Scala list to tuple conversion

Question

 def toPairs[A](xs : Seq[A]) : Seq[Tuple2[A,A]] = {
 @tailrec
 def accumulator(as : Seq[A], accum: Seq[Tuple2[A,A]]) : Seq[Tuple2[A,A]] = as match {
 case Nil => accum
 case x :: tail => tail match {
 case Nil => accum
 case _ => accumulator(as.tail, accum :+ (x,tail.head))
 }
 }
 accumulator(xs, Seq.empty[Tuple2[A,A]])
 }

I use this code for the following use case: for a Seq[A] i.e. List(1,2,3) I want a Seq of Tuples of A i.e. List((1,2)(2,3)). Is this the most optimal, idiomatic way in Scala, what could be improved?

For the record: the order of the elements is important and is expected to be congruent with the example in the preceding paragraph.

Answer 1

I find the way you go back and forth between as and x a bit annoying. You should be able to collapse the nested match into one match that covers three cases.

The :+ operator, which appends an element to a sequence, should be avoided at all costs, because it involves traversing to the end of the sequence. That makes your algorithm O(n²), when it should ideally be O(n).

Anyway, the solution can be much simpler than that.

def toPairs[A](xs: Seq[A]): Seq[(A,A)] = xs.zip(xs.tail)

• Quant: Awesome, esp. the short syntax of Tuple typeargs: (A,A) vs Tuple2[A,A]!