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

问题

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, 4, 5]

In Swift 1.1 I'd do a filter followed by a map like this:

let filtermap = possibles.filter({ return $0 != nil }).map({ return $0! })
// filtermap = [1, 2, 3, 4, 5]

I've been trying to do this using flatMap a couple ways:

var flatmap1 = possibles.flatMap({
    return $0 == nil ? [] : [$0!]
})

and

var flatmap2:[Int] = possibles.flatMap({
    if let exercise = $0 { return [exercise] }
    return []
})

I prefer the last approach (because I don't have to do a forced unwrap $0!... I'm terrified for these and avoid them at all costs) except that I need to specify the Array type.

Is there an alternative away that figures out the type by context, but doesn't have the forced unwrap?

回答1:

Since Swift 4.1 you can use compactMap:

let possibles:[Int?] = [nil, 1, 2, 3, nil, nil, 4, 5]
let actuals = possibles.compactMap { $0 }

(Swift 4.1 replaced some overloads of flatMap with compactmap. If you are interested in more detail on this then see for example: https://useyourloaf.com/blog/replacing-flatmap-with-compactmap/ )

With Swift 2 b1, you can simply do

let possibles:[Int?] = [nil, 1, 2, 3, nil, nil, 4, 5]
let actuals = possibles.flatMap { $0 }

For earlier versions, you can shim this with the following extension:

extension Array {
    func flatMap<U>(transform: Element -> U?) -> [U] {
        var result = [U]()
        result.reserveCapacity(self.count)
        for item in map(transform) {
            if let item = item {
                result.append(item)
            }
        }
        return result
    }
}

One caveat (which is also true for Swift 2) is that you might need to explicitly type the return value of the transform:

let actuals = ["a", "1"].flatMap { str -> Int? in
    if let int = str.toInt() {
        return int
    } else {
        return nil
    }
}
assert(actuals == [1])

For more info, see http://airspeedvelocity.net/2015/07/23/changes-to-the-swift-standard-library-in-2-0-betas-2-5/

回答2:

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

and the inner flatMap is the function

func flatMap<T, U>(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

回答3:

You could use reduce:

let flattened = possibles.reduce([Int]()) { 
        if let x = $1 { return $0 + [x] } else { return $0 } 
    }

You are still kind of declaring the type, but it's slightly less obtrusive.

回答4:

Since this is something I seem to end up doing quite a lot I'm exploring a generic function to do this.

I tried to add an extension to Array so I could do something like possibles.unwraped but couldn't figure out how to make an extension on an Array. Instead used a custom operator -- hardest part here was trying to figure out which operator to choose. In the end I chose >! to show that the array is being filtered > and then unwrapped !.

let possibles:[Int?] = [nil, 1, 2, 3, nil, nil, 4, 5]

postfix operator >! {}

postfix func >! <T>(array: Array<T?>) -> Array<T> {
    return array.filter({ $0 != nil }).map({ $0! })
}

possibles>!
// [1, 2, 3, 4, 5]

回答5:

Related to the question. If you are applying flatMap to an optional array, do not forget to optionally or force unwrap your array otherwise it will call flatMap on Optional and not objects conforming to Sequence protocol. I made that mistake once, E.g. when you want to remove empty strings:

var texts: [String]? = ["one", "two", "", "three"] // has unwanted empty string

let notFlatMapped = texts.flatMap({ $0.count > 0 ? $0 : nil })
// ["one", "two", "", "three"], not what we want - calls flatMap on Optional

let flatMapped = texts?.flatMap({ $0.count > 0 ? $0 : nil })
// ["one", "two", "three"], that's what we want, calls flatMap on Array

来源：https://stackoverflow.com/questions/29870365/how-to-use-swift-flatmap-to-filter-out-optionals-from-an-array

标签

swift

functional-programming

optional

higher-order-functions

flatmap