Styjun

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?

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 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 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]

In Swift 3 and Swift 4 you can do that:

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

extension Array where Element: Equatable 

 func indexes(of item: Element) -> [Int] 
 return enumerated().compactMap $0.element == item ? $0.offset : nil 
 


items.indexes(of: "A")

I hope my answer was helpful 😊

you can use it like that :

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

 let indexes = items.enumerated().filter 
 $0.element == "A"
 .map$0.offset

 print(indexes)