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] 

Does Swift 3 support a function like whatFunction(_: Element)?

If not, what is the most efficient logic?

Answer 1

You can filter the indices of the array directly, it avoids the extra mapping.

let items = ["A", "B", "A", "C", "A", "D"] let filteredIndices = items.indices.filter {items[$0] == "A"} 

or as Array extension:

extension Array where Element: Equatable {      func whatFunction(_ value :  Element) -> [Int] {         return self.indices.filter {self[$0] == value}     }  }  items.whatFunction("A") // -> [0, 2, 4] items.whatFunction("B") // -> [1] 

or still more generic

extension Collection where Element: Equatable {      func whatFunction(_ value :  Element) -> [Index] {         return self.indices.filter {self[$0] == value}     }  }