[SOLVED] multiple pattern matching on iteration

multiple pattern matching on iteration

val subjectpairs = IndexedSeq((8,0),(3,4),(0,9),(6,1))
val priors =  subjectpairs.map { case( f,_) => f}.filter{ _ >0.0}
val posts =  subjectpairs.map { case( _,l) => l}.filter{ _ >0.0}

This creates 2 sequences from the first , with the first and second members of tuples as non-zero respectively.
However , this needs 2 iterations of the source sequence - which is quite inefficient.

Any suggestion on achieving this with a single iteration ( + any other improvement) - given that the sequence itself will be non-trivial.

Solution

Just use mutable ListBuffers. As long as they don't escape the scope of your helper function, there is nothing wrong with using fast mutable buffers and good ol' for-loops and if-elses.

import scala.collection.mutable.ListBuffer 
 
def unzipAndFilter[A, B](input: List[(A, B)])(pA: A => Boolean)(pB: B => Boolean): (List[A], List[B]) = {
  val aBldr = new ListBuffer[A]
  val bBldr = new ListBuffer[B]
  for (a, b) <- input do
    if (pA(a)) aBldr += a
    if (pB(b)) bBldr += b
  (aBldr.result, bBldr.result)
}

Usage:

unzipAndFilter(List((8,0),(3,4),(0,9),(6,1)))(_ > 0)(_ > 0)

// (List(8, 3, 6),List(4, 9, 1))

Or with Scala 3 extension syntax:

extension [A, B](input: List[(A, B)])
  def unzipAndFilter[A, B](pA: A => Boolean)(pB: B => Boolean): (List[A], List[B]) = {
    val aBldr = new ListBuffer[A]
    val bBldr = new ListBuffer[B]
    for (a, b) <- input do
      if (pA(a)) aBldr += a
      if (pB(b)) bBldr += b
    (aBldr.result, bBldr.result)
  }

and then

input.unzipAndFilter(_ > 0)(_ > 0)

There seems to be no unzipCollect in the standard library, unfortunately.