Collections (Pairs and Tuples)

Pair and Tuples

A pair in scala is written (x,y), where x and y are the elements of the pair.

val pair = ("answer", 42)
val pair2 = ("answer" -> 43)

The type of the above pair is (String, Int)

Pairs can be decomposed by pattern matching:

val pair = ("answer", 42)
val (label, value) = pair // label: String = answer; value: Int = 42

Also, the fields of the pair can be directly accessed by _1,_2:

val pair = ("answer", 42)
label = pair._1
value = pair._2

This works the same with tuples of more than two elements; you can have triples, quadruples, etc.

Translation of Tuples

So far, all the types we've encountered are actually abbreviations for some instance of a class type. Tuples are no exception.

• A tuple type (T1, ..., Tn) is an abbreviation of the parameterized type scala.Tuplen[T1, ..., Tn]
• A tuple expression of (e1, ..., en) is equivalent to the function application scala.Tuplen(e1, ..., en)
• A tuple pattern of (p1, ..., pn) is equivalent to the constructor pattern scala.Tuplen(p1, ..., pn)

Merge sort using Pairs

Insertion sort was slow (quadratic).
Previous implementation of merge in Merge Sort had issues as mentioned.

Here is a better implementation using pattern matching over pairs:

def msort(xs List[Int]): List[Int] = {
  val n = xs.length / 2
  if (n == 0) xs
  else {
    def merge(xs: List[Int], ys: List[Int]): List[Int] = (xs, ys) match {
      case (Nil, ys) => ys
      case (xs, Nil) => xs
      case (x :: xs1, y :: ys1) =>        // both lists are non-nil case
        if (x < y) x :: merge(xs1, ys)
        else y :: merge(xs, ys1)
    }
    val (fst, snd) = xs splitAt n
    merge(msort(fst), msort(snd))
  }
}