protocol Collection
A sequence whose elements can be traversed multiple times, nondestructively, and accessed by an indexed subscript.
Inheritance  Sequence 

Conforming Types  AnyCollection, BidirectionalCollection, CollectionDifference, DefaultIndices, Dictionary, LazyCollectionProtocol, MutableCollection, RangeReplaceableCollection, Set, Slice, UnsafeBufferPointer, UnsafeRawBufferPointer 
Associated Types 
@available(*, deprecated, message: "all index distances are now of type Int") typealias IndexDistance = Int

Instance Variables
The number of elements in the collection.
To check whether a collection is empty, use its isEmpty
property
instead of comparing count
to zero. Unless the collection guarantees
randomaccess performance, calculating count
can be an O(n)
operation.
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(n), where n is the length of the collection.
Declaration
var count: Int
The collection's "past the end" positionthat is, the position one greater than the last valid subscript argument.
When you need a range that includes the last element of a collection, use
the halfopen range operator (..<
) with endIndex
. The ..<
operator
creates a range that doesn't include the upper bound, so it's always
safe to use with endIndex
. For example:
let numbers = [10, 20, 30, 40, 50]
if let index = numbers.firstIndex(of: 30) {
print(numbers[index ..< numbers.endIndex])
}
// Prints "[30, 40, 50]"
If the collection is empty, endIndex
is equal to startIndex
.
Declaration
var endIndex: Self.Index
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 nonuniquely referenced.
If you mutate the collection while iterating over its indices, a strong
reference can result in 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: Self.Indices
A Boolean value indicating whether the collection is empty.
When you need to check whether your collection is empty, use the
isEmpty
property instead of checking that the count
property is
equal to zero. For collections that don't conform to
RandomAccessCollection
, accessing the count
property iterates
through the elements of the collection.
let horseName = "Silver"
if horseName.isEmpty {
print("I've been through the desert on a horse with no name.")
} else {
print("Hi ho, \(horseName)!")
}
// Prints "Hi ho, Silver!"
Complexity: O(1)
Declaration
var isEmpty: Bool
The position of the first element in a nonempty collection.
If the collection is empty, startIndex
is equal to endIndex
.
Declaration
var startIndex: Self.Index
Subscripts
Accesses a contiguous subrange of the collection's elements.
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..<5]
print(streetsSlice)
// ["Channing", "Douglas", "Evarts"]
The accessed slice uses the same indices for the same elements as the
original collection. 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 may result in a runtime
error, even if that index is valid for the original collection.
print(streetsSlice.startIndex)
// 2
print(streetsSlice[2])
// "Channing"
print(streetsSlice[0])
// error: Index out of bounds
 Parameter bounds: A range of the collection's indices. The bounds of the range must be valid indices of the collection.
Complexity: O(1)
Declaration
subscript(bounds: Range<Self.Index>) > Self.SubSequence
Accesses the element at the specified position.
The following example accesses an element of an array through its subscript to print its value:
var streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
print(streets[1])
// Prints "Bryant"
You can subscript a collection with any valid index other than the collection's end index. The end index refers to the position one past the last element of a collection, so it doesn't correspond with an element.
 Parameter position: The position of the element to access.
position
must be a valid index of the collection that is not equal to theendIndex
property.
Complexity: O(1)
Declaration
subscript(position: Self.Index) > Self.Element
Instance Methods
Returns the distance between two indices.
Unless the collection conforms to the BidirectionalCollection
protocol,
start
must be less than or equal to end
.
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(k), where k is the resulting distance.
Declaration
func distance(from start: Self.Index, to end: Self.Index) > Int
Replaces the given index with its successor.
 Parameter i: A valid index of the collection.
i
must be less thanendIndex
.
Declaration
func formIndex(after i: inout Self.Index)
Returns an index that is the specified distance from the given index.
The following example obtains an index advanced four positions from a string's starting index and then prints the character at that position.
let s = "Swift"
let i = s.index(s.startIndex, offsetBy: 4)
print(s[i])
// Prints "t"
The value passed as distance
must not offset i
beyond the bounds of
the collection.
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(k), where k is the absolute value ofdistance
.
Declaration
func index(_ i: Self.Index, offsetBy distance: Int) > Self.Index
Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index.
The following example obtains an index advanced four positions from a
string's starting index and then prints the character at that position.
The operation doesn't require going beyond the limiting s.endIndex
value, so it succeeds.
let s = "Swift"
if let i = s.index(s.startIndex, offsetBy: 4, limitedBy: s.endIndex) {
print(s[i])
}
// Prints "t"
The next example attempts to retrieve an index six positions from
s.startIndex
but fails, because that distance is beyond the index
passed as limit
.
let j = s.index(s.startIndex, offsetBy: 6, limitedBy: s.endIndex)
print(j)
// Prints "nil"
The value passed as distance
must not offset i
beyond the bounds of
the collection, unless the index passed as limit
prevents offsetting
beyond those bounds.
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(k), where k is the absolute value ofdistance
.
Declaration
func index(_ i: Self.Index, offsetBy distance: Int, limitedBy limit: Self.Index) > Self.Index?
Returns the position immediately after the given index.
The successor of an index must be well defined. For an index i
into a
collection c
, calling c.index(after: i)
returns the same index every
time.
 Parameter i: A valid index of the collection.
i
must be less thanendIndex
.
Declaration
func index(after i: Self.Index) > Self.Index
Returns an iterator over the elements of the collection.
Declaration
override func makeIterator() > Self.Iterator
Default Implementations
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 nonnil
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]
 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>
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 zerobased,
integerindexed 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 singlelevel 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:
 You cannot use a
break
orcontinue
statement to exit the current call of thebody
closure or skip subsequent calls.  Using the
return
statement in thebody
closure will exit only from the current call tobody
, 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
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>
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 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 keyvalue 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 keyvalue 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:
 The
nextPartialResult
closure is called withinitialResult
0
in this caseand the first element ofnumbers
, 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
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 copyonwrite 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 caseand the first character ofletters
, modifying the accumulating value by setting1
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
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 greaterthan 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 lessthan 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]
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 ofCollection.underestimatedCount
.
Declaration
var underestimatedCount: Int
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?
Collections are used extensively throughout the standard library. When you use arrays, dictionaries, and other collections, you benefit from the operations that the
Collection
protocol declares and implements. In addition to the operations that collections inherit from theSequence
protocol, you gain access to methods that depend on accessing an element at a specific position in a collection.For example, if you want to print only the first word in a string, you can search for the index of the first space, and then create a substring up to that position.
The
firstSpace
constant is an index into thetext
stringthe position of the first space in the string. You can store indices in variables, and pass them to collection algorithms or use them later to access the corresponding element. In the example above,firstSpace
is used to extract the prefix that contains elements up to that index.Accessing Individual Elements
You can access an element of a collection through its subscript by using any valid index except the collection's
endIndex
property. This property is a "past the end" index that does not correspond with any element of the collection.Here's an example of accessing the first character in a string through its subscript:
The
Collection
protocol declares and provides default implementations for many operations that depend on elements being accessible by their subscript. For example, you can also access the first character oftext
using thefirst
property, which has the value of the first element of the collection, ornil
if the collection is empty.You can pass only valid indices to collection operations. You can find a complete set of a collection's valid indices by starting with the collection's
startIndex
property and finding every successor up to, and including, theendIndex
property. All other values of theIndex
type, such as thestartIndex
property of a different collection, are invalid indices for this collection.Saved indices may become invalid as a result of mutating operations. For more information about index invalidation in mutable collections, see the reference for the
MutableCollection
andRangeReplaceableCollection
protocols, as well as for the specific type you're using.Accessing Slices of a Collection
You can access a slice of a collection through its ranged subscript or by calling methods like
prefix(while:)
orsuffix(_:)
. A slice of a collection can contain zero or more of the original collection's elements and shares the original collection's semantics.The following example creates a
firstWord
constant by using theprefix(while:)
method to get a slice of thetext
string.You can retrieve the same slice using the string's ranged subscript, which takes a range expression.
The retrieved slice of
text
is equivalent in each of these cases.Slices Share Indices
A collection and its slices share the same indices. An element of a collection is located under the same index in a slice as in the base collection, as long as neither the collection nor the slice has been mutated since the slice was created.
For example, suppose you have an array holding the number of absences from each class during a session.
You're tasked with finding the day with the most absences in the second half of the session. To find the index of the day in question, follow these steps:
absences
array that holds the second half of the days.max(by:)
method to determine the index of the day with the most absences.absences
array.Here's an implementation of those steps:
Slices Inherit Collection Semantics
A slice inherits the value or reference semantics of its base collection. That is, when working with a slice of a mutable collection that has value semantics, such as an array, mutating the original collection triggers a copy of that collection and does not affect the contents of the slice.
For example, if you update the last element of the
absences
array from0
to2
, thesecondHalf
slice is unchanged.Traversing a Collection
Although a sequence can be consumed as it is traversed, a collection is guaranteed to be multipass: Any element can be repeatedly accessed by saving its index. Moreover, a collection's indices form a finite range of the positions of the collection's elements. The fact that all collections are finite guarantees the safety of many sequence operations, such as using the
contains(_:)
method to test whether a collection includes an element.Iterating over the elements of a collection by their positions yields the same elements in the same order as iterating over that collection using its iterator. This example demonstrates that the
characters
view of a string returns the same characters in the same order whether the view's indices or the view itself is being iterated.Conforming to the Collection Protocol
If you create a custom sequence that can provide repeated access to its elements, make sure that its type conforms to the
Collection
protocol in order to give a more useful and more efficient interface for sequence and collection operations. To addCollection
conformance to your type, you must declare at least the following requirements:Expected Performance
Types that conform to
Collection
are expected to provide thestartIndex
andendIndex
properties and subscript access to elements as O(1) operations. Types that are not able to guarantee this performance must document the departure, because many collection operations depend on O(1) subscripting performance for their own performance guarantees.The performance of some collection operations depends on the type of index that the collection provides. For example, a randomaccess collection, which can measure the distance between two indices in O(1) time, can calculate its
count
property in O(1) time. Conversely, because a forward or bidirectional collection must traverse the entire collection to count the number of contained elements, accessing itscount
property is an O(n) operation.