protocol LazySequenceProtocol A sequence on which normally-eager sequence operations are implemented lazily. Inheritance Sequence Conforming Types LazyCollectionProtocol, LazyDropWhileSequence, LazyMapSequence, LazyPrefixWhileSequence, LazySequence Associated Types associatedtype Elements See also: elements Lazy sequences can be used to avoid needless storage allocation and computation, because they use an underlying sequence for storage and compute their elements on demand. For example, doubled in this code sample is a sequence containing the values 2, 4, and 6. let doubled = [1, 2, 3].lazy.map { $0 * 2 } Each time an element of the lazy sequence doubled is accessed, the closure accesses and transforms an element of the underlying array. Sequence operations that take closure arguments, such as map(_:) and filter(_:), are normally eager: They use the closure immediately and return a new array. When you use the lazy property, you give the standard library explicit permission to store the closure and the sequence in the result, and defer computation until it is needed. Adding New Lazy Operations To add a new lazy sequence operation, extend this protocol with a method that returns a lazy wrapper that itself conforms to LazySequenceProtocol. For example, an eager scan(_:_:) method is defined as follows: extension Sequence { /// Returns an array containing the results of /// /// p.reduce(initial, nextPartialResult) /// /// for each prefix `p` of `self`, in order from shortest to /// longest. For example: /// /// (1..<6).scan(0, +) // [0, 1, 3, 6, 10, 15] /// /// - Complexity: O(n) func scan<Result>( _ initial: Result, _ nextPartialResult: (Result, Element) -> Result ) -> [Result] { var result = [initial] for x in self { result.append(nextPartialResult(result.last!, x)) } return result } } You can build a sequence type that lazily computes the elements in the result of a scan: struct LazyScanSequence<Base: Sequence, Result> : LazySequenceProtocol { let initial: Result let base: Base let nextPartialResult: (Result, Base.Element) -> Result struct Iterator: IteratorProtocol { var base: Base.Iterator var nextElement: Result? let nextPartialResult: (Result, Base.Element) -> Result mutating func next() -> Result? { return nextElement.map { result in nextElement = base.next().map { nextPartialResult(result, $0) } return result } } } func makeIterator() -> Iterator { return Iterator( base: base.makeIterator(), nextElement: initial as Result?, nextPartialResult: nextPartialResult) } } Finally, you can give all lazy sequences a lazy scan(_:_:) method: extension LazySequenceProtocol { func scan<Result>( _ initial: Result, _ nextPartialResult: @escaping (Result, Element) -> Result ) -> LazyScanSequence<Self, Result> { return LazyScanSequence( initial: initial, base: self, nextPartialResult: nextPartialResult) } } With this type and extension method, you can call .lazy.scan(_:_:) on any sequence to create a lazily computed scan. The resulting LazyScanSequence is itself lazy, too, so further sequence operations also defer computation. The explicit permission to implement operations lazily applies only in contexts where the sequence is statically known to conform to LazySequenceProtocol. In the following example, because the extension applies only to Sequence, side-effects such as the accumulation of result are never unexpectedly dropped or deferred: extension Sequence where Element == Int { func sum() -> Int { var result = 0 _ = self.map { result += $0 } return result } } Don't actually use map for this purpose, however, because it creates and discards the resulting array. Instead, use reduce for summing operations, or forEach or a for-in loop for operations with side effects. Instance Variables var elements Required A sequence containing the same elements as this one, possibly with a simpler type. When implementing lazy operations, wrapping elements instead of self can prevent result types from growing an extra LazySequence layer. For example, prext example needed Note: this property need not be implemented by conforming types, it has a default implementation in a protocol extension that just returns self. Declaration var elements: Self.Elements Default Implementations func allSatisfy(_ predicate: (Self.Element) throws -> Bool) rethrows -> Bool Returns a Boolean value indicating whether every element of a sequence satisfies a given predicate. The following code uses this method to test whether all the names in an array have at least five characters: let names = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"] let allHaveAtLeastFive = names.allSatisfy({ $0.count >= 5 }) // allHaveAtLeastFive == true Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value that indicates whether the passed element satisfies a condition. Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func allSatisfy(_ predicate: (Self.Element) throws -> Bool) rethrows -> Bool func compactMap(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [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 non-optional values when your transformation produces an optional value. In this example, note the difference in the result of using map and compactMap with a transformation that returns an optional Int value. let possibleNumbers = ["1", "2", "three", "///4///", "5"] let mapped: [Int?] = possibleNumbers.map { str in Int(str) } // [1, 2, nil, nil, 5] let compactMapped: [Int] = possibleNumbers.compactMap { str in Int(str) } // [1, 2, 5] Parameter transform: A closure that accepts an element of this sequence as its argument and returns an optional value. Complexity: O(m + n), where n is the length of this sequence and m is the length of the result. Declaration @inlinable public func compactMap<ElementOfResult>(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult] func contains(where predicate: (Self.Element) throws -> Bool) rethrows -> Bool Returns a Boolean value indicating whether the sequence contains an element that satisfies the given predicate. You can use the predicate to check for an element of a type that doesn't conform to the Equatable protocol, such as the HTTPResponse enumeration in this example. enum HTTPResponse { case ok case error(Int) } let lastThreeResponses: [HTTPResponse] = [.ok, .ok, .error(404)] let hadError = lastThreeResponses.contains { element in if case .error = element { return true } else { return false } } // 'hadError' == true Alternatively, a predicate can be satisfied by a range of Equatable elements or a general condition. This example shows how you can check an array for an expense greater than $100. let expenses = [21.37, 55.21, 9.32, 10.18, 388.77, 11.41] let hasBigPurchase = expenses.contains { $0 > 100 } // 'hasBigPurchase' == true Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value that indicates whether the passed element represents a match. Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func contains(where predicate: (Self.Element) throws -> Bool) rethrows -> Bool func drop(while predicate: (Self.Element) throws -> Bool) rethrows -> DropWhileSequence<Self> Returns a sequence by skipping the initial, consecutive elements that satisfy the given predicate. The following example uses the drop(while:) method to skip over the positive numbers at the beginning of the numbers array. The result begins with the first element of numbers that does not satisfy predicate. let numbers = [3, 7, 4, -2, 9, -6, 10, 1] let startingWithNegative = numbers.drop(while: { $0 > 0 }) // startingWithNegative == [-2, 9, -6, 10, 1] If predicate matches every element in the sequence, the result is an empty sequence. Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the result. Complexity: O(k), where k is the number of elements to drop from the beginning of the sequence. Declaration @inlinable public func drop(while predicate: (Self.Element) throws -> Bool) rethrows -> DropWhileSequence<Self> func dropFirst(_ k: Int = 1) -> DropFirstSequence<Self> Returns a sequence containing all but the given number of initial elements. If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty sequence. let numbers = [1, 2, 3, 4, 5] print(numbers.dropFirst(2)) // Prints "[3, 4, 5]" print(numbers.dropFirst(10)) // Prints "[]" Parameter k: The number of elements to drop from the beginning of the sequence. k must be greater than or equal to zero. Complexity: O(1), with O(k) deferred to each iteration of the result, where k is the number of elements to drop from the beginning of the sequence. Declaration @inlinable public func dropFirst(_ k: Int = 1) -> DropFirstSequence<Self> func dropLast(_ k: Int = 1) -> [Self.Element] Returns a sequence containing all but the given number of final elements. The sequence must be finite. If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty sequence. let numbers = [1, 2, 3, 4, 5] print(numbers.dropLast(2)) // Prints "[1, 2, 3]" print(numbers.dropLast(10)) // Prints "[]" Parameter n: The number of elements to drop off the end of the sequence. n must be greater than or equal to zero. Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func dropLast(_ k: Int = 1) -> [Self.Element] func elementsEqual(_ other: OtherSequence, by areEquivalent: (Self.Element, OtherSequence.Element) throws -> Bool) rethrows -> Bool Returns a Boolean value indicating whether this sequence and another sequence contain equivalent elements in the same order, using the given predicate as the equivalence test. At least one of the sequences must be finite. The predicate must be a equivalence relation over the elements. That is, for any elements a, b, and c, the following conditions must hold: Complexity: O(m), where m is the lesser of the length of the sequence and the length of other. Declaration @inlinable public func elementsEqual<OtherSequence>(_ other: OtherSequence, by areEquivalent: (Self.Element, OtherSequence.Element) throws -> Bool) rethrows -> Bool where OtherSequence: Sequence func enumerated() -> EnumeratedSequence<Self> Returns a sequence of pairs (n, x), where n represents a consecutive integer starting at zero and x represents an element of the sequence. This example enumerates the characters of the string "Swift" and prints each character along with its place in the string. for (n, c) in "Swift".enumerated() { print("\(n): '\(c)'") } // Prints "0: 'S'" // Prints "1: 'w'" // Prints "2: 'i'" // Prints "3: 'f'" // Prints "4: 't'" When you enumerate a collection, the integer part of each pair is a counter for the enumeration, but is not necessarily the index of the paired value. These counters can be used as indices only in instances of zero-based, integer-indexed collections, such as Array and ContiguousArray. For other collections the counters may be out of range or of the wrong type to use as an index. To iterate over the elements of a collection with its indices, use the zip(_:_:) function. This example iterates over the indices and elements of a set, building a list consisting of indices of names with five or fewer letters. let names: Set = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"] var shorterIndices: [Set<String>.Index] = [] for (i, name) in zip(names.indices, names) { if name.count <= 5 { shorterIndices.append(i) } } Now that the shorterIndices array holds the indices of the shorter names in the names set, you can use those indices to access elements in the set. for i in shorterIndices { print(names[i]) } // Prints "Sofia" // Prints "Mateo" Complexity: O(1) Declaration @inlinable public func enumerated() -> EnumeratedSequence<Self> func filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element] Returns an array containing, in order, the elements of the sequence that satisfy the given predicate. In this example, filter(_:) is used to include only names shorter than five characters. let cast = ["Vivien", "Marlon", "Kim", "Karl"] let shortNames = cast.filter { $0.count < 5 } print(shortNames) // Prints "["Kim", "Karl"]" Parameter isIncluded: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the returned array. Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element] func first(where predicate: (Self.Element) throws -> Bool) rethrows -> Self.Element? Returns the first element of the sequence that satisfies the given predicate. The following example uses the first(where:) method to find the first negative number in an array of integers: let numbers = [3, 7, 4, -2, 9, -6, 10, 1] if let firstNegative = numbers.first(where: { $0 < 0 }) { print("The first negative number is \(firstNegative).") } // Prints "The first negative number is -2." Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element is a match. Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func first(where predicate: (Self.Element) throws -> Bool) rethrows -> Self.Element? func flatMap(_ transform: (Self.Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] Returns an array containing the concatenated results of calling the given transformation with each element of this sequence. Use this method to receive a single-level collection when your transformation produces a sequence or collection for each element. In this example, note the difference in the result of using map and flatMap with a transformation that returns an array. let numbers = [1, 2, 3, 4] let mapped = numbers.map { Array(repeating: $0, count: $0) } // [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]] let flatMapped = numbers.flatMap { Array(repeating: $0, count: $0) } // [1, 2, 2, 3, 3, 3, 4, 4, 4, 4] In fact, s.flatMap(transform) is equivalent to Array(s.map(transform).joined()). Parameter transform: A closure that accepts an element of this sequence as its argument and returns a sequence or collection. Complexity: O(m + n), where n is the length of this sequence and m is the length of the result. Declaration @inlinable public func flatMap<SegmentOfResult>(_ transform: (Self.Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult: Sequence func flatMap(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult] Declaration @available(swift, deprecated: 4.1, renamed: "compactMap(_:)", message: "Please use compactMap(_:) for the case where closure returns an optional value") public func flatMap<ElementOfResult>(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult] func forEach(_ body: (Self.Element) throws -> Void) rethrows Calls the given closure on each element in the sequence in the same order as a for-in loop. The two loops in the following example produce the same output: let numberWords = ["one", "two", "three"] for word in numberWords { print(word) } // Prints "one" // Prints "two" // Prints "three" numberWords.forEach { word in print(word) } // Same as above Using the forEach method is distinct from a for-in loop in two important ways: You cannot use a break or continue statement to exit the current call of the body closure or skip subsequent calls. Using the return statement in the body closure will exit only from the current call to body, not from any outer scope, and won't skip subsequent calls. Parameter body: A closure that takes an element of the sequence as a parameter. Declaration @inlinable public func forEach(_ body: (Self.Element) throws -> Void) rethrows var lazy A sequence containing the same elements as this sequence, but on which some operations, such as map and filter, are implemented lazily. Declaration var lazy: LazySequence<Self> func lexicographicallyPrecedes(_ other: OtherSequence, by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Bool Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the given predicate to compare elements. The predicate must be a strict weak ordering over the elements. That is, for any elements a, b, and c, the following conditions must hold: Note: This method implements the mathematical notion of lexicographical ordering, which has no connection to Unicode. If you are sorting strings to present to the end user, use String APIs that perform localized comparison instead. Complexity: O(m), where m is the lesser of the length of the sequence and the length of other. Declaration @inlinable public func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence, by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Bool where OtherSequence: Sequence, Self.Element == OtherSequence.Element func map(_ transform: (Self.Element) throws -> T) rethrows -> [T] Returns an array containing the results of mapping the given closure over the sequence's elements. In this example, map is used first to convert the names in the array to lowercase strings and then to count their characters. let cast = ["Vivien", "Marlon", "Kim", "Karl"] let lowercaseNames = cast.map { $0.lowercased() } // 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"] let letterCounts = cast.map { $0.count } // 'letterCounts' == [6, 6, 3, 4] Parameter transform: A mapping closure. transform accepts an element of this sequence as its parameter and returns a transformed value of the same or of a different type. Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func map<T>(_ transform: (Self.Element) throws -> T) rethrows -> [T] func max(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Self.Element? Returns the maximum element in the sequence, using the given predicate as the comparison between elements. The predicate must be a strict weak ordering over the elements. That is, for any elements a, b, and c, the following conditions must hold: This example shows how to use the max(by:) method on a dictionary to find the key-value pair with the highest value. let hues = ["Heliotrope": 296, "Coral": 16, "Aquamarine": 156] let greatestHue = hues.max { a, b in a.value < b.value } print(greatestHue) // Prints "Optional(("Heliotrope", 296))" Parameter areInIncreasingOrder: A predicate that returns true if its first argument should be ordered before its second argument; otherwise, false. Complexity: O(n), where n is the length of the sequence. Declaration @warn_unqualified_access @inlinable public func max(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Self.Element? func min(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Self.Element? Returns the minimum element in the sequence, using the given predicate as the comparison between elements. The predicate must be a strict weak ordering over the elements. That is, for any elements a, b, and c, the following conditions must hold: This example shows how to use the min(by:) method on a dictionary to find the key-value pair with the lowest value. let hues = ["Heliotrope": 296, "Coral": 16, "Aquamarine": 156] let leastHue = hues.min { a, b in a.value < b.value } print(leastHue) // Prints "Optional(("Coral", 16))" Parameter areInIncreasingOrder: A predicate that returns true if its first argument should be ordered before its second argument; otherwise, false. Complexity: O(n), where n is the length of the sequence. Declaration @warn_unqualified_access @inlinable public func min(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Self.Element? func prefix(_ maxLength: Int) -> PrefixSequence<Self> Returns a sequence, up to the specified maximum length, containing the initial elements of the sequence. If the maximum length exceeds the number of elements in the sequence, the result contains all the elements in the sequence. let numbers = [1, 2, 3, 4, 5] print(numbers.prefix(2)) // Prints "[1, 2]" print(numbers.prefix(10)) // Prints "[1, 2, 3, 4, 5]" Parameter maxLength: The maximum number of elements to return. The value of maxLength must be greater than or equal to zero. Complexity: O(1) Declaration @inlinable public func prefix(_ maxLength: Int) -> PrefixSequence<Self> func prefix(while predicate: (Self.Element) throws -> Bool) rethrows -> [Self.Element] Returns a sequence containing the initial, consecutive elements that satisfy the given predicate. The following example uses the prefix(while:) method to find the positive numbers at the beginning of the numbers array. Every element of numbers up to, but not including, the first negative value is included in the result. let numbers = [3, 7, 4, -2, 9, -6, 10, 1] let positivePrefix = numbers.prefix(while: { $0 > 0 }) // positivePrefix == [3, 7, 4] If predicate matches every element in the sequence, the resulting sequence contains every element of the sequence. Parameter predicate: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the result. Complexity: O(k), where k is the length of the result. Declaration @inlinable public func prefix(while predicate: (Self.Element) throws -> Bool) rethrows -> [Self.Element] func reduce(_ initialResult: Result, _ nextPartialResult: (Result, Self.Element) throws -> Result) rethrows -> Result Returns the result of combining the elements of the sequence using the given closure. Use the reduce(_:_:) method to produce a single value from the elements of an entire sequence. For example, you can use this method on an array of numbers to find their sum or product. The nextPartialResult closure is called sequentially with an accumulating value initialized to initialResult and each element of the sequence. This example shows how to find the sum of an array of numbers. let numbers = [1, 2, 3, 4] let numberSum = numbers.reduce(0, { x, y in x + y }) // numberSum == 10 When numbers.reduce(_:_:) is called, the following steps occur: The nextPartialResult closure is called with initialResult---0 in this case---and the first element of numbers, returning the sum: 1. The closure is called again repeatedly with the previous call's return value and each element of the sequence. When the sequence is exhausted, the last value returned from the closure is returned to the caller. If the sequence has no elements, nextPartialResult is never executed and initialResult is the result of the call to reduce(_:_:). Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Self.Element) throws -> Result) rethrows -> Result func reduce(into initialResult: Result, _ updateAccumulatingResult: (inout Result, Self.Element) throws -> ()) rethrows -> Result Returns the result of combining the elements of the sequence using the given closure. Use the reduce(into:_:) method to produce a single value from the elements of an entire sequence. For example, you can use this method on an array of integers to filter adjacent equal entries or count frequencies. This method is preferred over reduce(_:_:) for efficiency when the result is a copy-on-write type, for example an Array or a Dictionary. The updateAccumulatingResult closure is called sequentially with a mutable accumulating value initialized to initialResult and each element of the sequence. This example shows how to build a dictionary of letter frequencies of a string. let letters = "abracadabra" let letterCount = letters.reduce(into: [:]) { counts, letter in counts[letter, default: 0] += 1 } // letterCount == ["a": 5, "b": 2, "r": 2, "c": 1, "d": 1] When letters.reduce(into:_:) is called, the following steps occur: The updateAccumulatingResult closure is called with the initial accumulating value---[:] in this case---and the first character of letters, modifying the accumulating value by setting 1 for the key "a". The closure is called again repeatedly with the updated accumulating value and each element of the sequence. When the sequence is exhausted, the accumulating value is returned to the caller. If the sequence has no elements, updateAccumulatingResult is never executed and initialResult is the result of the call to reduce(into:_:). Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func reduce<Result>(into initialResult: Result, _ updateAccumulatingResult: (inout Result, Self.Element) throws -> ()) rethrows -> Result func reversed() -> [Self.Element] 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. Declaration @inlinable public func reversed() -> [Self.Element] func shuffled() -> [Self.Element] Returns the elements of the sequence, shuffled. For example, you can shuffle the numbers between 0 and 9 by calling the shuffled() method on that range: let numbers = 0...9 let shuffledNumbers = numbers.shuffled() // shuffledNumbers == [1, 7, 6, 2, 8, 9, 4, 3, 5, 0] This method is equivalent to calling shuffled(using:), passing in the system's default random generator. Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func shuffled() -> [Self.Element] func shuffled(using generator: inout T) -> [Self.Element] Returns the elements of the sequence, shuffled using the given generator as a source for randomness. You use this method to randomize the elements of a sequence when you are using a custom random number generator. For example, you can shuffle the numbers between 0 and 9 by calling the shuffled(using:) method on that range: let numbers = 0...9 let shuffledNumbers = numbers.shuffled(using: &myGenerator) // shuffledNumbers == [8, 9, 4, 3, 2, 6, 7, 0, 5, 1] Parameter generator: The random number generator to use when shuffling the sequence. Complexity: O(n), where n is the length of the sequence. Note: The algorithm used to shuffle a sequence may change in a future version of Swift. If you're passing a generator that results in the same shuffled order each time you run your program, that sequence may change when your program is compiled using a different version of Swift. Declaration @inlinable public func shuffled<T>(using generator: inout T) -> [Self.Element] where T: RandomNumberGenerator func sorted(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> [Self.Element] Returns the elements of the sequence, sorted using the given predicate as the comparison between elements. When you want to sort a sequence of elements that don't conform to the Comparable protocol, pass a predicate to this method that returns true when the first element should be ordered before the second. The elements of the resulting array are ordered according to the given predicate. In the following example, the predicate provides an ordering for an array of a custom HTTPResponse type. The predicate orders errors before successes and sorts the error responses by their error code. enum HTTPResponse { case ok case error(Int) } let responses: [HTTPResponse] = [.error(500), .ok, .ok, .error(404), .error(403)] let sortedResponses = responses.sorted { switch ($0, $1) { // Order errors by code case let (.error(aCode), .error(bCode)): return aCode < bCode // All successes are equivalent, so none is before any other case (.ok, .ok): return false // Order errors before successes case (.error, .ok): return true case (.ok, .error): return false } } print(sortedResponses) // Prints "[.error(403), .error(404), .error(500), .ok, .ok]" You also use this method to sort elements that conform to the Comparable protocol in descending order. To sort your sequence in descending order, pass the greater-than operator (>) as the areInIncreasingOrder parameter. let students: Set = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"] let descendingStudents = students.sorted(by: >) print(descendingStudents) // Prints "["Peter", "Kweku", "Kofi", "Akosua", "Abena"]" Calling the related sorted() method is equivalent to calling this method and passing the less-than operator (<) as the predicate. print(students.sorted()) // Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]" print(students.sorted(by: <)) // Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]" 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: The sorting algorithm is not guaranteed to be stable. A stable sort preserves the relative order of elements for which areInIncreasingOrder does not establish an order. Parameter areInIncreasingOrder: A predicate that returns true if its first argument should be ordered before its second argument; otherwise, false. Complexity: O(n log n), where n is the length of the sequence. Declaration @inlinable public func sorted(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> [Self.Element] func split(maxSplits: Int = Int.max, omittingEmptySubsequences: Bool = true, whereSeparator isSeparator: (Self.Element) throws -> Bool) rethrows -> [ArraySlice<Self.Element>] Returns the longest possible subsequences of the sequence, in order, that don't contain elements satisfying the given predicate. Elements that are used to split the sequence are not returned as part of any subsequence. The following examples show the effects of the maxSplits and omittingEmptySubsequences parameters when splitting a string using a closure that matches spaces. The first use of split returns each word that was originally separated by one or more spaces. let line = "BLANCHE: I don't want realism. I want magic!" print(line.split(whereSeparator: { $0 == " " }) .map(String.init)) // Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]" The second example passes 1 for the maxSplits parameter, so the original string is split just once, into two new strings. print( line.split(maxSplits: 1, whereSeparator: { $0 == " " }) .map(String.init)) // Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]" The final example passes true for the allowEmptySlices parameter, so the returned array contains empty strings where spaces were repeated. print( line.split( omittingEmptySubsequences: false, whereSeparator: { $0 == " " } ).map(String.init)) // Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]" Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func split(maxSplits: Int = Int.max, omittingEmptySubsequences: Bool = true, whereSeparator isSeparator: (Self.Element) throws -> Bool) rethrows -> [ArraySlice<Self.Element>] func starts(with possiblePrefix: PossiblePrefix, by areEquivalent: (Self.Element, PossiblePrefix.Element) throws -> Bool) rethrows -> Bool Returns a Boolean value indicating whether the initial elements of the sequence are equivalent to the elements in another sequence, using the given predicate as the equivalence test. The predicate must be a equivalence relation over the elements. That is, for any elements a, b, and c, the following conditions must hold: Complexity: O(m), where m is the lesser of the length of the sequence and the length of possiblePrefix. Declaration @inlinable public func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix, by areEquivalent: (Self.Element, PossiblePrefix.Element) throws -> Bool) rethrows -> Bool where PossiblePrefix: Sequence func suffix(_ maxLength: Int) -> [Self.Element] Returns a subsequence, up to the given maximum length, containing the final elements of the sequence. The sequence must be finite. If the maximum length exceeds the number of elements in the sequence, the result contains all the elements in the sequence. let numbers = [1, 2, 3, 4, 5] print(numbers.suffix(2)) // Prints "[4, 5]" print(numbers.suffix(10)) // Prints "[1, 2, 3, 4, 5]" Parameter maxLength: The maximum number of elements to return. The value of maxLength must be greater than or equal to zero. Complexity: O(n), where n is the length of the sequence. Declaration @inlinable public func suffix(_ maxLength: Int) -> [Self.Element] var underestimatedCount A value less than or equal to the number of elements in the sequence, calculated nondestructively. The default implementation returns 0. If you provide your own implementation, make sure to compute the value nondestructively. Complexity: O(1), except if the sequence also conforms to Collection. In this case, see the documentation of Collection.underestimatedCount. Declaration var underestimatedCount: Int func withContiguousStorageIfAvailable(_ body: (UnsafeBufferPointer<Self.Element>) throws -> R) rethrows -> R? Call body(p), where p is a pointer to the collection's contiguous storage. If no such storage exists, it is first created. If the collection does not support an internal representation in a form of contiguous storage, body is not called and nil is returned. A Collection that provides its own implementation of this method must also guarantee that an equivalent buffer of its SubSequence can be generated by advancing the pointer by the distance to the slice's startIndex. Declaration @inlinable public func withContiguousStorageIfAvailable<R>(_ body: (UnsafeBufferPointer<Self.Element>) throws -> R) rethrows -> R?