protocol RandomAccessCollection
Inheritance 
BidirectionalCollection, Collection, Sequence
View Protocol Hierarchy →


Associated Types 
SubSequence : BidirectionalCollection = RandomAccessSlice<Self>
A collection that represents a contiguous subrange of the collection's elements. Indices : BidirectionalCollection = DefaultRandomAccessIndices<Self>
A type that represents the indices that are valid for subscripting the collection, in ascending order. IndexDistance : SignedInteger = Int
A type that represents the number of steps between a pair of indices. Iterator : IteratorProtocol = IndexingIterator<Self>
A type that provides the collection's iteration interface and encapsulates its iteration state. By default, a collection conforms to the 
Import  import Swift 
Instance Variables
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 { get }
Declared In
RandomAccessCollection
, BidirectionalCollection
, Collection
The last element of the collection.
If the collection is empty, the value of this property is nil
.
let numbers = [10, 20, 30, 40, 50]
if let lastNumber = numbers.last {
print(lastNumber)
}
// Prints "50"
Complexity: O(1)
Declaration
var last: Self.Iterator.Element? { get }
Declared In
BidirectionalCollection
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: Self.IndexDistance { get }
Declared In
Collection
The first element of the collection.
If the collection is empty, the value of this property is nil
.
let numbers = [10, 20, 30, 40, 50]
if let firstNumber = numbers.first {
print(firstNumber)
}
// Prints "10"
Declaration
var first: Self.Iterator.Element? { get }
Declared In
Collection
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.characters.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 { get }
Declared In
Collection
A value less than or equal to the number of elements in the sequence, calculated nondestructively.
Complexity: O(1)
Declaration
var underestimatedCount: Int { get }
Declared In
Sequence
Subscripts
Accesses a contiguous subrange of the collection's elements.
The accessed slice uses the same indices for the same elements as the
original collection uses. Always use the slice's startIndex
property
instead of assuming that its indices start at a particular value.
This example demonstrates getting a slice of an array of strings, finding the index of one of the strings in the slice, and then using that index in the original array.
let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2 ..< streets.endIndex]
print(streetsSlice)
// Prints "["Channing", "Douglas", "Evarts"]"
let index = streetsSlice.index(of: "Evarts") // 4
print(streets[index!])
// Prints "Evarts"
bounds
: A range of the collection's indices. The bounds of
the range must be valid indices of the collection.
Declaration
subscript(bounds: Range<Self.Index>) > Self.SubSequence { get }
Declared In
RandomAccessCollection
, BidirectionalCollection
, Collection
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.
position
: The position of the element to access. position
must be a valid index of the collection that is not equal to the
endIndex
property.
Complexity: O(1)
Declaration
subscript(position: Self.Index) > Self.Iterator.Element { get }
Declared In
Collection
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
.
Parameters:
start: A valid index of the collection.
end: Another valid index of the collection. If end
is equal to
start
, the result is zero.
Returns: The distance between start
and end
. The result can be
negative only if the collection conforms to the
BidirectionalCollection
protocol.
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(n), where n is the
resulting distance.
Declaration
func distance(from start: Self.Index, to end: Self.Index) > Self.IndexDistance
Declared In
Collection
Returns a subsequence by skipping elements while predicate
returns
true
and returning the remaining elements.
predicate
: A closure that takes an element of the
sequence as its argument and returns a Boolean value indicating
whether the element is a match.
Complexity: O(n), where n is the length of the collection.
Declaration
func drop(while predicate: (Self.Iterator.Element) throws > Bool) rethrows > Self.SubSequence
Declared In
Sequence
Returns a subsequence containing all but the given number of initial elements.
If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty subsequence.
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst(2))
// Prints "[3, 4, 5]"
print(numbers.dropFirst(10))
// Prints "[]"
n
: The number of elements to drop from the beginning of
the sequence. n
must be greater than or equal to zero.
Returns: A subsequence starting after the specified number of
elements.
Complexity: O(n), where n is the number of elements to drop from the beginning of the sequence.
Declaration
func dropFirst(_ n: Int) > Self.SubSequence
Declared In
Sequence
Returns a subsequence containing all but the specified number of final elements.
The sequence must be finite. If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty subsequence.
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast(2))
// Prints "[1, 2, 3]"
print(numbers.dropLast(10))
// Prints "[]"
n
: The number of elements to drop off the end of the
sequence. n
must be greater than or equal to zero.
Returns: A subsequence leaving off the specified number of elements.
Complexity: O(n), where n is the length of the sequence.
Declaration
func dropLast(_ n: Int) > Self.SubSequence
Declared In
Sequence
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.characters.count < 5 }
print(shortNames)
// Prints "["Kim", "Karl"]"
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.
Returns: An array of the elements that includeElement
allowed.
Declaration
func filter(_ isIncluded: (Self.Iterator.Element) throws > Bool) rethrows > [Self.Iterator.Element]
Declared In
Sequence
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.
body
: A closure that takes an element of the sequence as a
parameter.
Declaration
func forEach(_ body: (Self.Iterator.Element) throws > Swift.Void) rethrows
Declared In
Sequence
Replaces the given index with its predecessor.
i
: A valid index of the collection. i
must be greater than
startIndex
.
Declaration
func formIndex(before i: inout Self.Index)
Declared In
BidirectionalCollection
Returns an index that is the specified distance from the given index.
The following example obtains an index advanced four positions from a string's starting index and then prints the character at that position.
let s = "Swift"
let i = s.index(s.startIndex, offsetBy: 4)
print(s[i])
// Prints "t"
The value passed as n
must not offset i
beyond the bounds of the
collection.
Parameters:
i: A valid index of the collection.
n: The distance to offset i
. n
must not be negative unless the
collection conforms to the BidirectionalCollection
protocol.
Returns: An index offset by n
from the index i
. If n
is positive,
this is the same value as the result of n
calls to index(after:)
.
If n
is negative, this is the same value as the result of n
calls
to index(before:)
.
See Also: index(_:offsetBy:limitedBy:)
, formIndex(_:offsetBy:)
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(n), where n is the absolute
value of n
.
Declaration
func index(_ i: Self.Index, offsetBy n: Self.IndexDistance) > Self.Index
Declared In
Collection
Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index.
The following example obtains an index advanced four positions from a
string's starting index and then prints the character at that position.
The operation doesn't require going beyond the limiting s.endIndex
value, so it succeeds.
let s = "Swift"
if let i = s.index(s.startIndex, offsetBy: 4, limitedBy: s.endIndex) {
print(s[i])
}
// Prints "t"
The next example attempts to retrieve an index six positions from
s.startIndex
but fails, because that distance is beyond the index
passed as limit
.
let j = s.index(s.startIndex, offsetBy: 6, limitedBy: s.endIndex)
print(j)
// Prints "nil"
The value passed as n
must not offset i
beyond the bounds of the
collection, unless the index passed as limit
prevents offsetting
beyond those bounds.
Parameters:
i: A valid index of the collection.
n: The distance to offset i
. n
must not be negative unless the
collection conforms to the BidirectionalCollection
protocol.
limit: A valid index of the collection to use as a limit. If n > 0
,
a limit that is less than i
has no effect. Likewise, if n < 0
, a
limit that is greater than i
has no effect.
Returns: An index offset by n
from the index i
, unless that index
would be beyond limit
in the direction of movement. In that case,
the method returns nil
.
See Also: index(_:offsetBy:)
, formIndex(_:offsetBy:limitedBy:)
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(n), where n is the absolute
value of n
.
Declaration
func index(_ i: Self.Index, offsetBy n: Self.IndexDistance, limitedBy limit: Self.Index) > Self.Index?
Declared In
Collection
Returns the position immediately before the given index.
i
: A valid index of the collection. i
must be greater than
startIndex
.
Returns: The index value immediately before i
.
Declaration
func index(before i: Self.Index) > Self.Index
Declared In
BidirectionalCollection
Returns an iterator over the elements of the collection.
Declaration
func makeIterator() > Self.Iterator
Declared In
Collection
, Sequence
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.lowercaseString }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { $0.characters.count }
// 'letterCounts' == [6, 6, 3, 4]
transform
: A mapping closure. transform
accepts an
element of this sequence as its parameter and returns a transformed
value of the same or of a different type.
Returns: An array containing the transformed elements of this
sequence.
Declaration
func map<T>(_ transform: (Self.Iterator.Element) throws > T) rethrows > [T]
Declared In
Sequence
Returns a subsequence, up to the specified maximum length, containing the initial elements of the sequence.
If the maximum length exceeds the number of elements in the sequence, the result contains all the elements in the sequence.
let numbers = [1, 2, 3, 4, 5]
print(numbers.prefix(2))
// Prints "[1, 2]"
print(numbers.prefix(10))
// Prints "[1, 2, 3, 4, 5]"
maxLength
: The maximum number of elements to return.
maxLength
must be greater than or equal to zero.
Returns: A subsequence starting at the beginning of this sequence
with at most maxLength
elements.
Declaration
func prefix(_ maxLength: Int) > Self.SubSequence
Declared In
Sequence
Returns a subsequence from the start of the collection through the specified position.
The resulting subsequence includes the element at the position end
.
The following example searches for the index of the number 40
in an
array of integers, and then prints the prefix of the array up to, and
including, that index:
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.index(of: 40) {
print(numbers.prefix(through: i))
}
// Prints "[10, 20, 30, 40]"
end
: The index of the last element to include in the
resulting subsequence. end
must be a valid index of the collection
that is not equal to the endIndex
property.
Returns: A subsequence up to, and including, the end
position.
Complexity: O(1)
See Also: prefix(upTo:)
Declaration
func prefix(through position: Self.Index) > Self.SubSequence
Declared In
Collection
Returns a subsequence from the start of the collection up to, but not including, the specified position.
The resulting subsequence does not include the element at the position
end
. The following example searches for the index of the number 40
in an array of integers, and then prints the prefix of the array up to,
but not including, that index:
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.index(of: 40) {
print(numbers.prefix(upTo: i))
}
// Prints "[10, 20, 30]"
Passing the collection's starting index as the end
parameter results in
an empty subsequence.
print(numbers.prefix(upTo: numbers.startIndex))
// Prints "[]"
end
: The "past the end" index of the resulting subsequence.
end
must be a valid index of the collection.
Returns: A subsequence up to, but not including, the end
position.
Complexity: O(1)
See Also: prefix(through:)
Declaration
func prefix(upTo end: Self.Index) > Self.SubSequence
Declared In
Collection
Returns a subsequence containing the initial, consecutive elements that satisfy the given predicate.
The following example uses the prefix(while:)
method to find the
positive numbers at the beginning of the numbers
array. Every element
of numbers
up to, but not including, the first negative value is
included in the result.
let numbers = [3, 7, 4, 2, 9, 6, 10, 1]
let positivePrefix = numbers.prefix(while: { $0 > 0 })
// positivePrefix == [3, 7, 4]
If predicate
matches every element in the sequence, the resulting
sequence contains every element of the sequence.
predicate
: A closure that takes an element of the sequence as
its argument and returns a Boolean value indicating whether the
element should be included in the result.
Returns: A subsequence of the initial, consecutive elements that
satisfy predicate
.
Complexity: O(n), where n is the length of the collection.
Declaration
func prefix(while predicate: (Self.Iterator.Element) throws > Bool) rethrows > Self.SubSequence
Declared In
Sequence
Returns the longest possible subsequences of the sequence, in order, that don't contain elements satisfying the given predicate.
The resulting array consists of at most maxSplits + 1
subsequences.
Elements that are used to split the sequence are not returned as part of
any subsequence.
The following examples show the effects of the maxSplits
and
omittingEmptySubsequences
parameters when splitting a string using a
closure that matches spaces. The first use of split
returns each word
that was originally separated by one or more spaces.
let line = "BLANCHE: I don't want realism. I want magic!"
print(line.characters.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.characters.split(maxSplits: 1, whereSeparator: { $0 == " " })
.map(String.init))
// Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
The final example passes false
for the omittingEmptySubsequences
parameter, so the returned array contains empty strings where spaces
were repeated.
print(
line.characters.split(
omittingEmptySubsequences: false,
whereSeparator: { $0 == " " })
).map(String.init))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
Parameters:
maxSplits: The maximum number of times to split the sequence, or one
less than the number of subsequences to return. If maxSplits + 1
subsequences are returned, the last one is a suffix of the original
sequence containing the remaining elements. maxSplits
must be
greater than or equal to zero. The default value is Int.max
.
omittingEmptySubsequences: If false
, an empty subsequence is
returned in the result for each pair of consecutive elements
satisfying the isSeparator
predicate and for each element at the
start or end of the sequence satisfying the isSeparator
predicate.
If true
, only nonempty subsequences are returned. The default
value is true
.
isSeparator: A closure that returns true
if its argument should be
used to split the sequence; otherwise, false
.
Returns: An array of subsequences, split from this sequence's elements.
Declaration
func split(maxSplits: Int, omittingEmptySubsequences: Bool, whereSeparator isSeparator: (Self.Iterator.Element) throws > Bool) rethrows > [Self.SubSequence]
Declared In
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]"
maxLength
: The maximum number of elements to return. The
value of maxLength
must be greater than or equal to zero.
Returns: A subsequence terminating at the end of this sequence with
at most maxLength
elements.
Complexity: O(n), where n is the length of the sequence.
Declaration
func suffix(_ maxLength: Int) > Self.SubSequence
Declared In
Sequence
Returns a subsequence from the specified position to the end of the collection.
The following example searches for the index of the number 40
in an
array of integers, and then prints the suffix of the array starting at
that index:
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.index(of: 40) {
print(numbers.suffix(from: i))
}
// Prints "[40, 50, 60]"
Passing the collection's endIndex
as the start
parameter results in
an empty subsequence.
print(numbers.suffix(from: numbers.endIndex))
// Prints "[]"
start
: The index at which to start the resulting subsequence.
start
must be a valid index of the collection.
Returns: A subsequence starting at the start
position.
Complexity: O(1)
Declaration
func suffix(from start: Self.Index) > Self.SubSequence
Declared In
Collection
Default Implementations
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: Self.IndexDistance { get }
Declared In
BidirectionalCollection
, Collection
The first element of the collection.
If the collection is empty, the value of this property is nil
.
let numbers = [10, 20, 30, 40, 50]
if let firstNumber = numbers.first {
print(firstNumber)
}
// Prints "10"
Declaration
var first: Self.Iterator.Element? { get }
Declared In
BidirectionalCollection
, Collection
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 cause an unexpected copy of the collection. To avoid the
unexpected copy, use the index(after:)
method starting with
startIndex
to produce indices instead.
var c = MyFancyCollection([10, 20, 30, 40, 50])
var i = c.startIndex
while i != c.endIndex {
c[i] /= 5
i = c.index(after: i)
}
// c == MyFancyCollection([2, 4, 6, 8, 10])
Declaration
var indices: DefaultRandomAccessIndices<Self> { get }
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.characters.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 { get }
Declared In
BidirectionalCollection
, Collection
The last element of the collection.
If the collection is empty, the value of this property is nil
.
let numbers = [10, 20, 30, 40, 50]
if let lastNumber = numbers.last {
print(lastNumber)
}
// Prints "50"
Declaration
var last: Self.Iterator.Element? { get }
Declared In
BidirectionalCollection
A view onto this collection that provides lazy implementations of
normally eager operations, such as map
and filter
.
Use the lazy
property when chaining operations to prevent
intermediate operations from allocating storage, or when you only
need a part of the final collection to avoid unnecessary computation.
See Also: LazySequenceProtocol
, LazyCollectionProtocol
.
Declaration
var lazy: LazyRandomAccessCollection<Self> { get }
Identical to self
.
Declaration
var lazy: Self { get }
Declared In
RandomAccessCollection
, BidirectionalCollection
, Collection
A value less than or equal to the number of elements in the collection.
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(n), where n is the length
of the collection.
Declaration
var underestimatedCount: Int { get }
Declared In
BidirectionalCollection
, Collection
, Sequence
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
predicate
: A closure that takes an element of the sequence
as its argument and returns a Boolean value that indicates whether
the passed element represents a match.
Returns: true
if the sequence contains an element that satisfies
predicate
; otherwise, false
.
Declaration
func contains(where predicate: (Self.Iterator.Element) throws > Bool) rethrows > Bool
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence by skipping elements while predicate
returns
true
and returning the remaining elements.
predicate
: A closure that takes an element of the
sequence as its argument and returns true
if the element should
be skipped or false
if it should be included. Once the predicate
returns false
it will not be called again.
Complexity: O(n), where n is the length of the collection.
Declaration
func drop(while predicate: (Self.Iterator.Element) throws > Bool) rethrows > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
Returns a subsequence containing all but the first element of the sequence.
The following example drops the first element from an array of integers.
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst())
// Prints "[2, 3, 4, 5]"
If the sequence has no elements, the result is an empty subsequence.
let empty: [Int] = []
print(empty.dropFirst())
// Prints "[]"
Returns: A subsequence starting after the first element of the sequence.
Complexity: O(1)
Declaration
func dropFirst() > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence containing all but the given number of initial elements.
If the number of elements to drop exceeds the number of elements in the collection, the result is an empty subsequence.
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst(2))
// Prints "[3, 4, 5]"
print(numbers.dropFirst(10))
// Prints "[]"
n
: The number of elements to drop from the beginning of
the collection. n
must be greater than or equal to zero.
Returns: A subsequence starting after the specified number of
elements.
Complexity: O(n), where n is the number of elements to drop from the beginning of the collection.
Declaration
func dropFirst(_ n: Int) > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
Returns a subsequence containing all but the last element of the sequence.
The sequence must be finite.
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast())
// Prints "[1, 2, 3, 4]"
If the sequence has no elements, the result is an empty subsequence.
let empty: [Int] = []
print(empty.dropLast())
// Prints "[]"
Returns: A subsequence leaving off the last element of the sequence.
Complexity: O(n), where n is the length of the sequence.
Declaration
func dropLast() > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence containing all but the specified number of final elements.
If the number of elements to drop exceeds the number of elements in the collection, the result is an empty subsequence.
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast(2))
// Prints "[1, 2, 3]"
print(numbers.dropLast(10))
// Prints "[]"
n
: The number of elements to drop off the end of the
collection. n
must be greater than or equal to zero.
Returns: A subsequence that leaves off n
elements from the end.
Complexity: O(n), where n is the number of elements to drop.
Declaration
func dropLast(_ n: Int) > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
Returns a Boolean value indicating whether this sequence and another sequence contain equivalent elements, using the given predicate as the equivalence test.
At least one of the sequences must be finite.
The predicate must be a equivalence relation over the elements. That
is, for any elements a
, b
, and c
, the following conditions must
hold:
areEquivalent(a, a)
is alwaystrue
. (Reflexivity)areEquivalent(a, b)
impliesareEquivalent(b, a)
. (Symmetry) If
areEquivalent(a, b)
andareEquivalent(b, c)
are bothtrue
, thenareEquivalent(a, c)
is alsotrue
. (Transitivity)
Parameters:
other: A sequence to compare to this sequence.
areEquivalent: A predicate that returns true
if its two arguments
are equivalent; otherwise, false
.
Returns: true
if this sequence and other
contain equivalent items,
using areEquivalent
as the equivalence test; otherwise, false.
See Also: elementsEqual(_:)
Declaration
func elementsEqual<OtherSequence where OtherSequence : Sequence, OtherSequence.Iterator.Element == Self.Iterator.Element>(_ other: OtherSequence, by areEquivalent: (Self.Iterator.Element, Self.Iterator.Element) throws > Bool) rethrows > Bool
Declared In
BidirectionalCollection
, Collection
, 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".characters.enumerated() {
print("\(n): '\(c)'")
}
// Prints "0: 'S'"
// Prints "1: 'w'"
// Prints "2: 'i'"
// Prints "3: 'f'"
// Prints "4: 't'"
When enumerating a collection, the integer part of each pair is a counter
for the enumeration, not necessarily the index of the paired value.
These counters can only be used as indices 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 of indices of names with five or fewer letters.
let names: Set = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"]
var shorterIndices: [SetIndex<String>] = []
for (i, name) in zip(names.indices, names) {
if name.characters.count <= 5 {
shorterIndices.append(i)
}
}
Now that the shorterIndices
array holds the indices of the shorter
names in the names
set, you can use those indices to access elements in
the set.
for i in shorterIndices {
print(names[i])
}
// Prints "Sofia"
// Prints "Mateo"
Returns: A sequence of pairs enumerating the sequence.
Declaration
func enumerated() > EnumeratedSequence<Self>
Declared In
BidirectionalCollection
, Collection
, Sequence
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.characters.count < 5 }
print(shortNames)
// Prints "["Kim", "Karl"]"
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.
Returns: An array of the elements that includeElement
allowed.
Declaration
func filter(_ isIncluded: (Self.Iterator.Element) throws > Bool) rethrows > [Self.Iterator.Element]
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the first element of the sequence that satisfies the given predicate.
The following example uses the first(where:)
method to find the first
negative number in an array of integers:
let numbers = [3, 7, 4, 2, 9, 6, 10, 1]
if let firstNegative = numbers.first(where: { $0 < 0 }) {
print("The first negative number is \(firstNegative).")
}
// Prints "The first negative number is 2."
predicate
: A closure that takes an element of the sequence as
its argument and returns a Boolean value indicating whether the
element is a match.
Returns: The first element of the sequence that satisfies predicate
,
or nil
if there is no element that satisfies predicate
.
Declaration
func first(where predicate: (Self.Iterator.Element) throws > Bool) rethrows > Self.Iterator.Element?
Declared In
BidirectionalCollection
, Collection
, Sequence
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
flatMap
with a transformation that returns an optional Int
value.
let possibleNumbers = ["1", "2", "three", "///4///", "5"]
let mapped: [Int?] = possibleNumbers.map { str in Int(str) }
// [1, 2, nil, nil, 5]
let flatMapped: [Int] = possibleNumbers.flatMap { str in Int(str) }
// [1, 2, 5]
transform
: A closure that accepts an element of this
sequence as its argument and returns an optional value.
Returns: An array of the nonnil
results of calling transform
with each element of the sequence.
Complexity: O(m + n), where m is the length of this sequence and n is the length of the result.
Declaration
func flatMap<ElementOfResult>(_ transform: (Self.Iterator.Element) throws > ElementOfResult?) rethrows > [ElementOfResult]
Declared In
BidirectionalCollection
, Collection
, Sequence
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(count: $0, repeatedValue: $0) }
// [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]]
let flatMapped = numbers.flatMap { Array(count: $0, repeatedValue: $0) }
// [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
In fact, s.flatMap(transform)
is equivalent to
Array(s.map(transform).joined())
.
transform
: A closure that accepts an element of this
sequence as its argument and returns a sequence or collection.
Returns: The resulting flattened array.
Complexity: O(m + n), where m is the length of this sequence
and n is the length of the result.
See Also: joined()
, map(_:)
Declaration
func flatMap<SegmentOfResult where SegmentOfResult : Sequence>(_ transform: (Self.Iterator.Element) throws > SegmentOfResult) rethrows > [SegmentOfResult.Iterator.Element]
Declared In
BidirectionalCollection
, Collection
, Sequence
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.
body
: A closure that takes an element of the sequence as a
parameter.
Declaration
func forEach(_ body: (Self.Iterator.Element) throws > Swift.Void) rethrows
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the first index in which an element of the collection satisfies the given predicate.
You can use the predicate to find an element of a type that doesn't
conform to the Equatable
protocol or to find an element that matches
particular criteria. Here's an example that finds a student name that
begins with the letter "A":
let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
if let i = students.index(where: { $0.hasPrefix("A") }) {
print("\(students[i]) starts with 'A'!")
}
// Prints "Abena starts with 'A'!"
predicate
: A closure that takes an element as its argument
and returns a Boolean value that indicates whether the passed element
represents a match.
Returns: The index of the first element for which predicate
returns
true
. If no elements in the collection satisfy the given predicate,
returns nil
.
See Also: index(of:)
Declaration
func index(where predicate: (Self.Iterator.Element) throws > Bool) rethrows > Self.Index?
Declared In
BidirectionalCollection
, Collection
Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the given predicate to compare elements.
The predicate must be a strict weak ordering over the elements. That
is, for any elements a
, b
, and c
, the following conditions must
hold:
areInIncreasingOrder(a, a)
is alwaysfalse
. (Irreflexivity) If
areInIncreasingOrder(a, b)
andareInIncreasingOrder(b, c)
are bothtrue
, thenareInIncreasingOrder(a, c)
is alsotrue
. (Transitive comparability)  Two elements are incomparable if neither is ordered before the other
according to the predicate. If
a
andb
are incomparable, andb
andc
are incomparable, thena
andc
are also incomparable. (Transitive incomparability)
Parameters:
other: A sequence to compare to this sequence.
areInIncreasingOrder: A predicate that returns true
if its first
argument should be ordered before its second argument; otherwise,
false
.
Returns: true
if this sequence precedes other
in a dictionary
ordering as ordered by areInIncreasingOrder
; otherwise, false
.
Note: This method implements the mathematical notion of lexicographical
ordering, which has no connection to Unicode. If you are sorting
strings to present to the end user, use String
APIs that perform
localized comparison instead.
See Also: lexicographicallyPrecedes(_:)
Declaration
func lexicographicallyPrecedes<OtherSequence where OtherSequence : Sequence, OtherSequence.Iterator.Element == Self.Iterator.Element>(_ other: OtherSequence, by areInIncreasingOrder: (Self.Iterator.Element, Self.Iterator.Element) throws > Bool) rethrows > Bool
Declared In
BidirectionalCollection
, Collection
, Sequence
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.lowercaseString }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { $0.characters.count }
// 'letterCounts' == [6, 6, 3, 4]
transform
: A mapping closure. transform
accepts an
element of this sequence as its parameter and returns a transformed
value of the same or of a different type.
Returns: An array containing the transformed elements of this
sequence.
Declaration
func map<T>(_ transform: (Self.Iterator.Element) throws > T) rethrows > [T]
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the maximum element in the sequence, using the given predicate as the comparison between elements.
The predicate must be a strict weak ordering over the elements. That
is, for any elements a
, b
, and c
, the following conditions must
hold:
areInIncreasingOrder(a, a)
is alwaysfalse
. (Irreflexivity) If
areInIncreasingOrder(a, b)
andareInIncreasingOrder(b, c)
are bothtrue
, thenareInIncreasingOrder(a, c)
is alsotrue
. (Transitive comparability)  Two elements are incomparable if neither is ordered before the other
according to the predicate. If
a
andb
are incomparable, andb
andc
are incomparable, thena
andc
are also incomparable. (Transitive incomparability)
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))"
areInIncreasingOrder
: A predicate that returns true
if its
first argument should be ordered before its second argument;
otherwise, false
.
Returns: The sequence's maximum element if the sequence is not empty;
otherwise, nil
.
See Also: max()
Declaration
@warn_unqualified_access
func max(by areInIncreasingOrder: (Self.Iterator.Element, Self.Iterator.Element) throws > Bool) rethrows > Self.Iterator.Element?
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the minimum element in the sequence, using the given predicate as the comparison between elements.
The predicate must be a strict weak ordering over the elements. That
is, for any elements a
, b
, and c
, the following conditions must
hold:
areInIncreasingOrder(a, a)
is alwaysfalse
. (Irreflexivity) If
areInIncreasingOrder(a, b)
andareInIncreasingOrder(b, c)
are bothtrue
, thenareInIncreasingOrder(a, c)
is alsotrue
. (Transitive comparability)  Two elements are incomparable if neither is ordered before the other
according to the predicate. If
a
andb
are incomparable, andb
andc
are incomparable, thena
andc
are also incomparable. (Transitive incomparability)
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))"
areInIncreasingOrder
: A predicate that returns true
if its first argument should be ordered before its second
argument; otherwise, false
.
Returns: The sequence's minimum element, according to
areInIncreasingOrder
. If the sequence has no elements, returns
nil
.
See Also: min()
Declaration
func min(by areInIncreasingOrder: (Self.Iterator.Element, Self.Iterator.Element) throws > Bool) rethrows > Self.Iterator.Element?
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence, up to the specified maximum length, containing the initial elements of the collection.
If the maximum length exceeds the number of elements in the collection, the result contains all the elements in the collection.
let numbers = [1, 2, 3, 4, 5]
print(numbers.prefix(2))
// Prints "[1, 2]"
print(numbers.prefix(10))
// Prints "[1, 2, 3, 4, 5]"
maxLength
: The maximum number of elements to return.
maxLength
must be greater than or equal to zero.
Returns: A subsequence starting at the beginning of this collection
with at most maxLength
elements.
Declaration
func prefix(_ maxLength: Int) > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
Returns a subsequence from the start of the collection through the specified position.
The resulting subsequence includes the element at the position end
.
The following example searches for the index of the number 40
in an
array of integers, and then prints the prefix of the array up to, and
including, that index:
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.index(of: 40) {
print(numbers.prefix(through: i))
}
// Prints "[10, 20, 30, 40]"
end
: The index of the last element to include in the
resulting subsequence. end
must be a valid index of the collection
that is not equal to the endIndex
property.
Returns: A subsequence up to, and including, the end
position.
Complexity: O(1)
See Also: prefix(upTo:)
Declaration
func prefix(through position: Self.Index) > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
Returns a subsequence from the start of the collection up to, but not including, the specified position.
The resulting subsequence does not include the element at the position
end
. The following example searches for the index of the number 40
in an array of integers, and then prints the prefix of the array up to,
but not including, that index:
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.index(of: 40) {
print(numbers.prefix(upTo: i))
}
// Prints "[10, 20, 30]"
Passing the collection's starting index as the end
parameter results in
an empty subsequence.
print(numbers.prefix(upTo: numbers.startIndex))
// Prints "[]"
end
: The "past the end" index of the resulting subsequence.
end
must be a valid index of the collection.
Returns: A subsequence up to, but not including, the end
position.
Complexity: O(1)
See Also: prefix(through:)
Declaration
func prefix(upTo end: Self.Index) > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
Returns a subsequence containing the initial elements until predicate
returns false
and skipping the remaining elements.
predicate
: A closure that takes an element of the
sequence as its argument and returns true
if the element should
be included or false
if it should be excluded. Once the predicate
returns false
it will not be called again.
Complexity: O(n), where n is the length of the collection.
Declaration
func prefix(while predicate: (Self.Iterator.Element) throws > Bool) rethrows > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
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(_:_:)
.
Parameters:
initialResult: The value to use as the initial accumulating value.
initialResult
is passed to nextPartialResult
the first time the
closure is executed.
nextPartialResult: A closure that combines an accumulating value and
an element of the sequence into a new accumulating value, to be used
in the next call of the nextPartialResult
closure or returned to
the caller.
Returns: The final accumulated value. If the sequence has no elements,
the result is initialResult
.
Declaration
func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Self.Iterator.Element) throws > Result) rethrows > Result
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a view presenting the elements of the collection in reverse order.
You can reverse a collection without allocating new space for its
elements by calling this reversed()
method. A
ReversedRandomAccessCollection
instance wraps an underlying collection
and provides access to its elements in reverse order. This example
prints the elements of an array in reverse order:
let numbers = [3, 5, 7]
for number in numbers.reversed() {
print(number)
}
// Prints "7"
// Prints "5"
// Prints "3"
If you need a reversed collection of the same type, you may be able to
use the collection's sequencebased or collectionbased initializer. For
example, to get the reversed version of an array, initialize a new
Array
instance from the result of this reversed()
method.
let reversedNumbers = Array(numbers.reversed())
print(reversedNumbers)
// Prints "[7, 5, 3]"
Complexity: O(1)
Declaration
func reversed() > ReversedRandomAccessCollection<Self>
Declared In
RandomAccessCollection
, BidirectionalCollection
, Collection
, Sequence
Returns the elements of the sequence, sorted using the given predicate as the comparison between elements.
When you want to sort a sequence of elements that don't conform to
the Comparable
protocol, pass a predicate to this method that returns
true
when the first element passed should be ordered before the
second. The elements of the resulting array are ordered according to the
given predicate.
The predicate must be a strict weak ordering over the elements. That
is, for any elements a
, b
, and c
, the following conditions must
hold:
areInIncreasingOrder(a, a)
is alwaysfalse
. (Irreflexivity) If
areInIncreasingOrder(a, b)
andareInIncreasingOrder(b, c)
are bothtrue
, thenareInIncreasingOrder(a, c)
is alsotrue
. (Transitive comparability)  Two elements are incomparable if neither is ordered before the other
according to the predicate. If
a
andb
are incomparable, andb
andc
are incomparable, thena
andc
are also incomparable. (Transitive incomparability)
The sorting algorithm is not stable. A nonstable sort may change the
relative order of elements for which areInIncreasingOrder
does not
establish an order.
In the following example, the predicate provides an ordering for an array
of a custom HTTPResponse
type. The predicate orders errors before
successes and sorts the error responses by their error code.
enum HTTPResponse {
case ok
case error(Int)
}
let responses: [HTTPResponse] = [.error(500), .ok, .ok, .error(404), .error(403)]
let sortedResponses = responses.sorted {
switch ($0, $1) {
// Order errors by code
case let (.error(aCode), .error(bCode)):
return aCode < bCode
// All successes are equivalent, so none is before any other
case (.ok, .ok): return false
// Order errors before successes
case (.error, .ok): return true
case (.ok, .error): return false
}
}
print(sortedResponses)
// Prints "[.error(403), .error(404), .error(500), .ok, .ok]"
You also use this method to sort elements that conform to the
Comparable
protocol in descending order. To sort your sequence
in descending order, pass the 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"]"
areInIncreasingOrder
: A predicate that returns true
if its first
argument should be ordered before its second argument; otherwise,
false
.
Returns: A sorted array of the sequence's elements.
See Also: sorted()
Declaration
func sorted(by areInIncreasingOrder: (Self.Iterator.Element, Self.Iterator.Element) > Bool) > [Self.Iterator.Element]
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the longest possible subsequences of the collection, in order, that don't contain elements satisfying the given predicate.
The resulting array consists of at most maxSplits + 1
subsequences.
Elements that are used to split the sequence are not returned as part of
any subsequence.
The following examples show the effects of the maxSplits
and
omittingEmptySubsequences
parameters when splitting a string using a
closure that matches spaces. The first use of split
returns each word
that was originally separated by one or more spaces.
let line = "BLANCHE: I don't want realism. I want magic!"
print(line.characters.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.characters.split(
maxSplits: 1, whereSeparator: { $0 == " " }
).map(String.init))
// Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
The final example passes false
for the omittingEmptySubsequences
parameter, so the returned array contains empty strings where spaces
were repeated.
print(line.characters.split(omittingEmptySubsequences: false, whereSeparator: { $0 == " " })
.map(String.init))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
Parameters:
maxSplits: The maximum number of times to split the collection, or
one less than the number of subsequences to return. If
maxSplits + 1
subsequences are returned, the last one is a suffix
of the original collection containing the remaining elements.
maxSplits
must be greater than or equal to zero. The default value
is Int.max
.
omittingEmptySubsequences: If false
, an empty subsequence is
returned in the result for each pair of consecutive elements
satisfying the isSeparator
predicate and for each element at the
start or end of the collection satisfying the isSeparator
predicate. The default value is true
.
isSeparator: A closure that takes an element as an argument and
returns a Boolean value indicating whether the collection should be
split at that element.
Returns: An array of subsequences, split from this collection's
elements.
Declaration
func split(maxSplits: Int = default, omittingEmptySubsequences: Bool = default, whereSeparator isSeparator: (Self.Iterator.Element) throws > Bool) rethrows > [Self.SubSequence]
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a Boolean value indicating whether the initial elements of the sequence are equivalent to the elements in another sequence, using the given predicate as the equivalence test.
The predicate must be a equivalence relation over the elements. That
is, for any elements a
, b
, and c
, the following conditions must
hold:
areEquivalent(a, a)
is alwaystrue
. (Reflexivity)areEquivalent(a, b)
impliesareEquivalent(b, a)
. (Symmetry) If
areEquivalent(a, b)
andareEquivalent(b, c)
are bothtrue
, thenareEquivalent(a, c)
is alsotrue
. (Transitivity)
Parameters:
possiblePrefix: A sequence to compare to this sequence.
areEquivalent: A predicate that returns true
if its two arguments
are equivalent; otherwise, false
.
Returns: true
if the initial elements of the sequence are equivalent
to the elements of possiblePrefix
; otherwise, false
. If
possiblePrefix
has no elements, the return value is true
.
See Also: starts(with:)
Declaration
func starts<PossiblePrefix where PossiblePrefix : Sequence, PossiblePrefix.Iterator.Element == Self.Iterator.Element>(with possiblePrefix: PossiblePrefix, by areEquivalent: (Self.Iterator.Element, Self.Iterator.Element) throws > Bool) rethrows > Bool
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence, up to the given maximum length, containing the final elements of the collection.
If the maximum length exceeds the number of elements in the collection, the result contains the entire collection.
let numbers = [1, 2, 3, 4, 5]
print(numbers.suffix(2))
// Prints "[4, 5]"
print(numbers.suffix(10))
// Prints "[1, 2, 3, 4, 5]"
maxLength
: The maximum number of elements to return.
maxLength
must be greater than or equal to zero.
Returns: A subsequence terminating at the end of the collection with at
most maxLength
elements.
Complexity: O(n), where n is equal to maxLength
.
Declaration
func suffix(_ maxLength: Int) > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence from the specified position to the end of the collection.
The following example searches for the index of the number 40
in an
array of integers, and then prints the suffix of the array starting at
that index:
let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.index(of: 40) {
print(numbers.suffix(from: i))
}
// Prints "[40, 50, 60]"
Passing the collection's endIndex
as the start
parameter results in
an empty subsequence.
print(numbers.suffix(from: numbers.endIndex))
// Prints "[]"
start
: The index at which to start the resulting subsequence.
start
must be a valid index of the collection.
Returns: A subsequence starting at the start
position.
Complexity: O(1)
Declaration
func suffix(from start: Self.Index) > Self.SubSequence
Declared In
BidirectionalCollection
, Collection
Where Base.Iterator == Iterator
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.characters.count < 5 }
print(shortNames)
// Prints "["Kim", "Karl"]"
includeElement
: 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.
Returns: An array of the elements that includeElement
allowed.
Declaration
func filter(_ isIncluded: (Self.Base.Iterator.Element) throws > Bool) rethrows > [Self.Base.Iterator.Element]
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns an iterator over the elements of this sequence.
Declaration
func makeIterator() > Self.Base.Iterator
Declared In
BidirectionalCollection
, Collection
, Sequence
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.lowercaseString }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { $0.characters.count }
// 'letterCounts' == [6, 6, 3, 4]
transform
: A mapping closure. transform
accepts an
element of this sequence as its parameter and returns a transformed
value of the same or of a different type.
Returns: An array containing the transformed elements of this
sequence.
Declaration
func map<T>(_ transform: (Self.Base.Iterator.Element) throws > T) rethrows > [T]
Declared In
BidirectionalCollection
, Collection
, Sequence
Where Index : Strideable, Indices == CountableRange, Index.Stride == IndexDistance, Indices.Index == Index, Indices.IndexDistance == IndexDistance, Indices.Indices == CountableRange, Indices.Iterator == IndexingIterator>, Indices.SubSequence == CountableRange, Indices._Element == Index, Indices.Indices.Index == Index, Indices.Indices.IndexDistance == IndexDistance, Indices.Indices.Indices == CountableRange, Indices.Indices.Iterator == IndexingIterator>, Indices.Indices.SubSequence == CountableRange, Indices.Indices._Element == Index, Indices.Iterator.Element == Index, Indices.SubSequence.Index == Index, Indices.SubSequence.IndexDistance == IndexDistance, Indices.SubSequence.Indices == CountableRange, Indices.SubSequence.Iterator == IndexingIterator>, Indices.SubSequence.SubSequence == CountableRange, Indices.SubSequence._Element == Index, Indices.Indices.Indices.Index == Index, Indices.Indices.Indices.IndexDistance == IndexDistance, Indices.Indices.Indices.Iterator == IndexingIterator>, Indices.Indices.Indices.SubSequence == CountableRange, Indices.Indices.Indices._Element == Index, Indices.Indices.Iterator.Element == Index, Indices.Indices.SubSequence.Index == Index, Indices.Indices.SubSequence.Iterator == IndexingIterator>, Indices.Indices.SubSequence.SubSequence == CountableRange, Indices.Indices.SubSequence._Element == Index, Indices.SubSequence.Indices.Index == Index, Indices.SubSequence.Indices.IndexDistance == IndexDistance, Indices.SubSequence.Indices.Iterator == IndexingIterator>, Indices.SubSequence.Indices.SubSequence == CountableRange, Indices.SubSequence.Indices._Element == Index, Indices.SubSequence.Iterator.Element == Index, Indices.SubSequence.SubSequence.Index == Index, Indices.SubSequence.SubSequence.Iterator == IndexingIterator>, Indices.SubSequence.SubSequence.SubSequence == CountableRange, Indices.SubSequence.SubSequence._Element == Index, Indices.Indices.Indices.Iterator.Element == Index, Indices.Indices.SubSequence.Iterator.Element == Index, Indices.SubSequence.Indices.Iterator.Element == Index, Indices.SubSequence.SubSequence.Iterator.Element == Index
The indices that are valid for subscripting the collection, in ascending order.
Declaration
var indices: CountableRange<Self.Index> { get }
Returns the distance between two indices.
Parameters:
start: A valid index of the collection.
end: Another valid index of the collection. If end
is equal to
start
, the result is zero.
Returns: The distance between start
and end
.
Complexity: O(1)
Declaration
func distance(from start: Self.Index, to end: Self.Index) > Self.Index.Stride
Returns an index that is the specified distance from the given index.
The following example obtains an index advanced four positions from an array's starting index and then prints the element at that position.
let numbers = [10, 20, 30, 40, 50]
let i = numbers.index(numbers.startIndex, offsetBy: 4)
print(numbers[i])
// Prints "50"
The value passed as n
must not offset i
beyond the bounds of the
collection.
Parameters:
i: A valid index of the collection.
n: The distance to offset i
.
Returns: An index offset by n
from the index i
. If n
is positive,
this is the same value as the result of n
calls to index(after:)
.
If n
is negative, this is the same value as the result of n
calls
to index(before:)
.
Complexity: O(1)
Declaration
func index(_ i: Self.Index, offsetBy n: Self.Index.Stride) > Self.Index
Returns the position immediately after the given index.
i
: A valid index of the collection. i
must be less than
endIndex
.
Returns: The index value immediately after i
.
Declaration
func index(after i: Self.Index) > Self.Index
Returns the position immediately after the given index.
i
: A valid index of the collection. i
must be greater than
startIndex
.
Returns: The index value immediately before i
.
Declaration
func index(before i: Self.Index) > Self.Index
Where Indices == DefaultBidirectionalIndices, Indices.Index == Index, Indices.IndexDistance == Int, Indices.Indices == DefaultBidirectionalIndices, Indices.Iterator == IndexingIterator>, Indices.SubSequence == DefaultBidirectionalIndices, Indices._Element == Index, Indices.IndexDistance.IntegerLiteralType == Int, Indices.IndexDistance.Stride == Int, Indices.IndexDistance._DisabledRangeIndex == Int._DisabledRangeIndex, Indices.Indices.Index == Index, Indices.Indices.IndexDistance == Int, Indices.Indices.Iterator == IndexingIterator>, Indices.Indices.SubSequence == DefaultBidirectionalIndices, Indices.Indices._Element == Index, Indices.Iterator.Element == Index, Indices.SubSequence.Index == Index, Indices.SubSequence.Iterator == IndexingIterator>, Indices.SubSequence.SubSequence == DefaultBidirectionalIndices, Indices.SubSequence._Element == Index, Indices.IndexDistance.Stride.IntegerLiteralType == Int, Indices.Indices.IndexDistance.IntegerLiteralType == Int, Indices.Indices.IndexDistance.Stride == Int, Indices.Indices.IndexDistance._DisabledRangeIndex == Int._DisabledRangeIndex, Indices.Indices.Iterator.Element == Index, Indices.SubSequence.Iterator.Element == Index, Indices.Indices.IndexDistance.Stride.IntegerLiteralType == Int
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 cause an unexpected copy of the collection. To avoid the
unexpected copy, use the index(after:)
method starting with
startIndex
to produce indices instead.
var c = MyFancyCollection([10, 20, 30, 40, 50])
var i = c.startIndex
while i != c.endIndex {
c[i] /= 5
i = c.index(after: i)
}
// c == MyFancyCollection([2, 4, 6, 8, 10])
Declaration
var indices: DefaultBidirectionalIndices<Self> { get }
Declared In
BidirectionalCollection
Where Indices == DefaultIndices, Indices.Index == Index, Indices.IndexDistance == Int, Indices.Iterator == IndexingIterator>, Indices.SubSequence == DefaultIndices, Indices._Element == Index, Indices.IndexDistance.IntegerLiteralType == Int, Indices.IndexDistance.Stride == Int, Indices.IndexDistance._DisabledRangeIndex == Int._DisabledRangeIndex, Indices.Iterator.Element == Index, Indices.IndexDistance.Stride.IntegerLiteralType == Int
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 cause an unexpected copy of the collection. To avoid the
unexpected copy, use the index(after:)
method starting with
startIndex
to produce indices instead.
var c = MyFancyCollection([10, 20, 30, 40, 50])
var i = c.startIndex
while i != c.endIndex {
c[i] /= 5
i = c.index(after: i)
}
// c == MyFancyCollection([2, 4, 6, 8, 10])
Declaration
var indices: DefaultIndices<Self> { get }
Declared In
BidirectionalCollection
, Collection
Where Iterator == IndexingIterator, Iterator.Element == _Element
Returns an iterator over the elements of the collection.
Declaration
func makeIterator() > IndexingIterator<Self>
Declared In
BidirectionalCollection
, Collection
Where Iterator == Self
Returns an iterator over the elements of this sequence.
Declaration
func makeIterator() > Self
Declared In
BidirectionalCollection
, Collection
, Sequence
Where Iterator.Element : BidirectionalCollection
Returns the elements of this collection of collections, concatenated.
In this example, an array of three ranges is flattened so that the elements of each range can be iterated in turn.
let ranges = [0..<3, 8..<10, 15..<17]
// A forin loop over 'ranges' accesses each range:
for range in ranges {
print(range)
}
// Prints "0..<3"
// Prints "8..<10"
// Prints "15..<17"
// Use 'joined()' to access each element of each range:
for index in ranges.joined() {
print(index, terminator: " ")
}
// Prints: "0 1 2 8 9 15 16"
Returns: A flattened view of the elements of this collection of collections.
See Also: flatMap(_:)
, joined(separator:)
Declaration
func joined() > FlattenBidirectionalCollection<Self>
Declared In
BidirectionalCollection
Where Iterator.Element : Collection
Returns the elements of this collection of collections, concatenated.
In this example, an array of three ranges is flattened so that the elements of each range can be iterated in turn.
let ranges = [0..<3, 8..<10, 15..<17]
// A forin loop over 'ranges' accesses each range:
for range in ranges {
print(range)
}
// Prints "0..<3"
// Prints "8..<10"
// Prints "15..<17"
// Use 'joined()' to access each element of each range:
for index in ranges.joined() {
print(index, terminator: " ")
}
// Prints: "0 1 2 8 9 15 16"
Returns: A flattened view of the elements of this collection of collections.
See Also: flatMap(_:)
, joined(separator:)
Declaration
func joined() > FlattenCollection<Self>
Declared In
BidirectionalCollection
, Collection
Where Iterator.Element : Comparable
Returns a Boolean value indicating whether the sequence precedes another
sequence in a lexicographical (dictionary) ordering, using the
lessthan operator (<
) to compare elements.
This example uses the lexicographicallyPrecedes
method to test which
array of integers comes first in a lexicographical ordering.
let a = [1, 2, 2, 2]
let b = [1, 2, 3, 4]
print(a.lexicographicallyPrecedes(b))
// Prints "true"
print(b.lexicographicallyPrecedes(b))
// Prints "false"
other
: A sequence to compare to this sequence.
Returns: true
if this sequence precedes other
in a dictionary
ordering; otherwise, false
.
Note: This method implements the mathematical notion of lexicographical
ordering, which has no connection to Unicode. If you are sorting
strings to present to the end user, use String
APIs that
perform localized comparison.
See Also: lexicographicallyPrecedes(_:by:)
Declaration
func lexicographicallyPrecedes<OtherSequence where OtherSequence : Sequence, OtherSequence.Iterator.Element == Self.Iterator.Element>(_ other: OtherSequence) > Bool
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the maximum element in the sequence.
This example finds the smallest value in an array of height measurements.
let heights = [67.5, 65.7, 64.3, 61.1, 58.5, 60.3, 64.9]
let greatestHeight = heights.max()
print(greatestHeight)
// Prints "Optional(67.5)"
Returns: The sequence's maximum element. If the sequence has no
elements, returns nil
.
See Also: max(by:)
Declaration
@warn_unqualified_access
func max() > Self.Iterator.Element?
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the minimum element in the sequence.
This example finds the smallest value in an array of height measurements.
let heights = [67.5, 65.7, 64.3, 61.1, 58.5, 60.3, 64.9]
let lowestHeight = heights.min()
print(lowestHeight)
// Prints "Optional(58.5)"
Returns: The sequence's minimum element. If the sequence has no
elements, returns nil
.
See Also: min(by:)
Declaration
@warn_unqualified_access
func min() > Self.Iterator.Element?
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the elements of the sequence, sorted.
You can sort any sequence of elements that conform to the
Comparable
protocol by calling this method. Elements are sorted in
ascending order.
The sorting algorithm is not stable. A nonstable sort may change the relative order of elements that compare equal.
Here's an example of sorting a list of students' names. Strings in Swift
conform to the Comparable
protocol, so the names are sorted in
ascending order according to the lessthan operator (<
).
let students: Set = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
let sortedStudents = students.sorted()
print(sortedStudents)
// Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]"
To sort the elements of your sequence in descending order, pass the
greaterthan operator (>
) to the sorted(by:)
method.
let descendingStudents = students.sorted(by: >)
print(descendingStudents)
// Prints "["Peter", "Kweku", "Kofi", "Akosua", "Abena"]"
Returns: A sorted array of the sequence's elements.
See Also: sorted(by:)
Declaration
func sorted() > [Self.Iterator.Element]
Declared In
BidirectionalCollection
, Collection
, Sequence
Where Iterator.Element : Equatable
Returns a Boolean value indicating whether the sequence contains the given element.
This example checks to see whether a favorite actor is in an array storing a movie's cast.
let cast = ["Vivien", "Marlon", "Kim", "Karl"]
print(cast.contains("Marlon"))
// Prints "true"
print(cast.contains("James"))
// Prints "false"
element
: The element to find in the sequence.
Returns: true
if the element was found in the sequence; otherwise,
false
.
Declaration
func contains(_ element: Self.Iterator.Element) > Bool
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a Boolean value indicating whether this sequence and another sequence contain the same elements in the same order.
At least one of the sequences must be finite.
This example tests whether one countable range shares the same elements as another countable range and an array.
let a = 1...3
let b = 1...10
print(a.elementsEqual(b))
// Prints "false"
print(a.elementsEqual([1, 2, 3]))
// Prints "true"
other
: A sequence to compare to this sequence.
Returns: true
if this sequence and other
contain the same elements
in the same order.
See Also: elementsEqual(_:by:)
Declaration
func elementsEqual<OtherSequence where OtherSequence : Sequence, OtherSequence.Iterator.Element == Self.Iterator.Element>(_ other: OtherSequence) > Bool
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the first index where the specified value appears in the collection.
After using index(of:)
to find the position of a particular element in
a collection, you can use it to access the element by subscripting. This
example shows how you can modify one of the names in an array of
students.
var students = ["Ben", "Ivy", "Jordell", "Maxime"]
if let i = students.index(of: "Maxime") {
students[i] = "Max"
}
print(students)
// Prints "["Ben", "Ivy", "Jordell", "Max"]"
element
: An element to search for in the collection.
Returns: The first index where element
is found. If element
is not
found in the collection, returns nil
.
See Also: index(where:)
Declaration
func index(of element: Self.Iterator.Element) > Self.Index?
Declared In
BidirectionalCollection
, Collection
Returns the longest possible subsequences of the collection, in order, around elements equal to the given element.
The resulting array consists of at most maxSplits + 1
subsequences.
Elements that are used to split the collection are not returned as part
of any subsequence.
The following examples show the effects of the maxSplits
and
omittingEmptySubsequences
parameters when splitting a string at each
space character (" "). The first use of split
returns each word that
was originally separated by one or more spaces.
let line = "BLANCHE: I don't want realism. I want magic!"
print(line.characters.split(separator: " ")
.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.characters.split(separator: " ", maxSplits: 1)
.map(String.init))
// Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
The final example passes false
for the omittingEmptySubsequences
parameter, so the returned array contains empty strings where spaces
were repeated.
print(line.characters.split(separator: " ", omittingEmptySubsequences: false)
.map(String.init))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
Parameters:
separator: The element that should be split upon.
maxSplits: The maximum number of times to split the collection, or
one less than the number of subsequences to return. If
maxSplits + 1
subsequences are returned, the last one is a suffix
of the original collection containing the remaining elements.
maxSplits
must be greater than or equal to zero. The default value
is Int.max
.
omittingEmptySubsequences: If false
, an empty subsequence is
returned in the result for each consecutive pair of separator
elements in the collection and for each instance of separator
at
the start or end of the collection. If true
, only nonempty
subsequences are returned. The default value is true
.
Returns: An array of subsequences, split from this collection's
elements.
Declaration
func split(separator: Self.Iterator.Element, maxSplits: Int = default, omittingEmptySubsequences: Bool = default) > [Self.SubSequence]
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a Boolean value indicating whether the initial elements of the sequence are the same as the elements in another sequence.
This example tests whether one countable range begins with the elements of another countable range.
let a = 1...3
let b = 1...10
print(b.starts(with: a))
// Prints "true"
Passing a sequence with no elements or an empty collection as
possiblePrefix
always results in true
.
print(b.starts(with: []))
// Prints "true"
possiblePrefix
: A sequence to compare to this sequence.
Returns: true
if the initial elements of the sequence are the same as
the elements of possiblePrefix
; otherwise, false
. If
possiblePrefix
has no elements, the return value is true
.
See Also: starts(with:by:)
Declaration
func starts<PossiblePrefix where PossiblePrefix : Sequence, PossiblePrefix.Iterator.Element == Self.Iterator.Element>(with possiblePrefix: PossiblePrefix) > Bool
Declared In
BidirectionalCollection
, Collection
, Sequence
Where Iterator.Element : Sequence
Returns the elements of this sequence of sequences, concatenated.
In this example, an array of three ranges is flattened so that the elements of each range can be iterated in turn.
let ranges = [0..<3, 8..<10, 15..<17]
// A forin loop over 'ranges' accesses each range:
for range in ranges {
print(range)
}
// Prints "0..<3"
// Prints "8..<10"
// Prints "15..<17"
// Use 'joined()' to access each element of each range:
for index in ranges.joined() {
print(index, terminator: " ")
}
// Prints: "0 1 2 8 9 15 16"
Returns: A flattened view of the elements of this sequence of sequences.
See Also: flatMap(_:)
, joined(separator:)
Declaration
func joined() > FlattenSequence<Self>
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns the concatenated elements of this sequence of sequences, inserting the given separator between each element.
This example shows how an array of [Int]
instances can be joined, using
another [Int]
instance as the separator:
let nestedNumbers = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let joined = nestedNumbers.joined(separator: [1, 2])
print(Array(joined))
// Prints "[1, 2, 3, 1, 2, 4, 5, 6, 1, 2, 7, 8, 9]"
separator
: A sequence to insert between each of this
sequence's elements.
Returns: The joined sequence of elements.
See Also: joined()
Declaration
func joined<Separator where Separator : Sequence, Separator.Iterator.Element == Self.Iterator.Element.Iterator.Element>(separator: Separator) > JoinedSequence<Self>
Declared In
BidirectionalCollection
, Collection
, Sequence
Where Iterator.Element == String
Returns a new string by concatenating the elements of the sequence, adding the given separator between each element.
The following example shows how an array of strings can be joined to a single, commaseparated string:
let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let list = cast.joined(separator: ", ")
print(list)
// Prints "Vivien, Marlon, Kim, Karl"
separator
: A string to insert between each of the elements
in this sequence. The default separator is an empty string.
Returns: A single, concatenated string.
Declaration
func joined(separator: String = default) > String
Declared In
BidirectionalCollection
, Collection
, Sequence
Where SubSequence : Sequence, SubSequence.SubSequence == SubSequence, SubSequence.Iterator.Element == Iterator.Element
Returns a subsequence by skipping the initial, consecutive elements that satisfy the given predicate.
The following example uses the drop(while:)
method to skip over the
positive numbers at the beginning of the numbers
array. The result
begins with the first element of numbers
that does not satisfy
predicate
.
let numbers = [3, 7, 4, 2, 9, 6, 10, 1]
let startingWithNegative = numbers.drop(while: { $0 > 0 })
// startingWithNegative == [2, 9, 6, 10, 1]
If predicate
matches every element in the sequence, the result is an
empty sequence.
predicate
: A closure that takes an element of the sequence as
its argument and returns a Boolean value indicating whether the
element should be included in the result.
Returns: A subsequence starting after the initial, consecutive elements
that satisfy predicate
.
Complexity: O(n), where n is the length of the collection.
See Also: prefix(while:)
Declaration
func drop(while predicate: (Self.Iterator.Element) throws > Bool) rethrows > AnySequence<Self.Iterator.Element>
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence containing all but the given number of initial elements.
If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty subsequence.
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst(2))
// Prints "[3, 4, 5]"
print(numbers.dropFirst(10))
// Prints "[]"
n
: The number of elements to drop from the beginning of
the sequence. n
must be greater than or equal to zero.
Returns: A subsequence starting after the specified number of
elements.
Complexity: O(1).
Declaration
func dropFirst(_ n: Int) > AnySequence<Self.Iterator.Element>
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence containing all but the given number of final elements.
The sequence must be finite. If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty subsequence.
let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast(2))
// Prints "[1, 2, 3]"
print(numbers.dropLast(10))
// Prints "[]"
n
: The number of elements to drop off the end of the
sequence. n
must be greater than or equal to zero.
Returns: A subsequence leaving off the specified number of elements.
Complexity: O(n), where n is the length of the sequence.
Declaration
func dropLast(_ n: Int) > AnySequence<Self.Iterator.Element>
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence, up to the specified maximum length, containing the initial elements of the sequence.
If the maximum length exceeds the number of elements in the sequence, the result contains all the elements in the sequence.
let numbers = [1, 2, 3, 4, 5]
print(numbers.prefix(2))
// Prints "[1, 2]"
print(numbers.prefix(10))
// Prints "[1, 2, 3, 4, 5]"
maxLength
: The maximum number of elements to return. The
value of maxLength
must be greater than or equal to zero.
Returns: A subsequence starting at the beginning of this sequence
with at most maxLength
elements.
Complexity: O(1)
Declaration
func prefix(_ maxLength: Int) > AnySequence<Self.Iterator.Element>
Declared In
BidirectionalCollection
, Collection
, Sequence
Returns a subsequence containing the initial, consecutive elements that satisfy the given predicate.
The following example uses the prefix(while:)
method to find the
positive numbers at the beginning of the numbers
array. Every element
of numbers
up to, but not including, the first negative value is
included in the result.
let numbers = [3, 7, 4, 2, 9, 6, 10, 1]
let positivePrefix = numbers.prefix(while: { $0 > 0 })
// positivePrefix == [3, 7, 4]
If predicate
matches every element in the sequence, the resulting
sequence contains every element of the sequence.
predicate
: A closure that takes an element of the sequence as
its argument and returns a Boolean value indicating whether the
element should be included in the result.
Returns: A subsequence of the initial, consecutive elements that
satisfy predicate
.
Complexity: O(n), where n is the length of the collection.
See Also: drop(while:)
Declaration
func prefix(while predicate: (Self.Iterator.Element) throws > Bool) rethrows > AnySequence<Self.Iterator.Element>
Declared In
BidirectionalCollection
, Collection
, Sequence
Where SubSequence == BidirectionalSlice, SubSequence.Index == Index, SubSequence.IndexDistance == IndexDistance, SubSequence.Indices == DefaultBidirectionalIndices>, SubSequence.Iterator == IndexingIterator>, SubSequence.SubSequence == BidirectionalSlice, SubSequence._Element == _Element, SubSequence.Indices.Index == Index, SubSequence.Indices.IndexDistance == Int, SubSequence.Indices.Iterator == IndexingIterator>>, SubSequence.Indices.SubSequence == DefaultBidirectionalIndices>, SubSequence.Indices._Element == Index, SubSequence.Iterator.Element == _Element, SubSequence.SubSequence.Index == Index, SubSequence.SubSequence.Iterator == IndexingIterator>, SubSequence.SubSequence.SubSequence == BidirectionalSlice, SubSequence.SubSequence._Element == _Element, SubSequence.Indices.IndexDistance.IntegerLiteralType == Int, SubSequence.Indices.IndexDistance.Stride == Int, SubSequence.Indices.IndexDistance._DisabledRangeIndex == Int._DisabledRangeIndex, SubSequence.Indices.Iterator.Element == Index, SubSequence.SubSequence.Iterator.Element == _Element, SubSequence.Indices.IndexDistance.Stride.IntegerLiteralType == Int
Accesses a contiguous subrange of the collection's elements.
The accessed slice uses the same indices for the same elements as the
original collection uses. Always use the slice's startIndex
property
instead of assuming that its indices start at a particular value.
This example demonstrates getting a slice of an array of strings, finding the index of one of the strings in the slice, and then using that index in the original array.
let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2 ..< streets.endIndex]
print(streetsSlice)
// Prints "["Channing", "Douglas", "Evarts"]"
let index = streetsSlice.index(of: "Evarts") // 4
print(streets[index!])
// Prints "Evarts"
bounds
: A range of the collection's indices. The bounds of
the range must be valid indices of the collection.
Declaration
subscript(bounds: Range<Self.Index>) > BidirectionalSlice<Self> { get }
Declared In
BidirectionalCollection
Where SubSequence == RandomAccessSlice, SubSequence.Index == Index, SubSequence.IndexDistance == IndexDistance, SubSequence.Indices == DefaultRandomAccessIndices>, SubSequence.Iterator == IndexingIterator>, SubSequence.SubSequence == RandomAccessSlice, SubSequence._Element == _Element, SubSequence.Indices.Index == Index, SubSequence.Indices.IndexDistance == Int, SubSequence.Indices.Indices == DefaultRandomAccessIndices>, SubSequence.Indices.Iterator == IndexingIterator>>, SubSequence.Indices.SubSequence == DefaultRandomAccessIndices>, SubSequence.Indices._Element == Index, SubSequence.Iterator.Element == _Element, SubSequence.SubSequence.Index == Index, SubSequence.SubSequence.IndexDistance == IndexDistance, SubSequence.SubSequence.Indices == DefaultRandomAccessIndices>, SubSequence.SubSequence.Iterator == IndexingIterator>, SubSequence.SubSequence.SubSequence == RandomAccessSlice, SubSequence.SubSequence._Element == _Element, SubSequence.Indices.IndexDistance.IntegerLiteralType == Int, SubSequence.Indices.IndexDistance.Stride == Int, SubSequence.Indices.IndexDistance._DisabledRangeIndex == Int._DisabledRangeIndex, SubSequence.Indices.Indices.Index == Index, SubSequence.Indices.Indices.IndexDistance == Int, SubSequence.Indices.Indices.Iterator == IndexingIterator>>, SubSequence.Indices.Indices.SubSequence == DefaultRandomAccessIndices>, SubSequence.Indices.Indices._Element == Index, SubSequence.Indices.Iterator.Element == Index, SubSequence.Indices.SubSequence.Index == Index, SubSequence.Indices.SubSequence.Iterator == IndexingIterator>>, SubSequence.Indices.SubSequence.SubSequence == DefaultRandomAccessIndices>, SubSequence.Indices.SubSequence._Element == Index, SubSequence.SubSequence.Indices.Index == Index, SubSequence.SubSequence.Indices.IndexDistance == Int, SubSequence.SubSequence.Indices.Iterator == IndexingIterator>>, SubSequence.SubSequence.Indices.SubSequence == DefaultRandomAccessIndices>, SubSequence.SubSequence.Indices._Element == Index, SubSequence.SubSequence.Iterator.Element == _Element, SubSequence.SubSequence.SubSequence.Index == Index, SubSequence.SubSequence.SubSequence.Iterator == IndexingIterator>, SubSequence.SubSequence.SubSequence.SubSequence == RandomAccessSlice, SubSequence.SubSequence.SubSequence._Element == _Element, SubSequence.Indices.IndexDistance.Stride.IntegerLiteralType == Int, SubSequence.Indices.Indices.IndexDistance.IntegerLiteralType == Int, SubSequence.Indices.Indices.IndexDistance.Stride == Int, SubSequence.Indices.Indices.IndexDistance._DisabledRangeIndex == Int._DisabledRangeIndex, SubSequence.Indices.Indices.Iterator.Element == Index, SubSequence.Indices.SubSequence.Iterator.Element == Index, SubSequence.SubSequence.Indices.IndexDistance.IntegerLiteralType == Int, SubSequence.SubSequence.Indices.IndexDistance.Stride == Int, SubSequence.SubSequence.Indices.IndexDistance._DisabledRangeIndex == Int._DisabledRangeIndex, SubSequence.SubSequence.Indices.Iterator.Element == Index, SubSequence.SubSequence.SubSequence.Iterator.Element == _Element, SubSequence.Indices.Indices.IndexDistance.Stride.IntegerLiteralType == Int, SubSequence.SubSequence.Indices.IndexDistance.Stride.IntegerLiteralType == Int
Accesses a contiguous subrange of the collection's elements.
The accessed slice uses the same indices for the same elements as the
original collection uses. Always use the slice's startIndex
property
instead of assuming that its indices start at a particular value.
This example demonstrates getting a slice of an array of strings, finding the index of one of the strings in the slice, and then using that index in the original array.
let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2 ..< streets.endIndex]
print(streetsSlice)
// Prints "["Channing", "Douglas", "Evarts"]"
let index = streetsSlice.index(of: "Evarts") // 4
print(streets[index!])
// Prints "Evarts"
bounds
: A range of the collection's indices. The bounds of
the range must be valid indices of the collection.
Declaration
subscript(bounds: Range<Self.Index>) > RandomAccessSlice<Self> { get }
Where SubSequence == Self
Removes and returns the first element of the collection.
Returns: The first element of the collection if the collection is
not empty; otherwise, nil
.
Complexity: O(1)
Declaration
mutating func popFirst() > Self.Iterator.Element?
Declared In
BidirectionalCollection
, Collection
Removes and returns the last element of the collection.
You can use popLast()
to remove the last element of a collection that
might be empty. The removeLast()
method must be used only on a
nonempty collection.
Returns: The last element of the collection if the collection has one
or more elements; otherwise, nil
.
Complexity: O(1).
See Also: removeLast()
Declaration
mutating func popLast() > Self.Iterator.Element?
Declared In
BidirectionalCollection
Removes and returns the first element of the collection.
The collection must not be empty.
Returns: The first element of the collection.
Complexity: O(1)
See Also: popFirst()
Declaration
mutating func removeFirst() > Self.Iterator.Element
Declared In
BidirectionalCollection
, Collection
Removes the specified number of elements from the beginning of the collection.
n
: The number of elements to remove. n
must be greater than
or equal to zero, and must be less than or equal to the number of
elements in the collection.
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(n).
Declaration
mutating func removeFirst(_ n: Int)
Declared In
BidirectionalCollection
, Collection
Removes and returns the last element of the collection.
The collection must not be empty. To remove the last element of a
collection that might be empty, use the popLast()
method instead.
Returns: The last element of the collection.
Complexity: O(1)
See Also: popLast()
Declaration
mutating func removeLast() > Self.Iterator.Element
Declared In
BidirectionalCollection
Removes the given number of elements from the end of the collection.
n
: The number of elements to remove. n
must be greater
than or equal to zero, and must be less than or equal to the number of
elements in the collection.
Complexity: O(1) if the collection conforms to
RandomAccessCollection
; otherwise, O(n), where n is the length
of the collection.
Declaration
mutating func removeLast(_ n: Int)
Declared In
BidirectionalCollection
Where SubSequence == Slice, SubSequence.Index == Index, SubSequence.Iterator == IndexingIterator>, SubSequence.SubSequence == Slice, SubSequence._Element == _Element, SubSequence.Iterator.Element == _Element
Accesses a contiguous subrange of the collection's elements.
The accessed slice uses the same indices for the same elements as the
original collection uses. Always use the slice's startIndex
property
instead of assuming that its indices start at a particular value.
This example demonstrates getting a slice of an array of strings, finding the index of one of the strings in the slice, and then using that index in the original array.
let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2 ..< streets.endIndex]
print(streetsSlice)
// Prints "["Channing", "Douglas", "Evarts"]"
let index = streetsSlice.index(of: "Evarts") // 4
print(streets[index!])
// Prints "Evarts"
bounds
: A range of the collection's indices. The bounds of
the range must be valid indices of the collection.
Complexity: O(1)
Declaration
subscript(bounds: Range<Self.Index>) > Slice<Self> { get }
Declared In
BidirectionalCollection
, Collection
A collection that supports efficient randomaccess index traversal.
Randomaccess collections can move indices any distance and measure the distance between indices in O(1) time. Therefore, the fundamental difference between randomaccess and bidirectional collections is that operations that depend on index movement or distance measurement offer significantly improved efficiency. For example, a randomaccess collection's
count
property is calculated in O(1) instead of requiring iteration of an entire collection.Conforming to the RandomAccessCollection Protocol
The
RandomAccessCollection
protocol adds further constraints on the associatedIndices
andSubSequence
types, but otherwise imposes no additional requirements over theBidirectionalCollection
protocol. However, in order to meet the complexity guarantees of a randomaccess collection, either the index for your custom type must conform to theStrideable
protocol or you must implement theindex(_:offsetBy:)
anddistance(from:to:)
methods with O(1) efficiency.