Avoid duplicates in Swift array

Question

I created this singleton to access a shared array throughout my app:

class TranslationItems {
    var delegate: TranslationItemsDelegate?

    static let shared = TranslationItems()

    var array = [Translation]() {
        didSet {
            delegate?.newItemAdded()
        }
    }
}

The problem is that this allows for duplication (the array may contain multiple items with the same hashValue). If I check for duplication inside the didSet setter and then change the array there (for example by doing array = Array(Set(array))) that leads to an infinite loop.

How do I remove duplicates in my class?

`Translation` has to be hashable` – ielyamani Dec 25 '18 at 11:41 — ielyamani, Dec 25 '18 at 11:41
I confirm it is hashable already – Cesare Dec 25 '18 at 11:59 — Cesare, Dec 25 '18 at 11:59
https://stackoverflow.com/a/46519116/2303865 – Leo Dabus Dec 25 '18 at 12:33 — Leo Dabus, Dec 25 '18 at 12:33

score 3 · Accepted Answer · answered Dec 25 '18 at 11:39

3

If you want to avoid duplicates why don't you use a Set anyway (Translation must conform to Hashable)?

var set = Set<Translation>()

However if you want to keep the array a more efficient way is to add an add method which filters the duplicates, Translation must conform to Equatable

func add(object: Translation) {
    if !array.contains(object) {
       array.append(object)
       delegate?.newItemAdded()
    }
}

Making a Set from the Array and then convert it back to Array is unnecessarily expensive.

answered Dec 25 '18 at 11:39

vadian

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`array.contains` will become [expensive](https://twitter.com/iklil_Elyamani/status/1077298360632135680) for a large number of elements. – ielyamani Dec 25 '18 at 11:43
1

@Carpsen90 Not as expensive as `Array(Set(array))` – vadian Dec 25 '18 at 11:44
1

Yes, that's like reaching for your ear with the wrong hand Not sure why such answers are upvoted... – ielyamani Dec 25 '18 at 11:49
@Carpsen90 Most people especially the newbies are looking for the easiest/shortest solution which is not necessarily the most efficient. – vadian Dec 25 '18 at 11:54
Great idea about using the Set. Thanks and Marry Christmas :) – Cesare Dec 25 '18 at 12:15
1

[OT] @Cesare *Marry* is *sposare* Buon Natale. – vadian Dec 25 '18 at 12:17

score 2 · Answer 2 · answered Dec 25 '18 at 11:29

2

You can do it exactly how you suggested. This doesn't lead to an infinite loop

didSet {
    array = Array(Set(array))
    ...
}

answered Dec 25 '18 at 11:29

Robert Dresler

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You're right, but isn't `didSet` supposed to be called every time the array is assigned to something? – Cesare Dec 25 '18 at 11:40
@Cesare you may find this post helpful: https://stackoverflow.com/questions/39819081/in-swift-does-resetting-the-property-inside-didset-trigger-another-didset – Robert Dresler Dec 25 '18 at 12:09

score 1 · Answer 3 · edited Dec 25 '18 at 11:34

1

Just add one instance method

class TranslationItems {

  var delegate: TranslationItemsDelegate?

  static let shared = TranslationItems()

  private(set) var array = [Translation]() {
      didSet {
          delegate?.newItemAdded()
      }
  } 

  func set(array:[Translation]) {
      self.array = Array(Set(array))
  }
}

edited Dec 25 '18 at 11:34

Shehata Gamal

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answered Dec 25 '18 at 11:30

SPatel

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Avoid duplicates in Swift array

3 Answers3