`struct Set`

An unordered collection of unique elements.

Inheritance | `Collection, CustomDebugStringConvertible, CustomReflectable, CustomStringConvertible, Equatable, ExpressibleByArrayLiteral, Hashable, Sequence, SetAlgebra` |
---|---|

Nested Types | `Set.Index, Set.Iterator` |

### Initializers

Creates an empty set.

This is equivalent to initializing with an empty array literal. For example:

```
var emptySet = Set<Int>()
print(emptySet.isEmpty)
// Prints "true"
emptySet = []
print(emptySet.isEmpty)
// Prints "true"
```

#### Declaration

`@inlinable public init()`

Creates a new set from a finite sequence of items.

Use this initializer to create a new set from an existing sequence, for example, an array or a range.

```
let validIndices = Set(0..<7).subtracting([2, 4, 5])
print(validIndices)
// Prints "[6, 0, 1, 3]"
```

This initializer can also be used to restore set methods after performing
sequence operations such as `filter(_:)`

or `map(_:)`

on a set. For
example, after filtering a set of prime numbers to remove any below 10,
you can create a new set by using this initializer.

```
let primes: Set = [2, 3, 5, 7, 11, 13, 17, 19, 23]
let laterPrimes = Set(primes.lazy.filter { $0 > 10 })
print(laterPrimes)
// Prints "[17, 19, 23, 11, 13]"
```

- Parameter sequence: The elements to use as members of the new set.

#### Declaration

`@inlinable public init<Source>(_ sequence: Source) where Element == Source.Element, Source: Sequence`

Creates a new set from a finite sequence of items.

Use this initializer to create a new set from an existing sequence, like an array or a range:

```
let validIndices = Set(0..<7).subtracting([2, 4, 5])
print(validIndices)
// Prints "[6, 0, 1, 3]"
```

- Parameter sequence: The elements to use as members of the new set.

#### Declaration

`@inlinable public init<S>(_ sequence: S) where S: Sequence, Self.Element == S.Element`

Creates a set containing the elements of the given array literal.

Do not call this initializer directly. It is used by the compiler when you use an array literal. Instead, create a new set using an array literal as its value by enclosing a comma-separated list of values in square brackets. You can use an array literal anywhere a set is expected by the type context.

Here, a set of strings is created from an array literal holding only strings.

```
let ingredients: Set = ["cocoa beans", "sugar", "cocoa butter", "salt"]
if ingredients.isSuperset(of: ["sugar", "salt"]) {
print("Whatever it is, it's bound to be delicious!")
}
// Prints "Whatever it is, it's bound to be delicious!"
```

- Parameter elements: A variadic list of elements of the new set.

#### Declaration

`@inlinable public init(arrayLiteral elements: Element)`

Creates an empty set with preallocated space for at least the specified number of elements.

Use this initializer to avoid intermediate reallocations of a set's storage buffer when you know how many elements you'll insert into the set after creation.

- Parameter minimumCapacity: The minimum number of elements that the newly created set should be able to store without reallocating its storage buffer.

#### Declaration

`public init(minimumCapacity: Int)`

### Instance Variables

The total number of elements that the set can contain without allocating new storage.

#### Declaration

`var capacity: Int`

The number of elements in the collection.

To check whether a collection is empty, use its `isEmpty`

property
instead of comparing `count`

to zero. Unless the collection guarantees
random-access performance, calculating `count`

can be an O(*n*)
operation.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(n), wherenis the length of the collection.

#### Declaration

`var count: Int`

A string that represents the contents of the set, suitable for debugging.

#### Declaration

`var debugDescription: String`

A string that represents the contents of the set.

#### Declaration

`var description: String`

The "past the end" position for the set---that is, the position one greater than the last valid subscript argument.

If the set is empty, `endIndex`

is equal to `startIndex`

.

#### Declaration

`var endIndex: Set<Element>.Index`

The first element of the collection.

If the collection is empty, the value of this property is `nil`

.

```
let numbers = [10, 20, 30, 40, 50]
if let firstNumber = numbers.first {
print(firstNumber)
}
// Prints "10"
```

#### Declaration

`var first: Self.Element?`

A Boolean value that indicates whether the set is empty.

#### Declaration

`var isEmpty: Bool`

A Boolean value indicating whether the collection is empty.

When you need to check whether your collection is empty, use the
`isEmpty`

property instead of checking that the `count`

property is
equal to zero. For collections that don't conform to
`RandomAccessCollection`

, accessing the `count`

property iterates
through the elements of the collection.

```
let horseName = "Silver"
if horseName.isEmpty {
print("I've been through the desert on a horse with no name.")
} else {
print("Hi ho, \(horseName)!")
}
// Prints "Hi ho, Silver!")
```

Complexity: O(1)

#### Declaration

`var isEmpty: Bool`

A Boolean value that indicates whether the set has no elements.

#### Declaration

`var isEmpty: Bool`

A sequence containing the same elements as this sequence,
but on which some operations, such as `map`

and `filter`

, are
implemented lazily.

#### Declaration

`var lazy: LazySequence<Self>`

The starting position for iterating members of the set.

If the set is empty, `startIndex`

is equal to `endIndex`

.

#### Declaration

`var startIndex: Set<Element>.Index`

A value less than or equal to the number of elements in the sequence, calculated nondestructively.

The default implementation returns 0. If you provide your own implementation, make sure to compute the value nondestructively.

Complexity: O(1), except if the sequence also conforms to

`Collection`

. In this case, see the documentation of`Collection.underestimatedCount`

.

#### Declaration

`var underestimatedCount: Int`

A value less than or equal to the number of elements in the collection.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(n), wherenis the length of the collection.

#### Declaration

`var underestimatedCount: Int`

### Subscripts

Accesses the member at the given position.

#### Declaration

`@inlinable public subscript(position: Set<Element>.Index) -> Element`

Accesses the contiguous subrange of the collection's elements specified by a range expression.

The range expression is converted to a concrete subrange relative to this
collection. For example, using a `PartialRangeFrom`

range expression
with an array accesses the subrange from the start of the range
expression until the end of the array.

```
let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2...]
print(streetsSlice)
// ["Channing", "Douglas", "Evarts"]
```

The accessed slice uses the same indices for the same elements as the
original collection uses. This example searches `streetsSlice`

for one
of the strings in the slice, and then uses that index in the original
array.

```
let index = streetsSlice.firstIndex(of: "Evarts") // 4
print(streets[index!])
// "Evarts"
```

Always use the slice's `startIndex`

property instead of assuming that its
indices start at a particular value. Attempting to access an element by
using an index outside the bounds of the slice's indices may result in a
runtime error, even if that index is valid for the original collection.

```
print(streetsSlice.startIndex)
// 2
print(streetsSlice[2])
// "Channing"
print(streetsSlice[0])
// error: Index out of bounds
```

- Parameter bounds: A range of the collection's indices. The bounds of the range must be valid indices of the collection.

Complexity: O(1)

#### Declaration

`@inlinable public subscript<R>(r: R) where R: RangeExpression, Self.Index == R.Bound -> Self.SubSequence`

#### Declaration

`@inlinable public subscript(x: (UnboundedRange_) -> ()) -> Self.SubSequence`

### Instance Methods

Returns a Boolean value indicating whether every element of a sequence satisfies a given predicate.

The following code uses this method to test whether all the names in an array have at least five characters:

```
let names = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"]
let allHaveAtLeastFive = names.allSatisfy({ $0.count >= 5 })
// allHaveAtLeastFive == true
```

- Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value that indicates whether the passed element satisfies a condition.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func allSatisfy(_ predicate: (Self.Element) throws -> Bool) rethrows -> Bool`

Returns an array containing the non-`nil`

results of calling the given
transformation with each element of this sequence.

Use this method to receive an array of non-optional values when your transformation produces an optional value.

In this example, note the difference in the result of using `map`

and
`compactMap`

with a transformation that returns an optional `Int`

value.

```
let possibleNumbers = ["1", "2", "three", "///4///", "5"]
let mapped: [Int?] = possibleNumbers.map { str in Int(str) }
// [1, 2, nil, nil, 5]
let compactMapped: [Int] = possibleNumbers.compactMap { str in Int(str) }
// [1, 2, 5]
```

- Parameter transform: A closure that accepts an element of this sequence as its argument and returns an optional value.

Complexity: O(

m+n), wherenis the length of this sequence andmis the length of the result.

#### Declaration

`@inlinable public func compactMap<ElementOfResult>(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]`

Returns a Boolean value that indicates whether the given element exists in the set.

This example uses the `contains(_:)`

method to test whether an integer is
a member of a set of prime numbers.

```
let primes: Set = [2, 3, 5, 7]
let x = 5
if primes.contains(x) {
print("\(x) is prime!")
} else {
print("\(x). Not prime.")
}
// Prints "5 is prime!"
```

- Parameter member: An element to look for in the set.

Complexity: O(1)

#### Declaration

`@inlinable public func contains(_ member: Element) -> Bool`

Returns a Boolean value indicating whether the sequence contains an element that satisfies the given predicate.

You can use the predicate to check for an element of a type that
doesn't conform to the `Equatable`

protocol, such as the
`HTTPResponse`

enumeration in this example.

```
enum HTTPResponse {
case ok
case error(Int)
}
let lastThreeResponses: [HTTPResponse] = [.ok, .ok, .error(404)]
let hadError = lastThreeResponses.contains { element in
if case .error = element {
return true
} else {
return false
}
}
// 'hadError' == true
```

Alternatively, a predicate can be satisfied by a range of `Equatable`

elements or a general condition. This example shows how you can check an
array for an expense greater than $100.

```
let expenses = [21.37, 55.21, 9.32, 10.18, 388.77, 11.41]
let hasBigPurchase = expenses.contains { $0 > 100 }
// 'hasBigPurchase' == true
```

- Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value that indicates whether the passed element represents a match.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func contains(where predicate: (Self.Element) throws -> Bool) rethrows -> Bool`

Returns the distance between two indices.

Unless the collection conforms to the `BidirectionalCollection`

protocol,
`start`

must be less than or equal to `end`

.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(k), wherekis the resulting distance.

#### Declaration

`@inlinable public func distance(from start: Self.Index, to end: Self.Index) -> Int`

Returns a sequence by skipping the initial, consecutive elements that satisfy the given predicate.

The following example uses the `drop(while:)`

method to skip over the
positive numbers at the beginning of the `numbers`

array. The result
begins with the first element of `numbers`

that does not satisfy
`predicate`

.

```
let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
let startingWithNegative = numbers.drop(while: { $0 > 0 })
// startingWithNegative == [-2, 9, -6, 10, 1]
```

If `predicate`

matches every element in the sequence, the result is an
empty sequence.

- Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the result.

Complexity: O(

k), wherekis the number of elements to drop from the beginning of the sequence.

#### Declaration

`@inlinable public func drop(while predicate: (Self.Element) throws -> Bool) rethrows -> DropWhileSequence<Self>`

Returns a subsequence by skipping elements while `predicate`

returns
`true`

and returning the remaining elements.

- Parameter predicate: A closure that takes an element of the
sequence as its argument and returns
`true`

if the element should be skipped or`false`

if it should be included. Once the predicate returns`false`

it will not be called again.

Complexity: O(

n), wherenis the length of the collection.

#### Declaration

`@inlinable public func drop(while predicate: (Self.Element) throws -> Bool) rethrows -> Self.SubSequence`

Returns a sequence containing all but the given number of initial elements.

If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty sequence.

```
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst(2))
// Prints "[3, 4, 5]"
print(numbers.dropFirst(10))
// Prints "[]"
```

- Parameter k: The number of elements to drop from the beginning of
the sequence.
`k`

must be greater than or equal to zero.

Complexity: O(1), with O(

k) deferred to each iteration of the result, wherekis the number of elements to drop from the beginning of the sequence.

#### Declaration

`@inlinable public func dropFirst(_ k: Int = 1) -> DropFirstSequence<Self>`

Returns a subsequence containing all but the given number of initial elements.

If the number of elements to drop exceeds the number of elements in the collection, the result is an empty subsequence.

```
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst(2))
// Prints "[3, 4, 5]"
print(numbers.dropFirst(10))
// Prints "[]"
```

- Parameter k: The number of elements to drop from the beginning of
the collection.
`k`

must be greater than or equal to zero.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(k), wherekis the number of elements to drop from the beginning of the collection.

#### Declaration

`@inlinable public func dropFirst(_ k: Int = 1) -> Self.SubSequence`

Returns a sequence containing all but the given number of final elements.

The sequence must be finite. If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty sequence.

```
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast(2))
// Prints "[1, 2, 3]"
print(numbers.dropLast(10))
// Prints "[]"
```

- Parameter n: The number of elements to drop off the end of the
sequence.
`n`

must be greater than or equal to zero.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func dropLast(_ k: Int = 1) -> [Self.Element]`

Returns a subsequence containing all but the specified number of final elements.

If the number of elements to drop exceeds the number of elements in the collection, the result is an empty subsequence.

```
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast(2))
// Prints "[1, 2, 3]"
print(numbers.dropLast(10))
// Prints "[]"
```

- Parameter k: The number of elements to drop off the end of the
collection.
`k`

must be greater than or equal to zero.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(n), wherenis the length of the collection.

#### Declaration

`@inlinable public func dropLast(_ k: Int = 1) -> Self.SubSequence`

Returns a Boolean value indicating whether this sequence and another sequence contain equivalent elements in the same order, using the given predicate as the equivalence test.

At least one of the sequences must be finite.

The predicate must be a *equivalence relation* over the elements. That
is, for any elements `a`

, `b`

, and `c`

, the following conditions must
hold:

Complexity: O(

m), wheremis the lesser of the length of the sequence and the length of`other`

.

#### Declaration

`@inlinable public func elementsEqual<OtherSequence>(_ other: OtherSequence, by areEquivalent: (Self.Element, OtherSequence.Element) throws -> Bool) rethrows -> Bool where OtherSequence: Sequence`

Returns a sequence of pairs (*n*, *x*), where *n* represents a
consecutive integer starting at zero and *x* represents an element of
the sequence.

This example enumerates the characters of the string "Swift" and prints each character along with its place in the string.

```
for (n, c) in "Swift".enumerated() {
print("\(n): '\(c)'")
}
// Prints "0: 'S'"
// Prints "1: 'w'"
// Prints "2: 'i'"
// Prints "3: 'f'"
// Prints "4: 't'"
```

When you enumerate a collection, the integer part of each pair is a counter
for the enumeration, but is not necessarily the index of the paired value.
These counters can be used as indices only in instances of zero-based,
integer-indexed collections, such as `Array`

and `ContiguousArray`

. For
other collections the counters may be out of range or of the wrong type
to use as an index. To iterate over the elements of a collection with its
indices, use the `zip(_:_:)`

function.

This example iterates over the indices and elements of a set, building a list consisting of indices of names with five or fewer letters.

```
let names: Set = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"]
var shorterIndices: [Set<String>.Index] = []
for (i, name) in zip(names.indices, names) {
if name.count <= 5 {
shorterIndices.append(i)
}
}
```

Now that the `shorterIndices`

array holds the indices of the shorter
names in the `names`

set, you can use those indices to access elements in
the set.

```
for i in shorterIndices {
print(names[i])
}
// Prints "Sofia"
// Prints "Mateo"
```

Complexity: O(1)

#### Declaration

`@inlinable public func enumerated() -> EnumeratedSequence<Self>`

Returns an array containing, in order, the elements of the sequence that satisfy the given predicate.

In this example, `filter(_:)`

is used to include only names shorter than
five characters.

```
let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let shortNames = cast.filter { $0.count < 5 }
print(shortNames)
// Prints "["Kim", "Karl"]"
```

- Parameter isIncluded: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the returned array.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element]`

Returns a new set containing the elements of the set that satisfy the given predicate.

In this example, `filter(_:)`

is used to include only names shorter than
five characters.

```
let cast: Set = ["Vivien", "Marlon", "Kim", "Karl"]
let shortNames = cast.filter { $0.count < 5 }
shortNames.isSubset(of: cast)
// true
shortNames.contains("Vivien")
// false
```

- Parameter isIncluded: A closure that takes an element as its argument and returns a Boolean value indicating whether the element should be included in the returned set.

#### Declaration

`@available(swift 4.0) @inlinable public func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> Set<Element>`

Returns the first element of the sequence that satisfies the given predicate.

The following example uses the `first(where:)`

method to find the first
negative number in an array of integers:

```
let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
if let firstNegative = numbers.first(where: { $0 < 0 }) {
print("The first negative number is \(firstNegative).")
}
// Prints "The first negative number is -2."
```

- Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element is a match.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func first(where predicate: (Self.Element) throws -> Bool) rethrows -> Self.Element?`

Returns the index of the given element in the set, or `nil`

if the
element is not a member of the set.

- Parameter member: An element to search for in the set.

Complexity: O(1)

#### Declaration

`@inlinable public func firstIndex(of member: Element) -> Set<Element>.Index?`

Returns the first index in which an element of the collection satisfies the given predicate.

You can use the predicate to find an element of a type that doesn't
conform to the `Equatable`

protocol or to find an element that matches
particular criteria. Here's an example that finds a student name that
begins with the letter "A":

```
let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
if let i = students.firstIndex(where: { $0.hasPrefix("A") }) {
print("\(students[i]) starts with 'A'!")
}
// Prints "Abena starts with 'A'!"
```

- Parameter predicate: A closure that takes an element as its argument and returns a Boolean value that indicates whether the passed element represents a match.

Complexity: O(

n), wherenis the length of the collection.

#### Declaration

`@inlinable public func firstIndex(where predicate: (Self.Element) throws -> Bool) rethrows -> Self.Index?`

Returns an array containing the concatenated results of calling the given transformation with each element of this sequence.

Use this method to receive a single-level collection when your transformation produces a sequence or collection for each element.

In this example, note the difference in the result of using `map`

and
`flatMap`

with a transformation that returns an array.

```
let numbers = [1, 2, 3, 4]
let mapped = numbers.map { Array(repeating: $0, count: $0) }
// [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]]
let flatMapped = numbers.flatMap { Array(repeating: $0, count: $0) }
// [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
```

In fact, `s.flatMap(transform)`

is equivalent to
`Array(s.map(transform).joined())`

.

- Parameter transform: A closure that accepts an element of this sequence as its argument and returns a sequence or collection.

Complexity: O(

m+n), wherenis the length of this sequence andmis the length of the result.

#### Declaration

`@inlinable public func flatMap<SegmentOfResult>(_ transform: (Self.Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult: Sequence`

#### Declaration

`@available(swift, deprecated: 4.1, renamed: "compactMap(_:)", message: "Please use compactMap(_:) for the case where closure returns an optional value") public func flatMap<ElementOfResult>(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]`

Calls the given closure on each element in the sequence in the same order
as a `for`

-`in`

loop.

The two loops in the following example produce the same output:

```
let numberWords = ["one", "two", "three"]
for word in numberWords {
print(word)
}
// Prints "one"
// Prints "two"
// Prints "three"
numberWords.forEach { word in
print(word)
}
// Same as above
```

Using the `forEach`

method is distinct from a `for`

-`in`

loop in two
important ways:

- You cannot use a
`break`

or`continue`

statement to exit the current call of the`body`

closure or skip subsequent calls. - Using the
`return`

statement in the`body`

closure will exit only from the current call to`body`

, not from any outer scope, and won't skip subsequent calls.

- Parameter body: A closure that takes an element of the sequence as a parameter.

#### Declaration

`@inlinable public func forEach(_ body: (Self.Element) throws -> Void) rethrows`

Offsets the given index by the specified distance.

The value passed as `distance`

must not offset `i`

beyond the bounds of
the collection.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(k), wherekis the absolute value of`distance`

.

#### Declaration

`@inlinable public func formIndex(_ i: inout Self.Index, offsetBy distance: Int)`

Offsets the given index by the specified distance, or so that it equals the given limiting index.

The value passed as `distance`

must not offset `i`

beyond the bounds of
the collection, unless the index passed as `limit`

prevents offsetting
beyond those bounds.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(k), wherekis the absolute value of`distance`

.

#### Declaration

`@inlinable public func formIndex(_ i: inout Self.Index, offsetBy distance: Int, limitedBy limit: Self.Index) -> Bool`

Replaces the given index with its successor.

- Parameter i: A valid index of the collection.
`i`

must be less than`endIndex`

.

#### Declaration

`@inlinable public func formIndex(after i: inout Set<Element>.Index)`

Replaces the given index with its successor.

- Parameter i: A valid index of the collection.
`i`

must be less than`endIndex`

.

#### Declaration

`@inlinable public func formIndex(after i: inout Self.Index)`

Removes the elements of the set that aren't also in the given sequence.

In the following example, the elements of the `employees`

set that are
not also members of the `neighbors`

set are removed. In particular, the
names `"Alicia"`

, `"Chris"`

, and `"Diana"`

are removed.

```
var employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors = ["Bethany", "Eric", "Forlani", "Greta"]
employees.formIntersection(neighbors)
print(employees)
// Prints "["Bethany", "Eric"]"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public mutating func formIntersection<S>(_ other: S) where Element == S.Element, S: Sequence`

Replace this set with the elements contained in this set or the given set, but not both.

In the following example, the elements of the `employees`

set that are
also members of `neighbors`

are removed from `employees`

, while the
elements of `neighbors`

that are not members of `employees`

are added to
`employees`

. In particular, the names `"Bethany"`

and `"Eric"`

are
removed from `employees`

while the name `"Forlani"`

is added.

```
var employees: Set = ["Alicia", "Bethany", "Diana", "Eric"]
let neighbors = ["Bethany", "Eric", "Forlani"]
employees.formSymmetricDifference(neighbors)
print(employees)
// Prints "["Diana", "Forlani", "Alicia"]"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public mutating func formSymmetricDifference<S>(_ other: S) where Element == S.Element, S: Sequence`

Removes the elements of the set that are also in the given sequence and adds the members of the sequence that are not already in the set.

In the following example, the elements of the `employees`

set that are
also members of `neighbors`

are removed from `employees`

, while the
elements of `neighbors`

that are not members of `employees`

are added to
`employees`

. In particular, the names `"Alicia"`

, `"Chris"`

, and
`"Diana"`

are removed from `employees`

while the names `"Forlani"`

and
`"Greta"`

are added.

```
var employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors: Set = ["Bethany", "Eric", "Forlani", "Greta"]
employees.formSymmetricDifference(neighbors)
print(employees)
// Prints "["Diana", "Chris", "Forlani", "Alicia", "Greta"]"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public mutating func formSymmetricDifference(_ other: Set<Element>)`

Inserts the elements of the given sequence into the set.

If the set already contains one or more elements that are also in
`other`

, the existing members are kept. If `other`

contains multiple
instances of equivalent elements, only the first instance is kept.

```
var attendees: Set = ["Alicia", "Bethany", "Diana"]
let visitors = ["Diana", "Marcia", "Nathaniel"]
attendees.formUnion(visitors)
print(attendees)
// Prints "["Diana", "Nathaniel", "Bethany", "Alicia", "Marcia"]"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public mutating func formUnion<S>(_ other: S) where Element == S.Element, S: Sequence`

Hashes the essential components of this value by feeding them into the given hasher.

- Parameter hasher: The hasher to use when combining the components of this instance.

#### Declaration

`@inlinable public func hash(into hasher: inout Hasher)`

Returns an index that is the specified distance from the given index.

The following example obtains an index advanced four positions from a string's starting index and then prints the character at that position.

```
let s = "Swift"
let i = s.index(s.startIndex, offsetBy: 4)
print(s[i])
// Prints "t"
```

The value passed as `distance`

must not offset `i`

beyond the bounds of
the collection.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(k), wherekis the absolute value of`distance`

.

#### Declaration

`@inlinable public func index(_ i: Self.Index, offsetBy distance: Int) -> Self.Index`

Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index.

The following example obtains an index advanced four positions from a
string's starting index and then prints the character at that position.
The operation doesn't require going beyond the limiting `s.endIndex`

value, so it succeeds.

```
let s = "Swift"
if let i = s.index(s.startIndex, offsetBy: 4, limitedBy: s.endIndex) {
print(s[i])
}
// Prints "t"
```

The next example attempts to retrieve an index six positions from
`s.startIndex`

but fails, because that distance is beyond the index
passed as `limit`

.

```
let j = s.index(s.startIndex, offsetBy: 6, limitedBy: s.endIndex)
print(j)
// Prints "nil"
```

The value passed as `distance`

must not offset `i`

beyond the bounds of
the collection, unless the index passed as `limit`

prevents offsetting
beyond those bounds.

`RandomAccessCollection`

; otherwise, O(k), wherekis the absolute value of`distance`

.

#### Declaration

`@inlinable public func index(_ i: Self.Index, offsetBy distance: Int, limitedBy limit: Self.Index) -> Self.Index?`

Returns the position immediately after the given index.

The successor of an index must be well defined. For an index `i`

into a
collection `c`

, calling `c.index(after: i)`

returns the same index every
time.

- Parameter i: A valid index of the collection.
`i`

must be less than`endIndex`

.

#### Declaration

`@inlinable public func index(after i: Set<Element>.Index) -> Set<Element>.Index`

Inserts the given element in the set if it is not already present.

If an element equal to `newMember`

is already contained in the set, this
method has no effect. In the following example, a new element is
inserted into `classDays`

, a set of days of the week. When an existing
element is inserted, the `classDays`

set does not change.

```
enum DayOfTheWeek: Int {
case sunday, monday, tuesday, wednesday, thursday,
friday, saturday
}
var classDays: Set<DayOfTheWeek> = [.wednesday, .friday]
print(classDays.insert(.monday))
// Prints "(true, .monday)"
print(classDays)
// Prints "[.friday, .wednesday, .monday]"
print(classDays.insert(.friday))
// Prints "(false, .friday)"
print(classDays)
// Prints "[.friday, .wednesday, .monday]"
```

- Parameter newMember: An element to insert into the set.

#### Declaration

`@inlinable public mutating func insert(_ newMember: Element) -> (inserted: Bool, memberAfterInsert: Element)`

Returns a new set with the elements that are common to both this set and the given sequence.

In the following example, the `bothNeighborsAndEmployees`

set is made up
of the elements that are in *both* the `employees`

and `neighbors`

sets.
Elements that are in only one or the other are left out of the result of
the intersection.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors = ["Bethany", "Eric", "Forlani", "Greta"]
let bothNeighborsAndEmployees = employees.intersection(neighbors)
print(bothNeighborsAndEmployees)
// Prints "["Bethany", "Eric"]"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public func intersection<S>(_ other: S) -> Set<Element> where Element == S.Element, S: Sequence`

Returns a new set with the elements that are common to both this set and the given sequence.

In the following example, the `bothNeighborsAndEmployees`

set is made up
of the elements that are in *both* the `employees`

and `neighbors`

sets.
Elements that are in only one or the other are left out of the result of
the intersection.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors: Set = ["Bethany", "Eric", "Forlani", "Greta"]
let bothNeighborsAndEmployees = employees.intersection(neighbors)
print(bothNeighborsAndEmployees)
// Prints "["Bethany", "Eric"]"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public func intersection(_ other: Set<Element>) -> Set<Element>`

Returns a Boolean value that indicates whether the set has no members in common with the given sequence.

In the following example, the `employees`

set is disjoint with the
elements of the `visitors`

array because no name appears in both.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let visitors = ["Marcia", "Nathaniel", "Olivia"]
print(employees.isDisjoint(with: visitors))
// Prints "true"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public func isDisjoint<S>(with other: S) -> Bool where Element == S.Element, S: Sequence`

Returns a Boolean value that indicates whether this set has no members in common with the given set.

In the following example, the `employees`

set is disjoint with the
`visitors`

set because no name appears in both sets.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let visitors: Set = ["Marcia", "Nathaniel", "Olivia"]
print(employees.isDisjoint(with: visitors))
// Prints "true"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public func isDisjoint(with other: Set<Element>) -> Bool`

Returns a Boolean value that indicates whether the set has no members in common with the given set.

In the following example, the `employees`

set is disjoint with the
`visitors`

set because no name appears in both sets.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let visitors: Set = ["Marcia", "Nathaniel", "Olivia"]
print(employees.isDisjoint(with: visitors))
// Prints "true"
```

- Parameter other: A set of the same type as the current set.

#### Declaration

`@inlinable public func isDisjoint(with other: Self) -> Bool`

Returns a Boolean value that indicates whether the set is a strict subset of the given sequence.

Set *A* is a strict subset of another set *B* if every member of *A* is
also a member of *B* and *B* contains at least one element that is not a
member of *A*.

```
let employees = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(attendees.isStrictSubset(of: employees))
// Prints "true"
// A set is never a strict subset of itself:
print(attendees.isStrictSubset(of: attendees))
// Prints "false"
```

- Parameter possibleStrictSuperset: A sequence of elements.
`possibleStrictSuperset`

must be finite.

#### Declaration

`@inlinable public func isStrictSubset<S>(of possibleStrictSuperset: S) -> Bool where Element == S.Element, S: Sequence`

Returns a Boolean value that indicates whether the set is a strict subset of the given sequence.

Set *A* is a strict subset of another set *B* if every member of *A* is
also a member of *B* and *B* contains at least one element that is not a
member of *A*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(attendees.isStrictSubset(of: employees))
// Prints "true"
// A set is never a strict subset of itself:
print(attendees.isStrictSubset(of: attendees))
// Prints "false"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public func isStrictSubset(of other: Set<Element>) -> Bool`

Returns a Boolean value that indicates whether this set is a strict subset of the given set.

Set *A* is a strict subset of another set *B* if every member of *A* is
also a member of *B* and *B* contains at least one element that is not a
member of *A*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(attendees.isStrictSubset(of: employees))
// Prints "true"
// A set is never a strict subset of itself:
print(attendees.isStrictSubset(of: attendees))
// Prints "false"
```

- Parameter other: A set of the same type as the current set.

#### Declaration

`@inlinable public func isStrictSubset(of other: Self) -> Bool`

Returns a Boolean value that indicates whether the set is a strict superset of the given sequence.

Set *A* is a strict superset of another set *B* if every member of *B* is
also a member of *A* and *A* contains at least one element that is *not*
a member of *B*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees = ["Alicia", "Bethany", "Diana"]
print(employees.isStrictSuperset(of: attendees))
// Prints "true"
print(employees.isStrictSuperset(of: employees))
// Prints "false"
```

- Parameter possibleStrictSubset: A sequence of elements.
`possibleStrictSubset`

must be finite.

#### Declaration

`@inlinable public func isStrictSuperset<S>(of possibleStrictSubset: S) -> Bool where Element == S.Element, S: Sequence`

Returns a Boolean value that indicates whether the set is a strict superset of the given sequence.

Set *A* is a strict superset of another set *B* if every member of *B* is
also a member of *A* and *A* contains at least one element that is *not*
a member of *B*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(employees.isStrictSuperset(of: attendees))
// Prints "true"
print(employees.isStrictSuperset(of: employees))
// Prints "false"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public func isStrictSuperset(of other: Set<Element>) -> Bool`

Returns a Boolean value that indicates whether this set is a strict superset of the given set.

Set *A* is a strict superset of another set *B* if every member of *B* is
also a member of *A* and *A* contains at least one element that is *not*
a member of *B*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(employees.isStrictSuperset(of: attendees))
// Prints "true"
// A set is never a strict superset of itself:
print(employees.isStrictSuperset(of: employees))
// Prints "false"
```

- Parameter other: A set of the same type as the current set.

#### Declaration

`@inlinable public func isStrictSuperset(of other: Self) -> Bool`

Returns a Boolean value that indicates whether the set is a subset of the given sequence.

Set *A* is a subset of another set *B* if every member of *A* is also a
member of *B*.

```
let employees = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(attendees.isSubset(of: employees))
// Prints "true"
```

- Parameter possibleSuperset: A sequence of elements.
`possibleSuperset`

must be finite.

#### Declaration

`@inlinable public func isSubset<S>(of possibleSuperset: S) -> Bool where Element == S.Element, S: Sequence`

Returns a Boolean value that indicates whether this set is a subset of the given set.

Set *A* is a subset of another set *B* if every member of *A* is also a
member of *B*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(attendees.isSubset(of: employees))
// Prints "true"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public func isSubset(of other: Set<Element>) -> Bool`

Returns a Boolean value that indicates whether the set is a subset of another set.

Set *A* is a subset of another set *B* if every member of *A* is also a
member of *B*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(attendees.isSubset(of: employees))
// Prints "true"
```

- Parameter other: A set of the same type as the current set.

#### Declaration

`@inlinable public func isSubset(of other: Self) -> Bool`

Returns a Boolean value that indicates whether the set is a superset of the given sequence.

Set *A* is a superset of another set *B* if every member of *B* is also a
member of *A*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees = ["Alicia", "Bethany", "Diana"]
print(employees.isSuperset(of: attendees))
// Prints "true"
```

- Parameter possibleSubset: A sequence of elements.
`possibleSubset`

must be finite.

#### Declaration

`@inlinable public func isSuperset<S>(of possibleSubset: S) -> Bool where Element == S.Element, S: Sequence`

Returns a Boolean value that indicates whether this set is a superset of the given set.

Set *A* is a superset of another set *B* if every member of *B* is also a
member of *A*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(employees.isSuperset(of: attendees))
// Prints "true"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public func isSuperset(of other: Set<Element>) -> Bool`

Returns a Boolean value that indicates whether the set is a superset of the given set.

Set *A* is a superset of another set *B* if every member of *B* is also a
member of *A*.

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let attendees: Set = ["Alicia", "Bethany", "Diana"]
print(employees.isSuperset(of: attendees))
// Prints "true"
```

- Parameter other: A set of the same type as the current set.

#### Declaration

`@inlinable public func isSuperset(of other: Self) -> Bool`

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the given predicate to compare elements.

The predicate must be a *strict weak ordering* over the elements. That
is, for any elements `a`

, `b`

, and `c`

, the following conditions must
hold:

Note: This method implements the mathematical notion of lexicographical ordering, which has no connection to Unicode. If you are sorting strings to present to the end user, use

`String`

APIs that perform localized comparison instead.

Complexity: O(

m), wheremis the lesser of the length of the sequence and the length of`other`

.

#### Declaration

`@inlinable public func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence, by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Bool where OtherSequence: Sequence, Self.Element == OtherSequence.Element`

Returns an iterator over the members of the set.

#### Declaration

`@inlinable public func makeIterator() -> Set<Element>.Iterator`

Returns an array containing the results of mapping the given closure over the sequence's elements.

In this example, `map`

is used first to convert the names in the array
to lowercase strings and then to count their characters.

```
let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let lowercaseNames = cast.map { $0.lowercased() }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { $0.count }
// 'letterCounts' == [6, 6, 3, 4]
```

- Parameter transform: A mapping closure.
`transform`

accepts an element of this sequence as its parameter and returns a transformed value of the same or of a different type.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func map<T>(_ transform: (Self.Element) throws -> T) rethrows -> [T]`

Returns an array containing the results of mapping the given closure over the sequence's elements.

In this example, `map`

is used first to convert the names in the array
to lowercase strings and then to count their characters.

```
let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let lowercaseNames = cast.map { $0.lowercased() }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { $0.count }
// 'letterCounts' == [6, 6, 3, 4]
```

- Parameter transform: A mapping closure.
`transform`

accepts an element of this sequence as its parameter and returns a transformed value of the same or of a different type.

#### Declaration

`@inlinable public func map<T>(_ transform: (Self.Element) throws -> T) rethrows -> [T]`

Returns the maximum element in the sequence, using the given predicate as the comparison between elements.

The predicate must be a *strict weak ordering* over the elements. That
is, for any elements `a`

, `b`

, and `c`

, the following conditions must
hold:

This example shows how to use the `max(by:)`

method on a
dictionary to find the key-value pair with the highest value.

```
let hues = ["Heliotrope": 296, "Coral": 16, "Aquamarine": 156]
let greatestHue = hues.max { a, b in a.value < b.value }
print(greatestHue)
// Prints "Optional(("Heliotrope", 296))"
```

- Parameter areInIncreasingOrder: A predicate that returns
`true`

if its first argument should be ordered before its second argument; otherwise,`false`

.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@warn_unqualified_access @inlinable public func max(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Self.Element?`

Returns the minimum element in the sequence, using the given predicate as the comparison between elements.

The predicate must be a *strict weak ordering* over the elements. That
is, for any elements `a`

, `b`

, and `c`

, the following conditions must
hold:

This example shows how to use the `min(by:)`

method on a
dictionary to find the key-value pair with the lowest value.

```
let hues = ["Heliotrope": 296, "Coral": 16, "Aquamarine": 156]
let leastHue = hues.min { a, b in a.value < b.value }
print(leastHue)
// Prints "Optional(("Coral", 16))"
```

- Parameter areInIncreasingOrder: A predicate that returns
`true`

if its first argument should be ordered before its second argument; otherwise,`false`

.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@warn_unqualified_access @inlinable public func min(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Self.Element?`

Removes and returns the first element of the set.

Because a set is not an ordered collection, the "first" element may not be the first element that was added to the set.

#### Declaration

`@inlinable public mutating func popFirst() -> Element?`

Returns a sequence, up to the specified maximum length, containing the initial elements of the sequence.

If the maximum length exceeds the number of elements in the sequence, the result contains all the elements in the sequence.

```
let numbers = [1, 2, 3, 4, 5]
print(numbers.prefix(2))
// Prints "[1, 2]"
print(numbers.prefix(10))
// Prints "[1, 2, 3, 4, 5]"
```

- Parameter maxLength: The maximum number of elements to return. The
value of
`maxLength`

must be greater than or equal to zero.

Complexity: O(1)

#### Declaration

`@inlinable public func prefix(_ maxLength: Int) -> PrefixSequence<Self>`

Returns a subsequence, up to the specified maximum length, containing the initial elements of the collection.

If the maximum length exceeds the number of elements in the collection, the result contains all the elements in the collection.

```
let numbers = [1, 2, 3, 4, 5]
print(numbers.prefix(2))
// Prints "[1, 2]"
print(numbers.prefix(10))
// Prints "[1, 2, 3, 4, 5]"
```

- Parameter maxLength: The maximum number of elements to return.
`maxLength`

must be greater than or equal to zero.

Complexity: O(1) if the collection conforms to

`RandomAccessCollection`

; otherwise, O(k), wherekis the number of elements to select from the beginning of the collection.

#### Declaration

`@inlinable public func prefix(_ maxLength: Int) -> Self.SubSequence`

Returns a subsequence from the start of the collection through the specified position.

The resulting subsequence *includes* the element at the position `end`

.
The following example searches for the index of the number `40`

in an
array of integers, and then prints the prefix of the array up to, and
including, that index:

```
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.firstIndex(of: 40) {
print(numbers.prefix(through: i))
}
// Prints "[10, 20, 30, 40]"
```

Using the `prefix(through:)`

method is equivalent to using a partial
closed range as the collection's subscript. The subscript notation is
preferred over `prefix(through:)`

.

```
if let i = numbers.firstIndex(of: 40) {
print(numbers[...i])
}
// Prints "[10, 20, 30, 40]"
```

- Parameter end: The index of the last element to include in the
resulting subsequence.
`end`

must be a valid index of the collection that is not equal to the`endIndex`

property.

Complexity: O(1)

#### Declaration

`@inlinable public func prefix(through position: Self.Index) -> Self.SubSequence`

Returns a subsequence from the start of the collection up to, but not including, the specified position.

The resulting subsequence *does not include* the element at the position
`end`

. The following example searches for the index of the number `40`

in an array of integers, and then prints the prefix of the array up to,
but not including, that index:

```
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.firstIndex(of: 40) {
print(numbers.prefix(upTo: i))
}
// Prints "[10, 20, 30]"
```

Passing the collection's starting index as the `end`

parameter results in
an empty subsequence.

```
print(numbers.prefix(upTo: numbers.startIndex))
// Prints "[]"
```

Using the `prefix(upTo:)`

method is equivalent to using a partial
half-open range as the collection's subscript. The subscript notation is
preferred over `prefix(upTo:)`

.

```
if let i = numbers.firstIndex(of: 40) {
print(numbers[..<i])
}
// Prints "[10, 20, 30]"
```

- Parameter end: The "past the end" index of the resulting subsequence.
`end`

must be a valid index of the collection.

Complexity: O(1)

#### Declaration

`@inlinable public func prefix(upTo end: Self.Index) -> Self.SubSequence`

Returns a sequence containing the initial, consecutive elements that satisfy the given predicate.

The following example uses the `prefix(while:)`

method to find the
positive numbers at the beginning of the `numbers`

array. Every element
of `numbers`

up to, but not including, the first negative value is
included in the result.

```
let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
let positivePrefix = numbers.prefix(while: { $0 > 0 })
// positivePrefix == [3, 7, 4]
```

If `predicate`

matches every element in the sequence, the resulting
sequence contains every element of the sequence.

- Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the result.

Complexity: O(

k), wherekis the length of the result.

#### Declaration

`@inlinable public func prefix(while predicate: (Self.Element) throws -> Bool) rethrows -> [Self.Element]`

Returns a subsequence containing the initial elements until `predicate`

returns `false`

and skipping the remaining elements.

- Parameter predicate: A closure that takes an element of the
sequence as its argument and returns
`true`

if the element should be included or`false`

if it should be excluded. Once the predicate returns`false`

it will not be called again.

Complexity: O(

n), wherenis the length of the collection.

#### Declaration

`@inlinable public func prefix(while predicate: (Self.Element) throws -> Bool) rethrows -> Self.SubSequence`

Returns a random element of the collection.

Call `randomElement()`

to select a random element from an array or
another collection. This example picks a name at random from an array:

```
let names = ["Zoey", "Chloe", "Amani", "Amaia"]
let randomName = names.randomElement()!
// randomName == "Amani"
```

This method is equivalent to calling `randomElement(using:)`

, passing in
the system's default random generator.

`RandomAccessCollection`

; otherwise, O(n), wherenis the length of the collection.

#### Declaration

`@inlinable public func randomElement() -> Self.Element?`

Returns a random element of the collection, using the given generator as a source for randomness.

Call `randomElement(using:)`

to select a random element from an array or
another collection when you are using a custom random number generator.
This example picks a name at random from an array:

```
let names = ["Zoey", "Chloe", "Amani", "Amaia"]
let randomName = names.randomElement(using: &myGenerator)!
// randomName == "Amani"
```

- Parameter generator: The random number generator to use when choosing a random element.

`RandomAccessCollection`

; otherwise, O(n), wherenis the length of the collection.

Note: The algorithm used to select a random element may change in a future version of Swift. If you're passing a generator that results in the same sequence of elements each time you run your program, that sequence may change when your program is compiled using a different version of Swift.

#### Declaration

`@inlinable public func randomElement<T>(using generator: inout T) -> Self.Element? where T: RandomNumberGenerator`

Returns the result of combining the elements of the sequence using the given closure.

Use the `reduce(_:_:)`

method to produce a single value from the elements
of an entire sequence. For example, you can use this method on an array
of numbers to find their sum or product.

The `nextPartialResult`

closure is called sequentially with an
accumulating value initialized to `initialResult`

and each element of
the sequence. This example shows how to find the sum of an array of
numbers.

```
let numbers = [1, 2, 3, 4]
let numberSum = numbers.reduce(0, { x, y in
x + y
})
// numberSum == 10
```

When `numbers.reduce(_:_:)`

is called, the following steps occur:

- The
`nextPartialResult`

closure is called with`initialResult`

---`0`

in this case---and the first element of`numbers`

, returning the sum:`1`

. - The closure is called again repeatedly with the previous call's return value and each element of the sequence.
- When the sequence is exhausted, the last value returned from the closure is returned to the caller.

If the sequence has no elements, `nextPartialResult`

is never executed
and `initialResult`

is the result of the call to `reduce(_:_:)`

.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Self.Element) throws -> Result) rethrows -> Result`

Returns the result of combining the elements of the sequence using the given closure.

Use the `reduce(into:_:)`

method to produce a single value from the
elements of an entire sequence. For example, you can use this method on an
array of integers to filter adjacent equal entries or count frequencies.

This method is preferred over `reduce(_:_:)`

for efficiency when the
result is a copy-on-write type, for example an Array or a Dictionary.

The `updateAccumulatingResult`

closure is called sequentially with a
mutable accumulating value initialized to `initialResult`

and each element
of the sequence. This example shows how to build a dictionary of letter
frequencies of a string.

```
let letters = "abracadabra"
let letterCount = letters.reduce(into: [:]) { counts, letter in
counts[letter, default: 0] += 1
}
// letterCount == ["a": 5, "b": 2, "r": 2, "c": 1, "d": 1]
```

When `letters.reduce(into:_:)`

is called, the following steps occur:

- The
`updateAccumulatingResult`

closure is called with the initial accumulating value---`[:]`

in this case---and the first character of`letters`

, modifying the accumulating value by setting`1`

for the key`"a"`

. - The closure is called again repeatedly with the updated accumulating value and each element of the sequence.
- When the sequence is exhausted, the accumulating value is returned to the caller.

If the sequence has no elements, `updateAccumulatingResult`

is never
executed and `initialResult`

is the result of the call to
`reduce(into:_:)`

.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func reduce<Result>(into initialResult: Result, _ updateAccumulatingResult: (inout Result, Self.Element) throws -> ()) rethrows -> Result`

Removes the specified element from the set.

This example removes the element `"sugar"`

from a set of ingredients.

```
var ingredients: Set = ["cocoa beans", "sugar", "cocoa butter", "salt"]
let toRemove = "sugar"
if let removed = ingredients.remove(toRemove) {
print("The recipe is now \(removed)-free.")
}
// Prints "The recipe is now sugar-free."
```

- Parameter member: The element to remove from the set.

#### Declaration

`@inlinable public mutating func remove(_ member: Element) -> Element?`

Removes the element at the given index of the set.

- Parameter position: The index of the member to remove.
`position`

must be a valid index of the set, and must not be equal to the set's end index.

#### Declaration

`@inlinable public mutating func remove(at position: Set<Element>.Index) -> Element`

Removes all members from the set.

- Parameter keepingCapacity: If
`true`

, the set's buffer capacity is preserved; if`false`

, the underlying buffer is released. The default is`false`

.

#### Declaration

`@inlinable public mutating func removeAll(keepingCapacity keepCapacity: Bool = false)`

Removes the first element of the set.

Because a set is not an ordered collection, the "first" element may not be the first element that was added to the set. The set must not be empty.

Complexity: Amortized O(1) if the set does not wrap a bridged

`NSSet`

. If the set wraps a bridged`NSSet`

, the performance is unspecified.

#### Declaration

`@inlinable public mutating func removeFirst() -> Element`

Reserves enough space to store the specified number of elements.

If you are adding a known number of elements to a set, use this method to avoid multiple reallocations. This method ensures that the set has unique, mutable, contiguous storage, with space allocated for at least the requested number of elements.

Calling the `reserveCapacity(_:)`

method on a set with bridged
storage triggers a copy to contiguous storage even if the existing
storage has room to store `minimumCapacity`

elements.

- Parameter minimumCapacity: The requested number of elements to store.

#### Declaration

`public mutating func reserveCapacity(_ minimumCapacity: Int)`

Returns an array containing the elements of this sequence in reverse order.

The sequence must be finite.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func reversed() -> [Self.Element]`

Returns the elements of the sequence, shuffled.

For example, you can shuffle the numbers between `0`

and `9`

by calling
the `shuffled()`

method on that range:

```
let numbers = 0...9
let shuffledNumbers = numbers.shuffled()
// shuffledNumbers == [1, 7, 6, 2, 8, 9, 4, 3, 5, 0]
```

This method is equivalent to calling `shuffled(using:)`

, passing in the
system's default random generator.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func shuffled() -> [Self.Element]`

Returns the elements of the sequence, shuffled using the given generator as a source for randomness.

You use this method to randomize the elements of a sequence when you are
using a custom random number generator. For example, you can shuffle the
numbers between `0`

and `9`

by calling the `shuffled(using:)`

method on
that range:

```
let numbers = 0...9
let shuffledNumbers = numbers.shuffled(using: &myGenerator)
// shuffledNumbers == [8, 9, 4, 3, 2, 6, 7, 0, 5, 1]
```

- Parameter generator: The random number generator to use when shuffling the sequence.

Complexity: O(

n), wherenis the length of the sequence.

Note: The algorithm used to shuffle a sequence may change in a future version of Swift. If you're passing a generator that results in the same shuffled order each time you run your program, that sequence may change when your program is compiled using a different version of Swift.

#### Declaration

`@inlinable public func shuffled<T>(using generator: inout T) -> [Self.Element] where T: RandomNumberGenerator`

Returns the elements of the sequence, sorted using the given predicate as the comparison between elements.

When you want to sort a sequence of elements that don't conform to the
`Comparable`

protocol, pass a predicate to this method that returns
`true`

when the first element should be ordered before the second. The
elements of the resulting array are ordered according to the given
predicate.

In the following example, the predicate provides an ordering for an array
of a custom `HTTPResponse`

type. The predicate orders errors before
successes and sorts the error responses by their error code.

```
enum HTTPResponse {
case ok
case error(Int)
}
let responses: [HTTPResponse] = [.error(500), .ok, .ok, .error(404), .error(403)]
let sortedResponses = responses.sorted {
switch ($0, $1) {
// Order errors by code
case let (.error(aCode), .error(bCode)):
return aCode < bCode
// All successes are equivalent, so none is before any other
case (.ok, .ok): return false
// Order errors before successes
case (.error, .ok): return true
case (.ok, .error): return false
}
}
print(sortedResponses)
// Prints "[.error(403), .error(404), .error(500), .ok, .ok]"
```

You also use this method to sort elements that conform to the
`Comparable`

protocol in descending order. To sort your sequence in
descending order, pass the greater-than operator (`>`

) as the
`areInIncreasingOrder`

parameter.

```
let students: Set = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
let descendingStudents = students.sorted(by: >)
print(descendingStudents)
// Prints "["Peter", "Kweku", "Kofi", "Akosua", "Abena"]"
```

Calling the related `sorted()`

method is equivalent to calling this
method and passing the less-than operator (`<`

) as the predicate.

```
print(students.sorted())
// Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]"
print(students.sorted(by: <))
// Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]"
```

*strict weak ordering* over the elements. That
is, for any elements `a`

, `b`

, and `c`

, the following conditions must
hold:

The sorting algorithm is not guaranteed to be stable. A stable sort
preserves the relative order of elements for which
`areInIncreasingOrder`

does not establish an order.

- Parameter areInIncreasingOrder: A predicate that returns
`true`

if its first argument should be ordered before its second argument; otherwise,`false`

.

Complexity: O(

nlogn), wherenis the length of the sequence.

#### Declaration

`@inlinable public func sorted(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> [Self.Element]`

Returns the longest possible subsequences of the sequence, in order, that don't contain elements satisfying the given predicate. Elements that are used to split the sequence are not returned as part of any subsequence.

The following examples show the effects of the `maxSplits`

and
`omittingEmptySubsequences`

parameters when splitting a string using a
closure that matches spaces. The first use of `split`

returns each word
that was originally separated by one or more spaces.

```
let line = "BLANCHE: I don't want realism. I want magic!"
print(line.split(whereSeparator: { $0 == " " })
.map(String.init))
// Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
```

The second example passes `1`

for the `maxSplits`

parameter, so the
original string is split just once, into two new strings.

```
print(
line.split(maxSplits: 1, whereSeparator: { $0 == " " })
.map(String.init))
// Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
```

The final example passes `true`

for the `allowEmptySlices`

parameter, so
the returned array contains empty strings where spaces were repeated.

```
print(
line.split(
omittingEmptySubsequences: false,
whereSeparator: { $0 == " " }
).map(String.init))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
```

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func split(maxSplits: Int = Int.max, omittingEmptySubsequences: Bool = true, whereSeparator isSeparator: (Self.Element) throws -> Bool) rethrows -> [ArraySlice<Self.Element>]`

Returns the longest possible subsequences of the collection, in order, that don't contain elements satisfying the given predicate.

The resulting array consists of at most `maxSplits + 1`

subsequences.
Elements that are used to split the sequence are not returned as part of
any subsequence.

The following examples show the effects of the `maxSplits`

and
`omittingEmptySubsequences`

parameters when splitting a string using a
closure that matches spaces. The first use of `split`

returns each word
that was originally separated by one or more spaces.

```
let line = "BLANCHE: I don't want realism. I want magic!"
print(line.split(whereSeparator: { $0 == " " }))
// Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
```

The second example passes `1`

for the `maxSplits`

parameter, so the
original string is split just once, into two new strings.

```
print(line.split(maxSplits: 1, whereSeparator: { $0 == " " }))
// Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
```

The final example passes `false`

for the `omittingEmptySubsequences`

parameter, so the returned array contains empty strings where spaces
were repeated.

```
print(line.split(omittingEmptySubsequences: false, whereSeparator: { $0 == " " }))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
```

Complexity: O(

n), wherenis the length of the collection.

#### Declaration

`@inlinable public func split(maxSplits: Int = Int.max, omittingEmptySubsequences: Bool = true, whereSeparator isSeparator: (Self.Element) throws -> Bool) rethrows -> [Self.SubSequence]`

Returns a Boolean value indicating whether the initial elements of the sequence are equivalent to the elements in another sequence, using the given predicate as the equivalence test.

The predicate must be a *equivalence relation* over the elements. That
is, for any elements `a`

, `b`

, and `c`

, the following conditions must
hold:

Complexity: O(

m), wheremis the lesser of the length of the sequence and the length of`possiblePrefix`

.

#### Declaration

`@inlinable public func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix, by areEquivalent: (Self.Element, PossiblePrefix.Element) throws -> Bool) rethrows -> Bool where PossiblePrefix: Sequence`

Removes the elements of the given sequence from the set.

In the following example, the elements of the `employees`

set that are
also elements of the `neighbors`

array are removed. In particular, the
names `"Bethany"`

and `"Eric"`

are removed from `employees`

.

```
var employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors = ["Bethany", "Eric", "Forlani", "Greta"]
employees.subtract(neighbors)
print(employees)
// Prints "["Chris", "Diana", "Alicia"]"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public mutating func subtract<S>(_ other: S) where Element == S.Element, S: Sequence`

Removes the elements of the given set from this set.

In the following example, the elements of the `employees`

set that are
also members of the `neighbors`

set are removed. In particular, the
names `"Bethany"`

and `"Eric"`

are removed from `employees`

.

```
var employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors: Set = ["Bethany", "Eric", "Forlani", "Greta"]
employees.subtract(neighbors)
print(employees)
// Prints "["Diana", "Chris", "Alicia"]"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public mutating func subtract(_ other: Set<Element>)`

Removes the elements of the given set from this set.

In the following example, the elements of the `employees`

set that are
also members of the `neighbors`

set are removed. In particular, the
names `"Bethany"`

and `"Eric"`

are removed from `employees`

.

```
var employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors: Set = ["Bethany", "Eric", "Forlani", "Greta"]
employees.subtract(neighbors)
print(employees)
// Prints "["Diana", "Chris", "Alicia"]"
```

- Parameter other: A set of the same type as the current set.

#### Declaration

`@inlinable public mutating func subtract(_ other: Self)`

Returns a new set containing the elements of this set that do not occur in the given sequence.

In the following example, the `nonNeighbors`

set is made up of the
elements of the `employees`

set that are not elements of `neighbors`

:

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors = ["Bethany", "Eric", "Forlani", "Greta"]
let nonNeighbors = employees.subtracting(neighbors)
print(nonNeighbors)
// Prints "["Chris", "Diana", "Alicia"]"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public func subtracting<S>(_ other: S) -> Set<Element> where Element == S.Element, S: Sequence`

Returns a new set containing the elements of this set that do not occur in the given set.

In the following example, the `nonNeighbors`

set is made up of the
elements of the `employees`

set that are not elements of `neighbors`

:

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors: Set = ["Bethany", "Eric", "Forlani", "Greta"]
let nonNeighbors = employees.subtracting(neighbors)
print(nonNeighbors)
// Prints "["Diana", "Chris", "Alicia"]"
```

- Parameter other: Another set.

#### Declaration

`@inlinable public func subtracting(_ other: Set<Element>) -> Set<Element>`

Returns a new set containing the elements of this set that do not occur in the given set.

In the following example, the `nonNeighbors`

set is made up of the
elements of the `employees`

set that are not elements of `neighbors`

:

```
let employees: Set = ["Alicia", "Bethany", "Chris", "Diana", "Eric"]
let neighbors: Set = ["Bethany", "Eric", "Forlani", "Greta"]
let nonNeighbors = employees.subtract(neighbors)
print(nonNeighbors)
// Prints "["Diana", "Chris", "Alicia"]"
```

- Parameter other: A set of the same type as the current set.

#### Declaration

`@inlinable public func subtracting(_ other: Self) -> Self`

Returns a subsequence, up to the given maximum length, containing the final elements of the sequence.

The sequence must be finite. If the maximum length exceeds the number of elements in the sequence, the result contains all the elements in the sequence.

```
let numbers = [1, 2, 3, 4, 5]
print(numbers.suffix(2))
// Prints "[4, 5]"
print(numbers.suffix(10))
// Prints "[1, 2, 3, 4, 5]"
```

- Parameter maxLength: The maximum number of elements to return. The
value of
`maxLength`

must be greater than or equal to zero.

Complexity: O(

n), wherenis the length of the sequence.

#### Declaration

`@inlinable public func suffix(_ maxLength: Int) -> [Self.Element]`

Returns a subsequence, up to the given maximum length, containing the final elements of the collection.

If the maximum length exceeds the number of elements in the collection, the result contains all the elements in the collection.

```
let numbers = [1, 2, 3, 4, 5]
print(numbers.suffix(2))
// Prints "[4, 5]"
print(numbers.suffix(10))
// Prints "[1, 2, 3, 4, 5]"
```

- Parameter maxLength: The maximum number of elements to return. The
value of
`maxLength`

must be greater than or equal to zero.

`RandomAccessCollection`

; otherwise, O(n), wherenis the length of the collection.

#### Declaration

`@inlinable public func suffix(_ maxLength: Int) -> Self.SubSequence`

Returns a subsequence from the specified position to the end of the collection.

The following example searches for the index of the number `40`

in an
array of integers, and then prints the suffix of the array starting at
that index:

```
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.firstIndex(of: 40) {
print(numbers.suffix(from: i))
}
// Prints "[40, 50, 60]"
```

Passing the collection's `endIndex`

as the `start`

parameter results in
an empty subsequence.

```
print(numbers.suffix(from: numbers.endIndex))
// Prints "[]"
```

Using the `suffix(from:)`

method is equivalent to using a partial range
from the index as the collection's subscript. The subscript notation is
preferred over `suffix(from:)`

.

```
if let i = numbers.firstIndex(of: 40) {
print(numbers[i...])
}
// Prints "[40, 50, 60]"
```

- Parameter start: The index at which to start the resulting subsequence.
`start`

must be a valid index of the collection.

Complexity: O(1)

#### Declaration

`@inlinable public func suffix(from start: Self.Index) -> Self.SubSequence`

Returns a new set with the elements that are either in this set or in the given sequence, but not in both.

In the following example, the `eitherNeighborsOrEmployees`

set is made up
of the elements of the `employees`

and `neighbors`

sets that are not in
both `employees`

*and* `neighbors`

. In particular, the names `"Bethany"`

and `"Eric"`

do not appear in `eitherNeighborsOrEmployees`

.

```
let employees: Set = ["Alicia", "Bethany", "Diana", "Eric"]
let neighbors = ["Bethany", "Eric", "Forlani"]
let eitherNeighborsOrEmployees = employees.symmetricDifference(neighbors)
print(eitherNeighborsOrEmployees)
// Prints "["Diana", "Forlani", "Alicia"]"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public func symmetricDifference<S>(_ other: S) -> Set<Element> where Element == S.Element, S: Sequence`

Returns a new set with the elements of both this set and the given sequence.

In the following example, the `attendeesAndVisitors`

set is made up
of the elements of the `attendees`

set and the `visitors`

array:

```
let attendees: Set = ["Alicia", "Bethany", "Diana"]
let visitors = ["Marcia", "Nathaniel"]
let attendeesAndVisitors = attendees.union(visitors)
print(attendeesAndVisitors)
// Prints "["Diana", "Nathaniel", "Bethany", "Alicia", "Marcia"]"
```

If the set already contains one or more elements that are also in
`other`

, the existing members are kept. If `other`

contains multiple
instances of equivalent elements, only the first instance is kept.

```
let initialIndices = Set(0..<5)
let expandedIndices = initialIndices.union([2, 3, 6, 6, 7, 7])
print(expandedIndices)
// Prints "[2, 4, 6, 7, 0, 1, 3]"
```

- Parameter other: A sequence of elements.
`other`

must be finite.

#### Declaration

`@inlinable public func union<S>(_ other: S) -> Set<Element> where Element == S.Element, S: Sequence`

Inserts the given element into the set unconditionally.

If an element equal to `newMember`

is already contained in the set,
`newMember`

replaces the existing element. In this example, an existing
element is inserted into `classDays`

, a set of days of the week.

```
enum DayOfTheWeek: Int {
case sunday, monday, tuesday, wednesday, thursday,
friday, saturday
}
var classDays: Set<DayOfTheWeek> = [.monday, .wednesday, .friday]
print(classDays.update(with: .monday))
// Prints "Optional(.monday)"
```

- Parameter newMember: An element to insert into the set.

#### Declaration

`@inlinable public mutating func update(with newMember: Element) -> Element?`

Call `body(p)`

, where `p`

is a pointer to the collection's
contiguous storage. If no such storage exists, it is
first created. If the collection does not support an internal
representation in a form of contiguous storage, `body`

is not
called and `nil`

is returned.

A `Collection`

that provides its own implementation of this method
must also guarantee that an equivalent buffer of its `SubSequence`

can be generated by advancing the pointer by the distance to the
slice's `startIndex`

.

#### Declaration

`@inlinable public func withContiguousStorageIfAvailable<R>(_ body: (UnsafeBufferPointer<Self.Element>) throws -> R) rethrows -> R?`

### Type Methods

#### Declaration

`public static func !=(lhs: Self, rhs: Self) -> Bool`

Returns a Boolean value indicating whether two sets have equal elements.

#### Declaration

`@inlinable public static func ==(lhs: Set<Element>, rhs: Set<Element>) -> Bool`

You use a set instead of an array when you need to test efficiently for membership and you aren't concerned with the order of the elements in the collection, or when you need to ensure that each element appears only once in a collection.

You can create a set with any element type that conforms to the

`Hashable`

protocol. By default, most types in the standard library are hashable, including strings, numeric and Boolean types, enumeration cases without associated values, and even sets themselves.Swift makes it as easy to create a new set as to create a new array. Simply assign an array literal to a variable or constant with the

`Set`

type specified.## Set Operations

Sets provide a suite of mathematical set operations. For example, you can efficiently test a set for membership of an element or check its intersection with another set:

You can also combine, exclude, or subtract the elements of two sets:

You can modify a set in place by using these methods' mutating counterparts:

`formUnion(_:)`

,`formIntersection(_:)`

,`formSymmetricDifference(_:)`

, and`subtract(_:)`

.Set operations are not limited to use with other sets. Instead, you can perform set operations with another set, an array, or any other sequence type.

## Sequence and Collection Operations

In addition to the

`Set`

type's set operations, you can use any nonmutating sequence or collection methods with a set.You can iterate through a set's unordered elements with a

`for`

-`in`

loop.Many sequence and collection operations return an array or a type-erasing collection wrapper instead of a set. To restore efficient set operations, create a new set from the result.

## Bridging Between Set and NSSet

You can bridge between

`Set`

and`NSSet`

using the`as`

operator. For bridging to be possible, the`Element`

type of a set must be a class, an`@objc`

protocol (a protocol imported from Objective-C or marked with the`@objc`

attribute), or a type that bridges to a Foundation type.Bridging from

`Set`

to`NSSet`

always takes O(1) time and space. When the set's`Element`

type is neither a class nor an`@objc`

protocol, any required bridging of elements occurs at the first access of each element, so the first operation that uses the contents of the set (for example, a membership test) can take O(n).Bridging from

`NSSet`

to`Set`

first calls the`copy(with:)`

method (`- copyWithZone:`

in Objective-C) on the set to get an immutable copy and then performs additional Swift bookkeeping work that takes O(1) time. For instances of`NSSet`

that are already immutable,`copy(with:)`

returns the same set in constant time; otherwise, the copying performance is unspecified. The instances of`NSSet`

and`Set`

share buffer using the same copy-on-write optimization that is used when two instances of`Set`

share buffer.