How to create array of unique object list in Swift

Question

How can we create unique object list in Swift language like NSSet & NSMutableSet in Objective-C.

Answer 1

Always in such a case the critical factor is how to compare objects and what types of objects go into the Set. Using a Swift Dictionary, where the Set objects are the dictionary keys, could be a problem based on the restrictions on the key type (String, Int, Double, Bool, valueless Enumerations or hashable).

If you can define a hash function on your object type then you can use a Dictionary. If the objects are orderable, then you could define a Tree. If the objects are only comparable with == then you'll need to iterate over the set elements to detect a preexisting object.

// When T is only Equatable
class Set<T: Equatable> {
  var items = Array<T>()

  func hasItem (that: T) {
   // No builtin Array method of hasItem... 
   //   because comparison is undefined in builtin Array   
   for this: T in items {
     if (this == that) {
       return true
     }
   }
   return false
  }

  func insert (that: T) {
    if (!hasItem (that))
      items.append (that)
  }
}

The above is an example of building a Swift Set; the example used objects that are only Equatable - which, while a common case, doesn't necessarily lead to an efficient Set implementations (O(N) search complexity - the above is an example).

Answer 2

I've built an extensive Set type similar to the built-in Array and Dictionary - here are blog posts one and two and a GitHub repository:

• Creating a Set Type in Swift
• Set Type Follow-up
• SwiftSets on GitHub

Answer 3

Swift has no concept of sets. Using NSMutableSet in Swift might be slower than using a Dictionary that holds dummy values. You could do this :

var mySet: Dictionary<String, Boolean> = [:]
mySet["something"]= 1

Then just iterate over the keys.

Answer 4

You can use any Objective-C class in Swift:

var set = NSMutableSet()
set.addObject(foo)

Answer 5

As of Swift 1.2 (Xcode 6.3 beta), Swift has a native set type. From the release notes:

A new Set data structure is included which provides a generic collection of unique elements, with full value semantics. It bridges with NSSet, providing functionality analogous to Array and Dictionary.

Here are some simple usage examples:

// Create set from array literal:
var set = Set([1, 2, 3, 2, 1])

// Add single elements:
set.insert(4)
set.insert(3)

// Add multiple elements:
set.unionInPlace([ 4, 5, 6 ])
// Swift 3: set.formUnion([ 4, 5, 6 ])

// Remove single element:
set.remove(2)

// Remove multiple elements:
set.subtractInPlace([ 6, 7 ])
// Swift 3: set.subtract([ 6, 7 ])

print(set) // [5, 3, 1, 4]

// Test membership:
if set.contains(5) {
    print("yes")
}

but there are far more methods available.

Update: Sets are now also documented in the "Collection Types" chapter of the Swift documentation.

Answer 6

You actually can create a Set object pretty easy (in contradiction to GoZoner, there is a built in contains method):

class Set<T : Equatable> {
    var items : T[] = []

    func add(item : T) {
        if !contains(items, {$0 == item}) {
            items += item
        }
    }
}

and you maybe even want to declare a custom operator:

@assignment @infix func += <T : Equatable> (inout set : Set<T>, items : T[]) -> Set<T> {
    for item in items {
        set.add(item)
    }
    return set
}