Scala filter on a list by index

Question

I wanted to write it functionally, and the best I could do was:

list.zipWithIndex.filter((tt:Tuple2[Thing,Int])=>(tt._2%3==0)).unzip._1

to g

Answer 1

I would do it like in Octave mathematical program.

val indices = 0 until n by 3  // Range 0,3,6,9 ...

and then I needed some way to select the indices from a collection. Obviously I had to have a collection with random-access O(1). Like Array or Vector. For example here I use Vector. To wrap the access into a nice DSL I'd add an implicit class:

implicit class VectorEnrichedWithIndices[T](v:Vector[T]) {
  def apply(indices:TraversableOnce[Int]):Vector[T] = {
    // some implementation 
    indices.toVector.map(v)
  }
}

The usage would look like:

val vector = list.toVector
val every3rdElement = vector(0 until vector.size by 3)