Unique values of array in swift

Question

I'm building an iOS app with swift and i need to get all unique values of array of strings.

I've been reading the apple developer docs but it doesn't seem to have a function for it.

Can someone give me an hint?

I use Dollar. `$.uniq(array)` https://github.com/ankurp/Dollar#uniq---uniq — Andrew, Dec 06 '16 at 19:00
No need to write extensions any more. Apple has provided `uniqued()` method in algorithms package. More info https://stackoverflow.com/a/70210590/1311902 — Kaunteya, Dec 03 '21 at 07:03

Logan · Answer 1 · 2021-03-16T19:54:35.220

209

There might be a more efficient way, but an extension would probably be most straightforward:

extension Array where Element: Equatable {
    var unique: [Element] {
        var uniqueValues: [Element] = []
        forEach { item in
            guard !uniqueValues.contains(item) else { return }
            uniqueValues.append(item)
        }
        return uniqueValues
    }
}

If order doesn't matter and objects are also hashable:

let array = ["one", "one", "two", "two", "three", "three"]
// order NOT guaranteed
let unique = Array(Set(array))
// ["three", "one", "two"]

edited Mar 16 '21 at 19:54

answered Dec 23 '14 at 16:41

Logan

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3

The first example is clean and simple. Works in Swift 3. Thanks! – Canucklesandwich Mar 06 '17 at 22:21
4

`Array(Set())` works in Swift 4 as well. Great solution. Thanks. – Anjan Biswas Dec 30 '17 at 07:16
Got to look this solution https://stackoverflow.com/a/48210756/5790492 – Nike Kov Mar 26 '18 at 00:05
I really like the Equatable version, as I don't have all elements to be Hashable as well. – d4Rk May 30 '18 at 15:53
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Negative. You may get a random order by using set. – BB9z Jun 23 '19 at 10:11
8

@BB9z I pointed that out in the answer, and also provided an extension that will preserve order, so don't know what you want here :) – Logan Jun 23 '19 at 13:04
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First variant has bad performance characteristics on big data set. Be cautious. – Aliaksandr Bialiauski Jun 12 '21 at 08:56
BigO n^2, really bad algorithm and performance, the correct way is using hash table. – user464230 Aug 19 '23 at 09:47

score 75 · Answer 2 · edited Mar 08 '17 at 22:51

75

There isn't a function to do this in the Swift standard library, but you could write one:

extension Sequence where Iterator.Element: Hashable {
    func unique() -> [Iterator.Element] {
        var seen: [Iterator.Element: Bool] = [:]
        return self.filter { seen.updateValue(true, forKey: $0) == nil }
    }
}

let a = ["four","one", "two", "one", "three","four", "four"]
a.unique // ["four", "one", "two", "three"]

This has the downside of requiring the contents of the sequence to be hashable, not just equatable, but then again most equatable things are, including strings.

It also preserves the original ordering unlike, say, putting the contents in a dictionary or set and then getting them back out again.

edited Mar 08 '17 at 22:51

tobiasdm

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answered Dec 23 '14 at 16:39

Airspeed Velocity

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Is there an error? ` Use of unresolved identifier 'seq'` – μολὼν.λαβέ Apr 28 '15 at 23:27
1

Ah, so there is, should be `filter(source)` – Airspeed Velocity Apr 28 '15 at 23:29
For my application, this method is at least ~3x faster than distinct. – μολὼν.λαβέ May 05 '15 at 20:16
See http://stackoverflow.com/a/33984316/616764 – Scott Gardner Nov 29 '15 at 15:14
1

This is throwing an error in Swift 2. It should be `source.filter`. – rob Dec 16 '15 at 19:53
Alternatively in an extension (Swift 2): `extension Array where Element: Hashable { func distinct() -> [Element] { var seen: [Element: Bool] = [:] return self.filter { seen.updateValue(true, forKey: $0) == nil } } }` – User Feb 02 '16 at 20:01
1

Not working to Swift 3. Are you have some ideas? – Bruno Oct 26 '16 at 18:05
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Set can be also used for seen ```var seen = Set() return filter { seen.update(with: $0) == nil }``` – Igor Palaguta Nov 17 '22 at 11:06

Ben Kane · Answer 3 · 2014-12-23T17:27:32.617

4

I don't know of a built in way. This generic function would do it:

func distinct<S: SequenceType, E: Equatable where E==S.Generator.Element>(source: S) -> [E]
{
    var unique = [E]()

    for item in source
    {
        if !contains(unique, item)
        {
            unique.append(item)
        }
    }
    return unique
}

The downside here is that this solution runs in O(n²).

edited Dec 23 '14 at 17:27

answered Dec 23 '14 at 16:44

Ben Kane

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`contains` runs in O(n) so this solution runs in quadratic time (though it does have the benefit of not requiring the elements be hashable). – Airspeed Velocity Dec 23 '14 at 16:47
@AirspeedVelocity Yes that's a good point. And being hashable won't normally be a problem like you've pointed out in your solution. Nonetheless it is an alternate, albeit slower, solution if having to be hashable is a game breaker. I prefer your solution but I think I'm going to keep this posted for that reason. – Ben Kane Dec 23 '14 at 16:53
Overloading means you can implement both, and the best one will be picked! (because Hashable conforms to Equatable, so is more specific, overload resolution will prefer it :) – Airspeed Velocity Dec 23 '14 at 16:55
(so long as you make both versions either take a sequence or an array, that is... otherwise they'll fight) – Airspeed Velocity Dec 23 '14 at 16:58
Yep! Gotta love Swift :) I'll edit my answer to take a sequence to be more generic and in case anyone wants to take the overloading approach. – Ben Kane Dec 23 '14 at 17:27

score 1 · Answer 4 · answered Dec 23 '14 at 16:38

1

Use a dictionary like var unique = [<yourtype>:Bool]() and fill in the values like unique[<array value>] = true in a loop. Now unique.keys has what you need.

answered Dec 23 '14 at 16:38

qwerty_so

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Unique values of array in swift

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