Swift - How to get indexes of filtered items of array

Question

let items: [String] = [\"A\", \"B\", \"A\", \"C\", \"A\", \"D\"]

items.whatFunction(\"A\") // -> [0, 2, 4]
items.whatFunction(\"B\") // -> [1]

Answer 1

just copy and paste

extension Array {
  func whatFunction(_ ids :  String) -> [Int] {

    var mutableArr = [Int]()
    for i in 0..