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

If efficiency is not an issue, you could do the following:

list.grouped(3).map(_.head)

Note that this constructs intermediate lists.

Alternatively you can use a for-comprehension:

for {
  (x,i) <- list zipWithIndex
  if i % 3 == 0
} yield x

This is of course almost identical to your original solution, just written differently.

My last alternative for you is the use of collect on the zipped list:

list.zipWithIndex.collect {
  case (x,i) if i % 3 == 0 => x
}