Filter a list by item index?

Question

val data = List("foo", "bar", "bash")
val selection = List(0, 2)
val selectedData = data.filter(datum => selection.contains(datum.MYINDEX))
//                                                  INVALID CODE HERE ^
// selectedData: List("foo", "bash")

Say I want to filter a List given a list of selected indices. If, in the filter method, I could reference the index of a list item then I could solve this as above, but datum.MYINDEX isn't valid in the above case.

How could I do this instead?

Answer 1

How about using zipWithIndex to keep a reference to the item's index, filtering as such, then mapping the index away?

data.zipWithIndex
    .filter{ case (datum, index) => selection.contains(index) }
    .map(_._1)

Answer 2

It's neater to do it the other way about (although potentially slow with Lists as indexing is slow (O(n)). Vectors would be better. On the other hand, the contains of the other solution for every item in data isn't exactly fast)

val data = List("foo", "bar", "bash")
         //> data  : List[String] = List(foo, bar, bash)
val selection = List(0, 2)  
         //> selection  : List[Int] = List(0, 2)
selection.map(index=>data(index))
         //> res0: List[String] = List(foo, bash)