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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>

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

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

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]

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

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>

Declaration

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

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>

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]

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

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

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

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

When you enumerate a collection, the integer part of each pair is a counter for the enumeration, but is not necessarily the index of the paired value. These counters can be used as indices only in instances of zero-based, integer-indexed collections, such as Array and ContiguousArray. For other collections the counters may be out of range or of the wrong type to use as an index. To iterate over the elements of a collection with its indices, use the zip(_:_:) function.

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

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

Now that the shorterIndices array holds the indices of the shorter names in the names set, you can use those indices to access elements in the set.

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

Complexity: O(1)

Declaration

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

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

In this example, filter(_:) is used to include only names shorter than five characters.

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

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

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

Declaration

@inlinable public func filter(_ isIncluded: (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?

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

Declaration

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

Calls the given closure on each element in the sequence in the same order as a for-in loop.

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

let numberWords = ["one", "two", "three"]
for word in numberWords {
    print(word)
}
// Prints "one"
// Prints "two"
// Prints "three"

numberWords.forEach { word in
    print(word)
}
// Same as above

Using the forEach method is distinct from a for-in loop in two important ways:

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.

• 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

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

Returns an iterator over the elements of this sequence.

Declaration

@inlinable public func makeIterator() -> DropWhileSequence<Base>.Iterator

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

In this example, map is used first to convert the names in the array to lowercase strings and then to count their characters.

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

• Parameter transform: A mapping closure. transform accepts an element of this sequence as its parameter and returns a transformed value of the same or of a different type.

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

Declaration

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

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

The predicate must be a strict weak ordering over the elements. That is, for any elements a, b, and c, the following conditions must hold:

This example shows how to use the max(by:) method on a dictionary to find the key-value pair with the highest value.

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

• Parameter areInIncreasingOrder: A predicate that returns true if its first argument should be ordered before its second argument; otherwise, false.

Complexity: O(n), 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?

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?

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

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

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

• Parameter maxLength: The maximum number of elements to return. The value of maxLength must be greater than or equal to zero.

Complexity: O(1)

Declaration

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

Returns a 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]

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(_:_:).

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

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:_:).

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

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]

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]

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

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]

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>]

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

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]

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?