explained this code is not tail recursive, thus allocating stack space to store local variables during the recursive call. A good way to make it tail recursive is to use another list that is filled by the elements extracted from the source one.

def reverse[A](l: List[A]): List[A] = {
    def _reverse(res: List[A], rem: List[A]): List[A] = rem match {
        case Nil => res
        case h :: tail => _reverse(h :: res, tail)
    _reverse(Nil, l)

Here we use an helper function in which we pick the first element of the remainder list rem and add it at the beginning of a list built from scratch. As you can see there is no computation left on the stack when we do the recursive call. The first solution, conversely, gave the result of the recursion as input to the :::() method.


Folding may greatly simplify the solution. Remember that foldLeft() is a method of List objects that visits each element in the list, applying a given function. The first use of the function receives the initial value passed to foldLeft(), while successive calls receive the result of the previous call.

def reverse[A](ls: List[A]): List[A] =
    ls.foldLeft(List[A]()) { (r, h) => h :: r }

The function passed to foldLeft() accepts the previous result and the current element. Since the initial value is an empty list we may apply the :: operator just like we did in the tail recursive solution.

Last nth element

def lastNth[A](n: Int, l:List[A]): A = {
    def reverse[A](l:List[A]):List[A] = {
        def _reverse[A](r:List[A], l:List[A]):List[A] = l match {
            case Nil => r
            case head::tail => _reverse(head::r, tail)
        _reverse(List(), l)

    def findKth[A](k:Int, l:List[A]):A = (k,l) match {
        case (0, h::_) => h
        case (k, _::tail) if k > 0 => findKth(k - 1, tail)
        case _ => throw new NoSuchElementException

    val r = reverse(l)
    findKth(n - 1, r)

Instead of computing the actual index of the last-nth element using the length() function, we may just reverse the list and get the (n - 1)-th element (the subtraction of 1 comes from the fact that last-nth elements are indexed from 1, while list indexes start from 0).

Final considerations

This problem led me into the list concatenation operators and the discovery of the :: class that allows list decomposition in pattern matching.


Feel free to use the blog Google+ page to comment the post. The GitHub issues page is the best place to submit corrections.