How to use swift flatMap to filter out optionals from an array

Question

I\'m a little confused around flatMap (added to Swift 1.2)

Say I have an array of some optional type e.g.

let possibles:[Int?] = [nil, 1, 2, 3, nil, nil         


        

Answer 1

I still like the first solution, which creates only one intermediate array. It can slightly more compact be written as

let filtermap = possibles.filter({ $0 != nil }).map({ $0! })

---

But flatMap() without type annotation and without forced unwrapping is possible:

var flatmap3 = possibles.flatMap {
    flatMap($0, { [$0] }) ?? []
}

The outer flatMap is the array method

func flatMap(transform: @noescape (T) -> [U]) -> [U]

and the inner flatMap is the function

func flatMap(x: T?, f: @noescape (T) -> U?) -> U?

---

Here is a simple performance comparison (compiled in Release mode). It shows that the first method is faster, approximately by a factor of 10:

let count = 1000000
let possibles : [Int?] = map(0 ..< count) { $0 % 2 == 0 ? $0 : nil }

let s1 = NSDate()
let result1 = possibles.filter({ $0 != nil }).map({ $0! })
let e1 = NSDate()
println(e1.timeIntervalSinceDate(s1))
// 0.0169369578361511

let s2 = NSDate()
var result2 = possibles.flatMap {
    flatMap($0, { [$0] }) ?? []
}
let e2 = NSDate()
println(e2.timeIntervalSinceDate(s2))
// 0.117663979530334