## Set

`struct Set<Element>`

An unordered collection of unique elements.

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.

``````let ingredients: Set = ["cocoa beans", "sugar", "cocoa butter", "salt"]
if ingredients.contains("sugar") {
print("No thanks, too sweet.")
}
// Prints "No thanks, too sweet."``````

# 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:

• Use the `contains(_:)` method to test whether a set contains a specific element.
• Use the "equal to" operator (`==`) to test whether two sets contain the same elements.
• Use the `isSubset(of:)` method to test whether a set contains all the elements of another set or sequence.
• Use the `isSuperset(of:)` method to test whether all elements of a set are contained in another set or sequence.
• Use the `isStrictSubset(of:)` and `isStrictSuperset(of:)` methods to test whether a set is a subset or superset of, but not equal to, another set.
• Use the `isDisjoint(with:)` method to test whether a set has any elements in common with another set.

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

• Use the `union(_:)` method to create a new set with the elements of a set and another set or sequence.
• Use the `intersection(_:)` method to create a new set with only the elements common to a set and another set or sequence.
• Use the `symmetricDifference(_:)` method to create a new set with the elements that are in either a set or another set or sequence, but not in both.
• Use the `subtracting(_:)` method to create a new set with the elements of a set that are not also in another set or sequence.

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.

``````var primes: Set = [2, 3, 5, 7]

// Tests whether primes is a subset of a Range<Int>
print(primes.isSubset(of: 0..<10))
// Prints "true"

// Performs an intersection with an Array<Int>
let favoriteNumbers = [5, 7, 15, 21]
print(primes.intersection(favoriteNumbers))
// Prints "[5, 7]"``````

# 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.

``````if primes.isEmpty {
print("No primes!")
} else {
print("We have \(primes.count) primes.")
}
// Prints "We have 4 primes."

let primesSum = primes.reduce(0, +)
// 'primesSum' == 17

let primeStrings = primes.sorted().map(String.init)
// 'primeStrings' == ["2", "3", "5", "7"]``````

You can iterate through a set's unordered elements with a `for`-`in` loop.

``````for number in primes {
print(number)
}
// Prints "5"
// Prints "7"
// Prints "2"
// Prints "3"``````

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.

``````let morePrimes = primes.union([11, 13, 17, 19])

let laterPrimes = morePrimes.filter { \$0 > 10 }
// 'laterPrimes' is of type Array<Int>

let laterPrimesSet = Set(morePrimes.filter { \$0 > 10 })
// 'laterPrimesSet' is of type Set<Int>``````

# 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.

Inheritance `Collection, CustomDebugStringConvertible, CustomReflectable, CustomStringConvertible, Decodable, Encodable, Equatable, ExpressibleByArrayLiteral, Hashable, Sequence, SetAlgebra` View Protocol Hierarchy → `Set.Index` `import Swift`

### Initializers

init()

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

`init()`
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]"``````

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

#### Declaration

`init<Source>(_ sequence: Source)`

#### Declared In

`Set` , `SetAlgebra`
init(arrayLiteral:)

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!"``````

`elements`: A variadic list of elements of the new set.

#### Declaration

`init(arrayLiteral elements: Set<Element>.Element...)`
init(from:)

Creates a new set by decoding from the given decoder.

This initializer throws an error if reading from the decoder fails, or if the data read is corrupted or otherwise invalid.

`decoder`: The decoder to read data from.

#### Declaration

`init(from decoder: Decoder)`
init(minimumCapacity:)

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.

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

#### Declaration

`init(minimumCapacity: Int)`

### Instance Variables

var capacity: Int

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

#### Declaration

`var capacity: Int { get }`
var count: Int

The number of elements in the set.

Complexity: O(1).

#### Declaration

`var count: Int { get }`

#### Declared In

`Set` , `Collection`
var customMirror: Mirror

A mirror that reflects the set.

#### Declaration

`var customMirror: Mirror { get }`
var debugDescription: String

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

#### Declaration

`var debugDescription: String { get }`
var description: String

A string that represents the contents of the set.

#### Declaration

`var description: String { get }`
var endIndex: Set<Element>.Index

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 { get }`
var first: Element?

The first element of the set.

The first element of the set is not necessarily the first element added to the set. Don't expect any particular ordering of set elements.

If the set is empty, the value of this property is `nil`.

#### Declaration

`var first: Element? { get }`
var hashValue: Int

The hash value for the set.

Two sets that are equal will always have equal hash values.

Hash values are not guaranteed to be equal across different executions of your program. Do not save hash values to use during a future execution.

#### Declaration

`var hashValue: Int { get }`
var isEmpty: Bool

A Boolean value that indicates whether the set is empty.

#### Declaration

`var isEmpty: Bool { get }`

#### Declared In

`Set` , `Collection` , `SetAlgebra`
var lazy: LazyCollection<Set<Element>>

A view onto this collection that provides lazy implementations of normally eager operations, such as `map` and `filter`.

Use the `lazy` property when chaining operations to prevent intermediate operations from allocating storage, or when you only need a part of the final collection to avoid unnecessary computation.

#### Declaration

`var lazy: LazyCollection<Set<Element>> { get }`

#### Declared In

`Collection`
var startIndex: Set<Element>.Index

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 { get }`
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), where n is the length of the collection.

#### Declaration

`var underestimatedCount: Int { get }`

#### Declared In

`Collection` , `Sequence`

### Subscripts

subscript(_: (UnboundedRange_)

#### Declaration

`subscript(x: (UnboundedRange_) -> ()) -> Set<Element>.SubSequence { get }`

#### Declared In

`Collection`
subscript(_: Set<Element>.Index)

Accesses the member at the given position.

#### Declaration

`subscript(position: Set<Element>.Index) -> Element { get }`
subscript<R>(_: R)

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``````

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

Complexity: O(1)

#### Declaration

`subscript<R>(r: R) -> Set<Element>.SubSequence where R : RangeExpression, Set<Element>.Index == R.Bound { get }`

#### Declared In

`Collection`

### Instance Methods

func allSatisfy(_:)

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

`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. Returns: `true` if the sequence contains only elements that satisfy `predicate`; otherwise, `false`.

#### Declaration

`func allSatisfy(_ predicate: (Set<Element>.Element) throws -> Bool) rethrows -> Bool`

#### Declared In

`Collection`, `Sequence`
func compactMap(_:)

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 nonoptional 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]``````

`transform`: A closure that accepts an element of this sequence as its argument and returns an optional value. Returns: An array of the non-`nil` results of calling `transform` with each element of the sequence.

Complexity: O(m + n), where m is the length of this sequence and n is the length of the result.

#### Declaration

`func compactMap<ElementOfResult>(_ transform: (Set<Element>.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]`

#### Declared In

`Collection`, `Sequence`
func contains(_:)

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!"``````

`member`: An element to look for in the set. Returns: `true` if `member` exists in the set; otherwise, `false`.

Complexity: O(1)

#### Declaration

`func contains(_ member: Set<Element>.Element) -> Bool`
func contains(where:)

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
}
}

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``````

`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. Returns: `true` if the sequence contains an element that satisfies `predicate`; otherwise, `false`.

#### Declaration

`func contains(where predicate: (Set<Element>.Element) throws -> Bool) rethrows -> Bool`

#### Declared In

`Collection`, `Sequence`
func distance(from: Set<Element>.Index, to: Set<Element>.Index)

Returns the distance between two indices.

Unless the collection conforms to the `BidirectionalCollection` protocol, `start` must be less than or equal to `end`.

Parameters: start: A valid index of the collection. end: Another valid index of the collection. If `end` is equal to `start`, the result is zero. Returns: The distance between `start` and `end`. The result can be negative only if the collection conforms to the `BidirectionalCollection` protocol.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the resulting distance.

#### Declaration

`func distance(from start: Set<Element>.Index, to end: Set<Element>.Index) -> Int`

#### Declared In

`Collection`
func distance<T>(from: Set<Element>.Index, to: Set<Element>.Index)

Deprecated: all index distances are now of type Int.

#### Declaration

`func distance<T>(from start: Set<Element>.Index, to end: Set<Element>.Index) -> T where T : BinaryInteger`

#### Declared In

`Collection`
func drop(while:)

Returns a subsequence by skipping elements while `predicate` returns `true` and returning the remaining elements.

`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), where n is the length of the collection.

#### Declaration

`func drop(while predicate: (Set<Element>.Element) throws -> Bool) rethrows -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func dropFirst()

Returns a subsequence containing all but the first element of the sequence.

The following example drops the first element from an array of integers.

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

If the sequence has no elements, the result is an empty subsequence.

``````let empty: [Int] = []
print(empty.dropFirst())
// Prints "[]"``````

Returns: A subsequence starting after the first element of the sequence.

Complexity: O(1)

#### Declaration

`func dropFirst() -> Set<Element>.SubSequence`

#### Declared In

`Collection`, `Sequence`
func dropFirst(_:)

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 "[]"``````

`n`: The number of elements to drop from the beginning of the collection. `n` must be greater than or equal to zero. Returns: A subsequence starting after the specified number of elements.

Complexity: O(n), where n is the number of elements to drop from the beginning of the collection.

#### Declaration

`func dropFirst(_ n: Int) -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func dropLast()

Returns a subsequence containing all but the last element of the sequence.

The sequence must be finite.

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

If the sequence has no elements, the result is an empty subsequence.

``````let empty: [Int] = []
print(empty.dropLast())
// Prints "[]"``````

Returns: A subsequence leaving off the last element of the sequence.

Complexity: O(n), where n is the length of the sequence.

#### Declaration

`func dropLast() -> Set<Element>.SubSequence`

#### Declared In

`Collection`, `Sequence`
func dropLast(_:)

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 "[]"``````

`n`: The number of elements to drop off the end of the collection. `n` must be greater than or equal to zero. Returns: A subsequence that leaves off the specified number of elements at the end.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`func dropLast(_ n: Int) -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func elementsEqual(_:by:)

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:

• `areEquivalent(a, a)` is always `true`. (Reflexivity)
• `areEquivalent(a, b)` implies `areEquivalent(b, a)`. (Symmetry)
• If `areEquivalent(a, b)` and `areEquivalent(b, c)` are both `true`, then `areEquivalent(a, c)` is also `true`. (Transitivity)

Parameters: other: A sequence to compare to this sequence. areEquivalent: A predicate that returns `true` if its two arguments are equivalent; otherwise, `false`. Returns: `true` if this sequence and `other` contain equivalent items, using `areEquivalent` as the equivalence test; otherwise, `false.`

#### Declaration

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

#### Declared In

`Collection`, `Sequence`
func encode(to:)

Encodes the elements of this set into the given encoder in an unkeyed container.

This function throws an error if any values are invalid for the given encoder's format.

`encoder`: The encoder to write data to.

#### Declaration

`func encode(to encoder: Encoder) throws`
func enumerated()

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: [SetIndex<String>] = []
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"``````

Returns: A sequence of pairs enumerating the sequence.

#### Declaration

`func enumerated() -> EnumeratedSequence<Set<Element>>`

#### Declared In

`Collection`, `Sequence`
func filter(_:)

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``````

`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. Returns: A set of the elements that `isIncluded` allows.

#### Declaration

`func filter(_ isIncluded: (Set<Element>.Element) throws -> Bool) rethrows -> Set<Element>`

#### Declared In

`Set`, `Collection`, `Sequence`
func first(where:)

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."``````

`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. Returns: The first element of the sequence that satisfies `predicate`, or `nil` if there is no element that satisfies `predicate`.

#### Declaration

`func first(where predicate: (Set<Element>.Element) throws -> Bool) rethrows -> Set<Element>.Element?`

#### Declared In

`Collection`, `Sequence`
func firstIndex(of:)

Returns the index of the given element in the set, or `nil` if the element is not a member of the set.

`member`: An element to search for in the set. Returns: The index of `member` if it exists in the set; otherwise, `nil`.

Complexity: O(1)

#### Declaration

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

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'!"``````

`predicate`: A closure that takes an element as its argument and returns a Boolean value that indicates whether the passed element represents a match. Returns: The index of the first element for which `predicate` returns `true`. If no elements in the collection satisfy the given predicate, returns `nil`.

#### Declaration

`func firstIndex(where predicate: (Set<Element>.Element) throws -> Bool) rethrows -> Set<Element>.Index?`

#### Declared In

`Collection`
func flatMap(_:)

#### Declaration

`func flatMap(_ transform: (Set<Element>.Element) throws -> String?) rethrows -> [String]`

#### Declared In

`Collection`
func flatMap<ElementOfResult>(_: (Set<Element>.Element) throws -> ElementOfResult?)

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 nonoptional values when your transformation produces an optional value.

In this example, note the difference in the result of using `map` and `flatMap` 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 flatMapped: [Int] = possibleNumbers.flatMap { str in Int(str) }
// [1, 2, 5]``````

`transform`: A closure that accepts an element of this sequence as its argument and returns an optional value. Returns: An array of the non-`nil` results of calling `transform` with each element of the sequence.

Complexity: O(m + n), where m is the length of this sequence and n is the length of the result.

#### Declaration

`func flatMap<ElementOfResult>(_ transform: (Set<Element>.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]`

#### Declared In

`Collection`, `Sequence`
func flatMap<SegmentOfResult>(_: (Set<Element>.Element) throws -> SegmentOfResult)

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())`.

`transform`: A closure that accepts an element of this sequence as its argument and returns a sequence or collection. Returns: The resulting flattened array.

Complexity: O(m + n), where m is the length of this sequence and n is the length of the result.

#### Declaration

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

#### Declared In

`Collection`, `Sequence`
func forEach(_:)

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:

1. You cannot use a `break` or `continue` statement to exit the current call of the `body` closure or skip subsequent calls.
2. 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.

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

#### Declaration

`func forEach(_ body: (Set<Element>.Element) throws -> Void) rethrows`

#### Declared In

`Collection`, `Sequence`
func formIndex(_: inout Set<Element>.Index, offsetBy: Int)

Offsets the given index by the specified distance.

The value passed as `n` must not offset `i` beyond the bounds of the collection.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func formIndex(_ i: inout Set<Element>.Index, offsetBy n: Int)`

#### Declared In

`Collection`
func formIndex<T>(_: inout Set<Element>.Index, offsetBy: T)

Deprecated: all index distances are now of type Int.

#### Declaration

`func formIndex<T>(_ i: inout Set<Element>.Index, offsetBy n: T)`

#### Declared In

`Collection`
func formIndex(_: inout Set<Element>.Index, offsetBy: Int, limitedBy: Set<Element>.Index)

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

The value passed as `n` must not offset `i` beyond the bounds of the collection, unless the index passed as `limit` prevents offsetting beyond those bounds.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. limit: A valid index of the collection to use as a limit. If `n > 0`, a limit that is less than `i` has no effect. Likewise, if `n < 0`, a limit that is greater than `i` has no effect. Returns: `true` if `i` has been offset by exactly `n` steps without going beyond `limit`; otherwise, `false`. When the return value is `false`, the value of `i` is equal to `limit`.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func formIndex(_ i: inout Set<Element>.Index, offsetBy n: Int, limitedBy limit: Set<Element>.Index) -> Bool`

#### Declared In

`Collection`
func formIndex<T>(_: inout Set<Element>.Index, offsetBy: T, limitedBy: Set<Element>.Index)

Deprecated: all index distances are now of type Int.

#### Declaration

`func formIndex<T>(_ i: inout Set<Element>.Index, offsetBy n: T, limitedBy limit: Set<Element>.Index) -> Bool where T : BinaryInteger`

#### Declared In

`Collection`
func formIndex(after:)

Replaces the given index with its successor.

`i`: A valid index of the collection. `i` must be less than `endIndex`.

#### Declaration

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

#### Declared In

`Collection`
mutating func formIntersection(_:)

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"]"``````

`other`: A sequence of elements. `other` must be finite.

#### Declaration

`mutating func formIntersection<S>(_ other: S)`
mutating func formSymmetricDifference(_: Set<Set<Element>.Element>)

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"]"``````

`other`: Another set.

#### Declaration

`mutating func formSymmetricDifference(_ other: Set<Set<Element>.Element>)`
mutating func formSymmetricDifference<S>(_: S)

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"]"``````

`other`: A sequence of elements. `other` must be finite.

#### Declaration

`mutating func formSymmetricDifference<S>(_ other: S)`
mutating func formUnion(_:)

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"]"``````

`other`: A sequence of elements. `other` must be finite.

#### Declaration

`mutating func formUnion<S>(_ other: S)`
func hash(into:)

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

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

#### Declaration

`func hash(into hasher: inout Hasher)`
func index(_: Set<Element>.Index, offsetBy: Int)

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 `n` must not offset `i` beyond the bounds of the collection.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. Returns: An index offset by `n` from the index `i`. If `n` is positive, this is the same value as the result of `n` calls to `index(after:)`. If `n` is negative, this is the same value as the result of `-n` calls to `index(before:)`.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func index(_ i: Set<Element>.Index, offsetBy n: Int) -> Set<Element>.Index`

#### Declared In

`Collection`
func index<T>(_: Set<Element>.Index, offsetBy: T)

Deprecated: all index distances are now of type Int.

#### Declaration

`func index<T>(_ i: Set<Element>.Index, offsetBy n: T) -> Set<Element>.Index where T : BinaryInteger`

#### Declared In

`Collection`
func index(_: Set<Element>.Index, offsetBy: Int, limitedBy: Set<Element>.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 `n` must not offset `i` beyond the bounds of the collection, unless the index passed as `limit` prevents offsetting beyond those bounds.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. limit: A valid index of the collection to use as a limit. If `n > 0`, a limit that is less than `i` has no effect. Likewise, if `n < 0`, a limit that is greater than `i` has no effect. Returns: An index offset by `n` from the index `i`, unless that index would be beyond `limit` in the direction of movement. In that case, the method returns `nil`.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func index(_ i: Set<Element>.Index, offsetBy n: Int, limitedBy limit: Set<Element>.Index) -> Set<Element>.Index?`

#### Declared In

`Collection`
func index<T>(_: Set<Element>.Index, offsetBy: T, limitedBy: Set<Element>.Index)

Deprecated: all index distances are now of type Int.

#### Declaration

`func index<T>(_ i: Set<Element>.Index, offsetBy n: T, limitedBy limit: Set<Element>.Index) -> Set<Element>.Index? where T : BinaryInteger`

#### Declared In

`Collection`
func index(after:)

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.

`i`: A valid index of the collection. `i` must be less than `endIndex`. Returns: The index value immediately after `i`.

#### Declaration

`func index(after i: Set<Element>.Index) -> Set<Element>.Index`
mutating func insert(_: Set<Element>.Element)

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]"``````

`newMember`: An element to insert into the set. Returns: `(true, newMember)` if `newMember` was not contained in the set. If an element equal to `newMember` was already contained in the set, the method returns `(false, oldMember)`, where `oldMember` is the element that was equal to `newMember`. In some cases, `oldMember` may be distinguishable from `newMember` by identity comparison or some other means.

#### Declaration

`mutating func insert(_ newMember: Set<Element>.Element) -> (inserted: Bool, memberAfterInsert: Set<Element>.Element)`
mutating func insert<ConcreteElement>(_: ConcreteElement)

#### Declaration

`mutating func insert<ConcreteElement>(_ newMember: ConcreteElement) -> (inserted: Bool, memberAfterInsert: ConcreteElement) where ConcreteElement : Hashable`
func intersection(_: Set<Set<Element>.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: Set = ["Bethany", "Eric", "Forlani", "Greta"]
let bothNeighborsAndEmployees = employees.intersection(neighbors)
print(bothNeighborsAndEmployees)
// Prints "["Bethany", "Eric"]"``````

`other`: Another set. Returns: A new set.

#### Declaration

`func intersection(_ other: Set<Set<Element>.Element>) -> Set<Set<Element>.Element>`
func intersection<S>(_: S)

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"]"``````

`other`: A sequence of elements. `other` must be finite. Returns: A new set.

#### Declaration

`func intersection<S>(_ other: S) -> Set<Set<Element>.Element> where Element == S.Element, S : Sequence`
func isDisjoint(with: Set<Set<Element>.Element>)

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"``````

`other`: Another set. Returns: `true` if the set has no elements in common with `other`; otherwise, `false`.

#### Declaration

`func isDisjoint(with other: Set<Set<Element>.Element>) -> Bool`
func isDisjoint(with:)

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"``````

`other`: A set of the same type as the current set. Returns: `true` if the set has no elements in common with `other`; otherwise, `false`.

#### Declaration

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

#### Declared In

`SetAlgebra`
func isDisjoint<S>(with: S)

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"``````

`other`: A sequence of elements. `other` must be finite. Returns: `true` if the set has no elements in common with `other`; otherwise, `false`.

#### Declaration

`func isDisjoint<S>(with other: S) -> Bool where Element == S.Element, S : Sequence`
func isStrictSubset(of: Set<Set<Element>.Element>)

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"``````

`other`: Another set. Returns: `true` if the set is a strict subset of `other`; otherwise, `false`.

#### Declaration

`func isStrictSubset(of other: Set<Set<Element>.Element>) -> Bool`
func isStrictSubset(of:)

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"``````

`other`: A set of the same type as the current set. Returns: `true` if the set is a strict subset of `other`; otherwise, `false`.

#### Declaration

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

#### Declared In

`SetAlgebra`
func isStrictSubset<S>(of: S)

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"``````

`possibleStrictSuperset`: A sequence of elements. `possibleStrictSuperset` must be finite. Returns: `true` is the set is strict subset of `possibleStrictSuperset`; otherwise, `false`.

#### Declaration

`func isStrictSubset<S>(of possibleStrictSuperset: S) -> Bool where Element == S.Element, S : Sequence`
func isStrictSuperset(of: Set<Set<Element>.Element>)

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"``````

`other`: Another set. Returns: `true` if the set is a strict superset of `other`; otherwise, `false`.

#### Declaration

`func isStrictSuperset(of other: Set<Set<Element>.Element>) -> Bool`
func isStrictSuperset(of:)

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"``````

`other`: A set of the same type as the current set. Returns: `true` if the set is a strict superset of `other`; otherwise, `false`.

#### Declaration

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

#### Declared In

`SetAlgebra`
func isStrictSuperset<S>(of: S)

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"``````

`possibleStrictSubset`: A sequence of elements. `possibleStrictSubset` must be finite. Returns: `true` if the set is a strict superset of `possibleStrictSubset`; otherwise, `false`.

#### Declaration

`func isStrictSuperset<S>(of possibleStrictSubset: S) -> Bool where Element == S.Element, S : Sequence`
func isSubset(of: Set<Set<Element>.Element>)

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"``````

`other`: Another set. Returns: `true` if the set is a subset of `other`; otherwise, `false`.

#### Declaration

`func isSubset(of other: Set<Set<Element>.Element>) -> Bool`
func isSubset(of:)

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"``````

`other`: A set of the same type as the current set. Returns: `true` if the set is a subset of `other`; otherwise, `false`.

#### Declaration

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

#### Declared In

`SetAlgebra`
func isSubset<S>(of: S)

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"``````

`possibleSuperset`: A sequence of elements. `possibleSuperset` must be finite. Returns: `true` if the set is a subset of `possibleSuperset`; otherwise, `false`.

#### Declaration

`func isSubset<S>(of possibleSuperset: S) -> Bool where Element == S.Element, S : Sequence`
func isSuperset(of: Set<Set<Element>.Element>)

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"``````

`other`: Another set. Returns: `true` if the set is a superset of `other`; otherwise, `false`.

#### Declaration

`func isSuperset(of other: Set<Set<Element>.Element>) -> Bool`
func isSuperset(of:)

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"``````

`other`: A set of the same type as the current set. Returns: `true` if the set is a superset of `other`; otherwise, `false`.

#### Declaration

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

#### Declared In

`SetAlgebra`
func isSuperset<S>(of: S)

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"``````

`possibleSubset`: A sequence of elements. `possibleSubset` must be finite. Returns: `true` if the set is a superset of `possibleSubset`; otherwise, `false`.

#### Declaration

`func isSuperset<S>(of possibleSubset: S) -> Bool where Element == S.Element, S : Sequence`
func lexicographicallyPrecedes(_:by:)

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:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

Parameters: other: A sequence to compare to this sequence. areInIncreasingOrder: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. Returns: `true` if this sequence precedes `other` in a dictionary ordering as ordered by `areInIncreasingOrder`; otherwise, `false`.

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.

#### Declaration

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

#### Declared In

`Collection`, `Sequence`
func makeIterator()

Returns an iterator over the members of the set.

#### Declaration

`func makeIterator() -> SetIterator<Element>`
func map(_:)

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]``````

`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. Returns: An array containing the transformed elements of this sequence.

#### Declaration

`func map<T>(_ transform: (Set<Element>.Element) throws -> T) rethrows -> [T]`

#### Declared In

`Collection`, `Sequence`
@warn_unqualified_access func max(by:)

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:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

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))"``````

`areInIncreasingOrder`: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. Returns: The sequence's maximum element if the sequence is not empty; otherwise, `nil`.

#### Declaration

```@warn_unqualified_access func max(by areInIncreasingOrder: (Set<Element>.Element, Set<Element>.Element) throws -> Bool) rethrows -> Set<Element>.Element?```

#### Declared In

`Collection`, `Sequence`
func min(by:)

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:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

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))"``````

`areInIncreasingOrder`: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. Returns: The sequence's minimum element, according to `areInIncreasingOrder`. If the sequence has no elements, returns `nil`.

#### Declaration

`func min(by areInIncreasingOrder: (Set<Element>.Element, Set<Element>.Element) throws -> Bool) rethrows -> Set<Element>.Element?`

#### Declared In

`Collection`, `Sequence`
mutating func popFirst()

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.

Returns: A member of the set. If the set is empty, returns `nil`.

#### Declaration

`mutating func popFirst() -> Set<Element>.Element?`
func prefix(_:)

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]"``````

`maxLength`: The maximum number of elements to return. `maxLength` must be greater than or equal to zero. Returns: A subsequence starting at the beginning of this collection with at most `maxLength` elements.

#### Declaration

`func prefix(_ maxLength: Int) -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func prefix(through:)

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]"``````

`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. Returns: A subsequence up to, and including, the `end` position.

Complexity: O(1)

#### Declaration

`func prefix(through position: Set<Element>.Index) -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func prefix(upTo:)

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]"``````

`end`: The "past the end" index of the resulting subsequence. `end` must be a valid index of the collection. Returns: A subsequence up to, but not including, the `end` position.

Complexity: O(1)

#### Declaration

`func prefix(upTo end: Set<Element>.Index) -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func prefix(while:)

Returns a subsequence containing the initial elements until `predicate` returns `false` and skipping the remaining elements.

`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), where n is the length of the collection.

#### Declaration

`func prefix(while predicate: (Set<Element>.Element) throws -> Bool) rethrows -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func randomElement()

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 uses the default random generator, `Random.default`. The call to `names.randomElement()` above is equivalent to calling `names.randomElement(using: &Random.default)`.

Returns: A random element from the collection. If the collection is empty, the method returns `nil`.

#### Declaration

`func randomElement() -> Set<Element>.Element?`

#### Declared In

`Collection`
func randomElement(using:)

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"``````

`generator`: The random number generator to use when choosing a random element. Returns: A random element from the collection. If the collection is empty, the method returns `nil`.

#### Declaration

`func randomElement<T>(using generator: inout T) -> Set<Element>.Element? where T : RandomNumberGenerator`

#### Declared In

`Collection`
func reduce(_:_:)

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:

1. The `nextPartialResult` closure is called with `initialResult`---`0` in this case---and the first element of `numbers`, returning the sum: `1`.
2. The closure is called again repeatedly with the previous call's return value and each element of the sequence.
3. 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(_:_:)`.

Parameters: initialResult: The value to use as the initial accumulating value. `initialResult` is passed to `nextPartialResult` the first time the closure is executed. nextPartialResult: A closure that combines an accumulating value and an element of the sequence into a new accumulating value, to be used in the next call of the `nextPartialResult` closure or returned to the caller. Returns: The final accumulated value. If the sequence has no elements, the result is `initialResult`.

#### Declaration

`func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Set<Element>.Element) throws -> Result) rethrows -> Result`

#### Declared In

`Collection`, `Sequence`
func reduce(into:_:)

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:

1. 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"`.
2. The closure is called again repeatedly with the updated accumulating value and each element of the sequence.
3. 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:_:)`.

Parameters: initialResult: The value to use as the initial accumulating value. updateAccumulatingResult: A closure that updates the accumulating value with an element of the sequence. Returns: The final accumulated value. If the sequence has no elements, the result is `initialResult`.

#### Declaration

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

#### Declared In

`Collection`, `Sequence`
mutating func remove(_: Set<Element>.Element)

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."``````

`member`: The element to remove from the set. Returns: The value of the `member` parameter if it was a member of the set; otherwise, `nil`.

#### Declaration

`mutating func remove(_ member: Set<Element>.Element) -> Set<Element>.Element?`
mutating func remove<ConcreteElement>(_: ConcreteElement)

#### Declaration

`mutating func remove<ConcreteElement>(_ member: ConcreteElement) -> ConcreteElement? where ConcreteElement : Hashable`
mutating func remove(at:)

Removes the element at the given index of the set.

`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. Returns: The element that was removed from the set.

#### Declaration

`mutating func remove(at position: Set<Element>.Index) -> Set<Element>.Element`
mutating func removeAll(keepingCapacity:)

Removes all members from the set.

`keepingCapacity`: If `true`, the set's buffer capacity is preserved; if `false`, the underlying buffer is released. The default is `false`.

#### Declaration

`mutating func removeAll(keepingCapacity keepCapacity: Bool = default)`
mutating func removeFirst()

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.

Returns: A member of the set.

#### Declaration

`mutating func removeFirst() -> Set<Element>.Element`
mutating func reserveCapacity(_:)

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.

`minimumCapacity`: The requested number of elements to store.

#### Declaration

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

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

The sequence must be finite.

Complexity: O(n), where n is the length of the sequence.

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

#### Declaration

`func reversed() -> [Set<Element>.Element]`

#### Declared In

`Collection`, `Sequence`
func shuffled()

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 uses the default random generator, `Random.default`. The call to `numbers.shuffled()` above is equivalent to calling `numbers.shuffled(using: &Random.default)`.

Returns: A shuffled array of this sequence's elements.

Complexity: O(n)

#### Declaration

`func shuffled() -> [Set<Element>.Element]`

#### Declared In

`Collection`, `Sequence`
func shuffled(using:)

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]``````

`generator`: The random number generator to use when shuffling the sequence. Returns: An array of this sequence's elements in a shuffled order.

Complexity: O(n)

#### Declaration

`func shuffled<T>(using generator: inout T) -> [Set<Element>.Element] where T : RandomNumberGenerator`

#### Declared In

`Collection`, `Sequence`
func sorted(by:)

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

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:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

The sorting algorithm is not stable. A nonstable sort may change the relative order of elements for which `areInIncreasingOrder` does not establish an order.

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"]"``````

`areInIncreasingOrder`: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. Returns: A sorted array of the sequence's elements.

#### Declaration

`func sorted(by areInIncreasingOrder: (Set<Element>.Element, Set<Element>.Element) throws -> Bool) rethrows -> [Set<Element>.Element]`

#### Declared In

`Collection`, `Sequence`
func split(_:omittingEmptySubsequences:whereSeparator:)

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!"]"``````

Parameters: maxSplits: The maximum number of times to split the collection, or one less than the number of subsequences to return. If `maxSplits + 1` subsequences are returned, the last one is a suffix of the original collection containing the remaining elements. `maxSplits` must be greater than or equal to zero. The default value is `Int.max`. omittingEmptySubsequences: If `false`, an empty subsequence is returned in the result for each pair of consecutive elements satisfying the `isSeparator` predicate and for each element at the start or end of the collection satisfying the `isSeparator` predicate. The default value is `true`. isSeparator: A closure that takes an element as an argument and returns a Boolean value indicating whether the collection should be split at that element. Returns: An array of subsequences, split from this collection's elements.

#### Declaration

`func split(maxSplits: Int = default, omittingEmptySubsequences: Bool = default, whereSeparator isSeparator: (Set<Element>.Element) throws -> Bool) rethrows -> [Set<Element>.SubSequence]`

#### Declared In

`Collection`
func starts(with:by:)

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:

• `areEquivalent(a, a)` is always `true`. (Reflexivity)
• `areEquivalent(a, b)` implies `areEquivalent(b, a)`. (Symmetry)
• If `areEquivalent(a, b)` and `areEquivalent(b, c)` are both `true`, then `areEquivalent(a, c)` is also `true`. (Transitivity)

Parameters: possiblePrefix: A sequence to compare to this sequence. areEquivalent: A predicate that returns `true` if its two arguments are equivalent; otherwise, `false`. Returns: `true` if the initial elements of the sequence are equivalent to the elements of `possiblePrefix`; otherwise, `false`. If `possiblePrefix` has no elements, the return value is `true`.

#### Declaration

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

#### Declared In

`Collection`, `Sequence`
mutating func subtract(_: Set<Set<Element>.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"]"``````

`other`: Another set.

#### Declaration

`mutating func subtract(_ other: Set<Set<Element>.Element>)`
mutating func subtract(_:)

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"]"``````

`other`: A set of the same type as the current set.

#### Declaration

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

#### Declared In

`SetAlgebra`
mutating func subtract<S>(_: S)

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"]"``````

`other`: A sequence of elements. `other` must be finite.

#### Declaration

`mutating func subtract<S>(_ other: S)`
func subtracting(_: Set<Set<Element>.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.subtracting(neighbors)
print(nonNeighbors)
// Prints "["Diana", "Chris", "Alicia"]"``````

`other`: Another set. Returns: A new set.

#### Declaration

`func subtracting(_ other: Set<Set<Element>.Element>) -> Set<Set<Element>.Element>`
func subtracting(_:)

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"]"``````

`other`: A set of the same type as the current set. Returns: A new set.

#### Declaration

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

#### Declared In

`SetAlgebra`
func subtracting<S>(_: S)

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"]"``````

`other`: A sequence of elements. `other` must be finite. Returns: A new set.

#### Declaration

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

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]"``````

`maxLength`: The maximum number of elements to return. The value of `maxLength` must be greater than or equal to zero. Returns: A subsequence terminating at the end of the collection with at most `maxLength` elements.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`func suffix(_ maxLength: Int) -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func suffix(from:)

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]"``````

`start`: The index at which to start the resulting subsequence. `start` must be a valid index of the collection. Returns: A subsequence starting at the `start` position.

Complexity: O(1)

#### Declaration

`func suffix(from start: Set<Element>.Index) -> Set<Element>.SubSequence`

#### Declared In

`Collection`
func symmetricDifference(_:)

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"]"``````

`other`: A sequence of elements. `other` must be finite. Returns: A new set.

#### Declaration

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

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]"``````

`other`: A sequence of elements. `other` must be finite. Returns: A new set with the unique elements of this set and `other`.

#### Declaration

`func union<S>(_ other: S) -> Set<Set<Element>.Element> where Element == S.Element, S : Sequence`
mutating func update(with: Set<Element>.Element)

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)"``````

`newMember`: An element to insert into the set. Returns: An element equal to `newMember` if the set already contained such a member; otherwise, `nil`. In some cases, the returned element may be distinguishable from `newMember` by identity comparison or some other means.

#### Declaration

`mutating func update(with newMember: Set<Element>.Element) -> Set<Element>.Element?`
mutating func update<ConcreteElement>(with: ConcreteElement)

#### Declaration

`mutating func update<ConcreteElement>(with newMember: ConcreteElement) -> ConcreteElement? where ConcreteElement : Hashable`

### Conditionally Inherited Items

The initializers, methods, and properties listed below may be available on this type under certain conditions (such as methods that are available on `Array` when its elements are `Equatable`) or may not ever be available if that determination is beyond SwiftDoc.org's capabilities. Please open an issue on GitHub if you see something out of place!

#### Where ArrayLiteralElement == Element

init(arrayLiteral:)

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!"``````

`arrayLiteral`: A list of elements of the new set.

#### Declaration

`init(arrayLiteral: Set<Element>.Element...)`

#### Declared In

`SetAlgebra`

#### Where Element : Collection

func joined()

Returns the elements of this collection of collections, concatenated.

In this example, an array of three ranges is flattened so that the elements of each range can be iterated in turn.

``````let ranges = [0..<3, 8..<10, 15..<17]

// A for-in loop over 'ranges' accesses each range:
for range in ranges {
print(range)
}
// Prints "0..<3"
// Prints "8..<10"
// Prints "15..<17"

// Use 'joined()' to access each element of each range:
for index in ranges.joined() {
print(index, terminator: " ")
}
// Prints: "0 1 2 8 9 15 16"``````

Returns: A flattened view of the elements of this collection of collections.

#### Declaration

`func joined() -> FlattenCollection<Set<Element>>`

#### Declared In

`Collection`

#### Where Element : Comparable

func lexicographicallyPrecedes(_:)

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the less-than operator (`<`) to compare elements.

This example uses the `lexicographicallyPrecedes` method to test which array of integers comes first in a lexicographical ordering.

``````let a = [1, 2, 2, 2]
let b = [1, 2, 3, 4]

print(a.lexicographicallyPrecedes(b))
// Prints "true"
print(b.lexicographicallyPrecedes(b))
// Prints "false"``````

`other`: A sequence to compare to this sequence. Returns: `true` if this sequence precedes `other` in a dictionary ordering; otherwise, `false`.

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.

#### Declaration

`func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence) -> Bool where OtherSequence : Sequence, Set<Element>.Element == OtherSequence.Element`

#### Declared In

`Collection`, `Sequence`
@warn_unqualified_access func max()

Returns the maximum element in the sequence.

This example finds the largest value in an array of height measurements.

``````let heights = [67.5, 65.7, 64.3, 61.1, 58.5, 60.3, 64.9]
let greatestHeight = heights.max()
print(greatestHeight)
// Prints "Optional(67.5)"``````

Returns: The sequence's maximum element. If the sequence has no elements, returns `nil`.

#### Declaration

```@warn_unqualified_access func max() -> Set<Element>.Element?```

#### Declared In

`Collection`, `Sequence`
@warn_unqualified_access func min()

Returns the minimum element in the sequence.

This example finds the smallest value in an array of height measurements.

``````let heights = [67.5, 65.7, 64.3, 61.1, 58.5, 60.3, 64.9]
let lowestHeight = heights.min()
print(lowestHeight)
// Prints "Optional(58.5)"``````

Returns: The sequence's minimum element. If the sequence has no elements, returns `nil`.

#### Declaration

```@warn_unqualified_access func min() -> Set<Element>.Element?```

#### Declared In

`Collection`, `Sequence`
func sorted()

Returns the elements of the sequence, sorted.

You can sort any sequence of elements that conform to the `Comparable` protocol by calling this method. Elements are sorted in ascending order.

The sorting algorithm is not stable. A nonstable sort may change the relative order of elements that compare equal.

Here's an example of sorting a list of students' names. Strings in Swift conform to the `Comparable` protocol, so the names are sorted in ascending order according to the less-than operator (`<`).

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

To sort the elements of your sequence in descending order, pass the greater-than operator (`>`) to the `sorted(by:)` method.

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

Returns: A sorted array of the sequence's elements.

#### Declaration

`func sorted() -> [Set<Element>.Element]`

#### Declared In

`Collection`, `Sequence`

#### Where Element : Equatable

func contains(_:)

Returns a Boolean value indicating whether the sequence contains the given element.

This example checks to see whether a favorite actor is in an array storing a movie's cast.

``````let cast = ["Vivien", "Marlon", "Kim", "Karl"]
print(cast.contains("Marlon"))
// Prints "true"
print(cast.contains("James"))
// Prints "false"``````

`element`: The element to find in the sequence. Returns: `true` if the element was found in the sequence; otherwise, `false`.

#### Declaration

`func contains(_ element: Set<Element>.Element) -> Bool`

#### Declared In

`Collection`, `Sequence`
func elementsEqual(_:)

Returns a Boolean value indicating whether this sequence and another sequence contain the same elements in the same order.

At least one of the sequences must be finite.

This example tests whether one countable range shares the same elements as another countable range and an array.

``````let a = 1...3
let b = 1...10

print(a.elementsEqual(b))
// Prints "false"
print(a.elementsEqual([1, 2, 3]))
// Prints "true"``````

`other`: A sequence to compare to this sequence. Returns: `true` if this sequence and `other` contain the same elements in the same order.

#### Declaration

`func elementsEqual<OtherSequence>(_ other: OtherSequence) -> Bool where OtherSequence : Sequence, Set<Element>.Element == OtherSequence.Element`

#### Declared In

`Collection`, `Sequence`
func firstIndex(of:)

Returns the first index where the specified value appears in the collection.

After using `firstIndex(of:)` to find the position of a particular element in a collection, you can use it to access the element by subscripting. This example shows how you can modify one of the names in an array of students.

``````var students = ["Ben", "Ivy", "Jordell", "Maxime"]
if let i = students.firstIndex(of: "Maxime") {
students[i] = "Max"
}
print(students)
// Prints "["Ben", "Ivy", "Jordell", "Max"]"``````

`element`: An element to search for in the collection. Returns: The first index where `element` is found. If `element` is not found in the collection, returns `nil`.

#### Declaration

`func firstIndex(of element: Set<Element>.Element) -> Set<Element>.Index?`

#### Declared In

`Collection`
func split(_:maxSplits:omittingEmptySubsequences:)

Returns the longest possible subsequences of the collection, in order, around elements equal to the given element.

The resulting array consists of at most `maxSplits + 1` subsequences. Elements that are used to split the collection are not returned as part of any subsequence.

The following examples show the effects of the `maxSplits` and `omittingEmptySubsequences` parameters when splitting a string at each space character (" "). 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(separator: " "))
// 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(separator: " ", maxSplits: 1))
// 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(separator: " ", omittingEmptySubsequences: false))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"``````

Parameters: separator: The element that should be split upon. maxSplits: The maximum number of times to split the collection, or one less than the number of subsequences to return. If `maxSplits + 1` subsequences are returned, the last one is a suffix of the original collection containing the remaining elements. `maxSplits` must be greater than or equal to zero. The default value is `Int.max`. omittingEmptySubsequences: If `false`, an empty subsequence is returned in the result for each consecutive pair of `separator` elements in the collection and for each instance of `separator` at the start or end of the collection. If `true`, only nonempty subsequences are returned. The default value is `true`. Returns: An array of subsequences, split from this collection's elements.

#### Declaration

`func split(separator: Set<Element>.Element, maxSplits: Int = default, omittingEmptySubsequences: Bool = default) -> [Set<Element>.SubSequence]`

#### Declared In

`Collection`, `Sequence`
func starts(with:)

Returns a Boolean value indicating whether the initial elements of the sequence are the same as the elements in another sequence.

This example tests whether one countable range begins with the elements of another countable range.

``````let a = 1...3
let b = 1...10

print(b.starts(with: a))
// Prints "true"``````

Passing a sequence with no elements or an empty collection as `possiblePrefix` always results in `true`.

``````print(b.starts(with: []))
// Prints "true"``````

`possiblePrefix`: A sequence to compare to this sequence. Returns: `true` if the initial elements of the sequence are the same as the elements of `possiblePrefix`; otherwise, `false`. If `possiblePrefix` has no elements, the return value is `true`.

#### Declaration

`func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix) -> Bool where PossiblePrefix : Sequence, Set<Element>.Element == PossiblePrefix.Element`

#### Declared In

`Collection`, `Sequence`

#### Where Element : Sequence

func joined()

Returns the elements of this sequence of sequences, concatenated.

In this example, an array of three ranges is flattened so that the elements of each range can be iterated in turn.

``````let ranges = [0..<3, 8..<10, 15..<17]

// A for-in loop over 'ranges' accesses each range:
for range in ranges {
print(range)
}
// Prints "0..<3"
// Prints "8..<10"
// Prints "15..<17"

// Use 'joined()' to access each element of each range:
for index in ranges.joined() {
print(index, terminator: " ")
}
// Prints: "0 1 2 8 9 15 16"``````

Returns: A flattened view of the elements of this sequence of sequences.

#### Declaration

`func joined() -> FlattenSequence<Set<Element>>`

#### Declared In

`Collection`, `Sequence`
func joined(_:)

Returns the concatenated elements of this sequence of sequences, inserting the given separator between each element.

This example shows how an array of `[Int]` instances can be joined, using another `[Int]` instance as the separator:

``````let nestedNumbers = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let joined = nestedNumbers.joined(separator: [-1, -2])
print(Array(joined))
// Prints "[1, 2, 3, -1, -2, 4, 5, 6, -1, -2, 7, 8, 9]"``````

`separator`: A sequence to insert between each of this sequence's elements. Returns: The joined sequence of elements.

#### Declaration

`func joined<Separator>(separator: Separator) -> JoinedSequence<Set<Element>> where Separator : Sequence, Separator.Element == Set<Element>.Element.Element`

#### Declared In

`Collection`, `Sequence`

#### Where Element : StringProtocol

func joined(_:)

Returns a new string by concatenating the elements of the sequence, adding the given separator between each element.

The following example shows how an array of strings can be joined to a single, comma-separated string:

``````let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let list = cast.joined(separator: ", ")
print(list)
// Prints "Vivien, Marlon, Kim, Karl"``````

`separator`: A string to insert between each of the elements in this sequence. The default separator is an empty string. Returns: A single, concatenated string.

#### Declaration

`func joined(separator: String = default) -> String`

#### Declared In

`Collection`, `Sequence`

#### Where Indices == DefaultIndices

var indices: DefaultIndices<Set<Element>>

The indices that are valid for subscripting the collection, in ascending order.

A collection's `indices` property can hold a strong reference to the collection itself, causing the collection to be non-uniquely referenced. If you mutate the collection while iterating over its indices, a strong reference can cause an unexpected copy of the collection. To avoid the unexpected copy, use the `index(after:)` method starting with `startIndex` to produce indices instead.

``````var c = MyFancyCollection([10, 20, 30, 40, 50])
var i = c.startIndex
while i != c.endIndex {
c[i] /= 5
i = c.index(after: i)
}
// c == MyFancyCollection([2, 4, 6, 8, 10])``````

#### Declaration

`var indices: DefaultIndices<Set<Element>> { get }`

#### Declared In

`Collection`

#### Where Iterator == IndexingIterator

func makeIterator()

Returns an iterator over the elements of the collection.

#### Declaration

`func makeIterator() -> IndexingIterator<Set<Element>>`

#### Declared In

`Collection`

#### Where SubSequence == AnySequence

func drop(while:)

Returns a subsequence 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.

`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. Returns: A subsequence starting after the initial, consecutive elements that satisfy `predicate`.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`func drop(while predicate: (Set<Element>.Element) throws -> Bool) rethrows -> AnySequence<Set<Element>.Element>`

#### Declared In

`Collection`, `Sequence`
func dropFirst(_:)

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 sequence, 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 "[]"``````

`n`: The number of elements to drop from the beginning of the sequence. `n` must be greater than or equal to zero. Returns: A subsequence starting after the specified number of elements.

Complexity: O(1).

#### Declaration

`func dropFirst(_ n: Int) -> AnySequence<Set<Element>.Element>`

#### Declared In

`Collection`, `Sequence`
func dropLast(_:)

Returns a subsequence 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 subsequence.

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

`n`: The number of elements to drop off the end of the sequence. `n` must be greater than or equal to zero. Returns: A subsequence leaving off the specified number of elements.

Complexity: O(n), where n is the length of the sequence.

#### Declaration

`func dropLast(_ n: Int) -> AnySequence<Set<Element>.Element>`

#### Declared In

`Collection`, `Sequence`
func prefix(_:)

Returns a subsequence, 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]"``````

`maxLength`: The maximum number of elements to return. The value of `maxLength` must be greater than or equal to zero. Returns: A subsequence starting at the beginning of this sequence with at most `maxLength` elements.

Complexity: O(1)

#### Declaration

`func prefix(_ maxLength: Int) -> AnySequence<Set<Element>.Element>`

#### Declared In

`Collection`, `Sequence`
func prefix(while:)

Returns a subsequence 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.

`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. Returns: A subsequence of the initial, consecutive elements that satisfy `predicate`.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`func prefix(while predicate: (Set<Element>.Element) throws -> Bool) rethrows -> AnySequence<Set<Element>.Element>`

#### Declared In

`Collection`, `Sequence`
func split(_:omittingEmptySubsequences:whereSeparator:)

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!"]"``````

Parameters: maxSplits: The maximum number of times to split the sequence, or one less than the number of subsequences to return. If `maxSplits + 1` subsequences are returned, the last one is a suffix of the original sequence containing the remaining elements. `maxSplits` must be greater than or equal to zero. The default value is `Int.max`. omittingEmptySubsequences: If `false`, an empty subsequence is returned in the result for each pair of consecutive elements satisfying the `isSeparator` predicate and for each element at the start or end of the sequence satisfying the `isSeparator` predicate. If `true`, only nonempty subsequences are returned. The default value is `true`. isSeparator: A closure that returns `true` if its argument should be used to split the sequence; otherwise, `false`. Returns: An array of subsequences, split from this sequence's elements.

#### Declaration

`func split(maxSplits: Int = default, omittingEmptySubsequences: Bool = default, whereSeparator isSeparator: (Set<Element>.Element) throws -> Bool) rethrows -> [AnySequence<Set<Element>.Element>]`

#### Declared In

`Collection`, `Sequence`
func suffix(_:)

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]"``````

`maxLength`: The maximum number of elements to return. The value of `maxLength` must be greater than or equal to zero. Complexity: O(n), where n is the length of the sequence.

#### Declaration

`func suffix(_ maxLength: Int) -> AnySequence<Set<Element>.Element>`

#### Declared In

`Collection`, `Sequence`

#### Where SubSequence == Slice

subscript(_: Range<Set<Element>.Index>)

Accesses a contiguous subrange of the collection's elements.

The accessed slice uses the same indices for the same elements as the original collection. Always use the slice's `startIndex` property instead of assuming that its indices start at a particular value.

This example demonstrates getting a slice of an array of strings, finding the index of one of the strings in the slice, and then using that index in the original array.

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

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

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

Complexity: O(1)

#### Declaration

`subscript(bounds: Range<Set<Element>.Index>) -> Slice<Set<Element>> { get }`

#### Declared In

`Collection`