Swift - How to get indexes of filtered items of array

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}    }}

You can create your own extension for arrays.

extension Array where Element: Equatable {    func indexes(of element: Element) -> [Int] {        return self.enumerated().filter({ element == $0.element }).map({ $0.offset })    }}

You can simply call it like this

items.indexes(of: "A") // [0, 2, 4]items.indexes(of: "B") // [1]

You can achieve this by chain of:

1. enumerated() - add indexes;
2. filter() out unnecessary items;
3. map() our indexes.

Example (works in Swift 3 - Swift 4.x):

let items: [String] = ["A", "B", "A", "C", "A", "D"]  print(items.enumerated().filter({ $0.element == "A" }).map({ $0.offset })) // -> [0, 2, 4]

Another way is using flatMap, which allows you to check the element and return index if needed in one closure.

Example (works in Swift 3 - Swift 4.0):

print(items.enumerated().flatMap { $0.element == "A" ? $0.offset : nil }) // -> [0, 2, 4]

But since Swift 4.1 flatMap that can return non-nil objects become deprecated and instead you should use compactMap.

Example (works since Swift 4.1):

print(items.enumerated().compactMap { $0.element == "A" ? $0.offset : nil }) // -> [0, 2, 4]

And the cleanest and the most memory-cheap way is to iterate through array indices and check if element of array at current index equals to required element.

Example (works in Swift 3 - Swift 4.x):

print(items.indices.filter({ items[$0] == "A" })) // -> [0, 2, 4]