Filter array by indices

Question

I have an array of elements. I also have an IndexSet that specifies which indices of the array need to be extracted into a new array. E.g.:

let array = ["sun", "moon", "star", "meteor"]
let indexSet: IndexSet = [2, 3]
// Some magic happens here to get:
let result = ["star", "meteor"]

I'm looking to use the swift filter function, but haven't got the answer yet. How can I do this?

Answer 1

IndexSet is a collection of increasing integers, therefore you can map each index to the corresponding array element:

let array = ["sun", "moon", "star", "meteor"]
let indexSet: IndexSet = [2, 3]

let result = indexSet.map { array[$0] } // Magic happening here!
print(result) // ["star", "meteor"]

This assumes that all indices are valid for the given array. If that is not guaranteed then you can filter the indices (as @dfri correctly remarked):

let result = indexSet.filteredIndexSet { $0 < array.count }.map { array[$0] }

Answer 2

You can use enumerated, filter and map like this

let result = array
    .enumerated()
    .filter { indexSet.contains($0.offset) }
    .map { $0.element }