protocol LazySequenceProtocol
Inheritance 
Sequence
View Protocol Hierarchy →


Associated Types 
Elements : Sequence = Self
A See Also: Element : where Self.Element == Self.Iterator.Element, Self.Iterator.Element == Self.SubSequence.Element, Self.SubSequence.Element = = Self.SubSequence.Iterator.Element
A type representing the sequence's elements. Iterator : IteratorProtocol
A type that provides the sequence's iteration interface and encapsulates its iteration state. SubSequence : Sequence = AnySequence<Self.Element>
A type that represents a subsequence of some of the sequence's elements. 
Import  import Swift 
Instance Variables
A sequence containing the same elements as this one, possibly with a simpler type.
When implementing lazy operations, wrapping elements
instead
of self
can prevent result types from growing an extra
LazySequence
layer. For example,
prext example needed
Note: this property need not be implemented by conforming types,
it has a default implementation in a protocol extension that
just returns self
.
Declaration
var elements: Self.Elements { get }
A value less than or equal to the number of elements in the sequence, calculated nondestructively.
The default implementation returns 0. If you provide your own implementation, make sure to compute the value nondestructively.
Complexity: O(1), except if the sequence also conforms to Collection
.
In this case, see the documentation of Collection.underestimatedCount
.
Declaration
var underestimatedCount: Int { get }
Declared In
Sequence
Instance Methods
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.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.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 isIncluded
allowed.
Declaration
func filter(_ isIncluded: (Self.Element) throws > Bool) rethrows > [Self.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.Element) throws > Void) rethrows
Declared In
Sequence
Returns an iterator over the elements of this sequence.
Declaration
func makeIterator() > Self.Iterator
Declared In
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.lowercased() }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { $0.count }
// 'letterCounts' == [6, 6, 3, 4]
transform
: A mapping closure. transform
accepts an
element of this sequence as its parameter and returns a transformed
value of the same or of a different type.
Returns: An array containing the transformed elements of this
sequence.
Declaration
func map<T>(_ transform: (Self.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 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.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.split(whereSeparator: { $0 == " " })
.map(String.init))
// Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
The second example passes 1
for the maxSplits
parameter, so the
original string is split just once, into two new strings.
print(
line.split(maxSplits: 1, whereSeparator: { $0 == " " })
.map(String.init))
// Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
The final example passes false
for the omittingEmptySubsequences
parameter, so the returned array contains empty strings where spaces
were repeated.
print(line.split(omittingEmptySubsequences: false,
whereSeparator: { $0 == " " })
).map(String.init))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
Parameters:
maxSplits: The maximum number of times to split the sequence, or one
less than the number of subsequences to return. If maxSplits + 1
subsequences are returned, the last one is a suffix of the original
sequence containing the remaining elements. maxSplits
must be
greater than or equal to zero. The default value is Int.max
.
omittingEmptySubsequences: If false
, an empty subsequence is
returned in the result for each pair of consecutive elements
satisfying the isSeparator
predicate and for each element at the
start or end of the sequence satisfying the isSeparator
predicate.
If true
, only nonempty subsequences are returned. The default
value is true
.
isSeparator: A closure that returns true
if its argument should be
used to split the sequence; otherwise, false
.
Returns: An array of subsequences, split from this sequence's elements.
Declaration
func split(maxSplits: Int, omittingEmptySubsequences: Bool, whereSeparator isSeparator: (Self.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
Default Implementations
Declaration
var lazy: LazySequence<Self.Elements> { get }
A value less than or equal to the number of elements in the sequence, calculated nondestructively.
The default implementation returns 0. If you provide your own implementation, make sure to compute the value nondestructively.
Complexity: O(1), except if the sequence also conforms to Collection
.
In this case, see the documentation of Collection.underestimatedCount
.
Declaration
var underestimatedCount: Int { get }
Declared In
Sequence
Returns a Boolean value indicating whether every element of a sequence satisfies a given predicate.
predicate
: A closure that takes an element of the sequence
as its argument and returns a Boolean value that indicates whether
the passed element satisfies a condition.
Returns: true
if the sequence contains only elements that satisfy
predicate
; otherwise, false
.
Declaration
func allSatisfy(_ predicate: (Self.Element) throws > Bool) rethrows > Bool
Declared In
Sequence
Returns the nonnil
results of mapping the given transformation over
this sequence.
Use this method to receive a sequence of nonoptional values when your transformation produces an optional value.
transform
: A closure that accepts an element of this sequence
as its argument and returns an optional value.
Complexity: O(1)
Declaration
func compactMap<ElementOfResult>(_ transform: @escaping (Self.Elements.Element) > ElementOfResult?) > LazyMapSequence<LazyFilterSequence<LazyMapSequence<Self.Elements, ElementOfResult?>>, ElementOfResult>
Declared In
LazySequenceProtocol
, 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.Element) throws > Bool) rethrows > Bool
Declared In
Sequence
Returns a lazy sequence that skips any initial elements that satisfy
predicate
.
predicate
: A closure that takes an element of the sequence as
its argument and returns true
if the element should be skipped or
false
otherwise. Once predicate
returns false
it will not be
called again.
Declaration
func drop(while predicate: @escaping (Self.Elements.Element) > Bool) > LazyDropWhileSequence<Self.Elements>
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
Sequence
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
Sequence
Returns a Boolean value indicating whether this sequence and another sequence contain equivalent elements in the same order, using the given predicate as the equivalence test.
At least one of the sequences must be finite.
The predicate must be a equivalence relation over the elements. That
is, for any elements a
, b
, and c
, the following conditions must
hold:
areEquivalent(a, a)
is 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.
Declaration
func elementsEqual<OtherSequence>(_ other: OtherSequence, by areEquivalent: (Self.Element, OtherSequence.Element) throws > Bool) rethrows > Bool where OtherSequence : Sequence
Declared In
Sequence
Returns a sequence of pairs (n, x), where n represents a consecutive integer starting at zero and x represents an element of the sequence.
This example enumerates the characters of the string "Swift" and prints each character along with its place in the string.
for (n, c) in "Swift".enumerated() {
print("\(n): '\(c)'")
}
// Prints "0: 'S'"
// Prints "1: 'w'"
// Prints "2: 'i'"
// Prints "3: 'f'"
// Prints "4: 't'"
When you enumerate a collection, the integer part of each pair is a counter
for the enumeration, but is not necessarily the index of the paired value.
These counters can be used as indices only in instances of zerobased,
integerindexed collections, such as Array
and ContiguousArray
. For
other collections the counters may be out of range or of the wrong type
to use as an index. To iterate over the elements of a collection with its
indices, use the zip(_:_:)
function.
This example iterates over the indices and elements of a set, building a list consisting of indices of names with five or fewer letters.
let names: Set = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"]
var shorterIndices: [SetIndex<String>] = []
for (i, name) in zip(names.indices, names) {
if name.count <= 5 {
shorterIndices.append(i)
}
}
Now that the shorterIndices
array holds the indices of the shorter
names in the names
set, you can use those indices to access elements in
the set.
for i in shorterIndices {
print(names[i])
}
// Prints "Sofia"
// Prints "Mateo"
Returns: A sequence of pairs enumerating the sequence.
Declaration
func enumerated() > EnumeratedSequence<Self>
Declared In
Sequence
Returns the elements of self
that satisfy isIncluded
.
Note: The elements of the result are computed ondemand, as
the result is used. No buffering storage is allocated and each
traversal step invokes predicate
on one or more underlying
elements.
Declaration
func filter(_ isIncluded: @escaping (Self.Elements.Element) > Bool) > LazyFilterSequence<Self.Elements>
Declared In
LazySequenceProtocol
, 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.Element) throws > Bool) rethrows > Self.Element?
Declared In
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.Element) throws > ElementOfResult?) rethrows > [ElementOfResult]
Declared In
Sequence
Returns the nonnil
results of mapping the given transformation over
this sequence.
Use this method to receive a sequence of nonoptional values when your transformation produces an optional value.
transform
: A closure that accepts an element of this sequence
as its argument and returns an optional value.
Complexity: O(1)
Declaration
func flatMap<ElementOfResult>(_ transform: @escaping (Self.Elements.Element) > ElementOfResult?) > LazyMapSequence<LazyFilterSequence<LazyMapSequence<Self.Elements, ElementOfResult?>>, ElementOfResult>
Returns an array containing the concatenated results of calling the given transformation with each element of this sequence.
Use this method to receive a singlelevel collection when your transformation produces a sequence or collection for each element.
In this example, note the difference in the result of using map
and
flatMap
with a transformation that returns an array.
let numbers = [1, 2, 3, 4]
let mapped = numbers.map { Array(repeating: $0, count: $0) }
// [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]]
let flatMapped = numbers.flatMap { Array(repeating: $0, count: $0) }
// [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
In fact, s.flatMap(transform)
is equivalent to
Array(s.map(transform).joined())
.
transform
: A closure that accepts an element of this
sequence as its argument and returns a sequence or collection.
Returns: The resulting flattened array.
Complexity: O(m + n), where m is the length of this sequence and n is the length of the result.
Declaration
func flatMap<SegmentOfResult>(_ transform: (Self.Element) throws > SegmentOfResult) rethrows > [SegmentOfResult.Element] where SegmentOfResult : Sequence
Declared In
Sequence
Returns the concatenated results of mapping the given transformation over this sequence.
Use this method to receive a singlelevel sequence when your
transformation produces a sequence or collection for each element.
Calling flatMap(_:)
on a sequence s
is equivalent to calling
s.map(transform).joined()
.
Complexity: O(1)
Declaration
func flatMap<SegmentOfResult>(_ transform: @escaping (Self.Elements.Element) > SegmentOfResult) > LazySequence<FlattenSequence<LazyMapSequence<Self.Elements, SegmentOfResult>>> where SegmentOfResult : 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.Element) throws > Void) rethrows
Declared In
Sequence
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.
Declaration
func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence, by areInIncreasingOrder: (Self.Element, Self.Element) throws > Bool) rethrows > Bool where OtherSequence : Sequence, Self.Element == OtherSequence.Element
Declared In
Sequence
Returns a LazyMapSequence
over this Sequence
. The elements of
the result are computed lazily, each time they are read, by
calling transform
function on a base element.
Declaration
func map<U>(_ transform: @escaping (Self.Elements.Element) > U) > LazyMapSequence<Self.Elements, U>
Declared In
LazySequenceProtocol
, 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
.
Declaration
@warn_unqualified_access
func max(by areInIncreasingOrder: (Self.Element, Self.Element) throws > Bool) rethrows > Self.Element?
Declared In
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
.
Declaration
func min(by areInIncreasingOrder: (Self.Element, Self.Element) throws > Bool) rethrows > Self.Element?
Declared In
Sequence
Returns a lazy sequence of the initial consecutive elements that satisfy
predicate
.
predicate
: A closure that takes an element of the sequence as
its argument and returns true
if the element should be included or
false
otherwise. Once predicate
returns false
it will not be
called again.
Declaration
func prefix(while predicate: @escaping (Self.Elements.Element) > Bool) > LazyPrefixWhileSequence<Self.Elements>
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.Element) throws > Result) rethrows > Result
Declared In
Sequence
Returns the result of combining the elements of the sequence using the given closure.
Use the reduce(into:_:)
method to produce a single value from the
elements of an entire sequence. For example, you can use this method on an
array of integers to filter adjacent equal entries or count frequencies.
This method is preferred over reduce(_:_:)
for efficiency when the
result is a copyonwrite type, for example an Array or a Dictionary.
The updateAccumulatingResult
closure is called sequentially with a
mutable accumulating value initialized to initialResult
and each element
of the sequence. This example shows how to build a dictionary of letter
frequencies of a string.
let letters = "abracadabra"
let letterCount = letters.reduce(into: [:]) { counts, letter in
counts[letter, default: 0] += 1
}
// letterCount == ["a": 5, "b": 2, "r": 2, "c": 1, "d": 1]
When letters.reduce(into:_:)
is called, the following steps occur:
 The
updateAccumulatingResult
closure is called with the initial accumulating value[:]
in this caseand the first character ofletters
, modifying the accumulating value by setting1
for the key"a"
.  The closure is called again repeatedly with the updated accumulating value and each element of the sequence.
 When the sequence is exhausted, the accumulating value is returned to the caller.
If the sequence has no elements, updateAccumulatingResult
is never
executed and initialResult
is the result of the call to
reduce(into:_:)
.
Parameters:
initialResult: The value to use as the initial accumulating value.
updateAccumulatingResult: A closure that updates the accumulating
value with an element of the sequence.
Returns: The final accumulated value. If the sequence has no elements,
the result is initialResult
.
Declaration
func reduce<Result>(into initialResult: Result, _ updateAccumulatingResult: (inout Result, Self.Element) throws > ()) rethrows > Result
Declared In
Sequence
Returns an array containing the elements of this sequence in reverse order.
The sequence must be finite.
Complexity: O(n), where n is the length of the sequence.
Returns: An array containing the elements of this sequence in reverse order.
Declaration
func reversed() > [Self.Element]
Declared In
Sequence
Returns the elements of the sequence, shuffled.
For example, you can shuffle the numbers between 0
and 9
by calling
the shuffled()
method on that range:
let numbers = 0...9
let shuffledNumbers = numbers.shuffled()
// shuffledNumbers == [1, 7, 6, 2, 8, 9, 4, 3, 5, 0]
This method uses the default random generator, Random.default
. The call
to numbers.shuffled()
above is equivalent to calling
numbers.shuffled(using: &Random.default)
.
Returns: A shuffled array of this sequence's elements.
Complexity: O(n)
Declaration
func shuffled() > [Self.Element]
Declared In
Sequence
Returns the elements of the sequence, shuffled using the given generator as a source for randomness.
You use this method to randomize the elements of a sequence when you
are using a custom random number generator. For example, you can shuffle
the numbers between 0
and 9
by calling the shuffled(using:)
method
on that range:
let numbers = 0...9
let shuffledNumbers = numbers.shuffled(using: &myGenerator)
// shuffledNumbers == [8, 9, 4, 3, 2, 6, 7, 0, 5, 1]
generator
: The random number generator to use when shuffling
the sequence.
Returns: An array of this sequence's elements in a shuffled order.
Complexity: O(n)
Declaration
func shuffled<T>(using generator: inout T) > [Self.Element] where T : RandomNumberGenerator
Declared In
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.
Declaration
func sorted(by areInIncreasingOrder: (Self.Element, Self.Element) throws > Bool) rethrows > [Self.Element]
Declared In
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
.
Declaration
func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix, by areEquivalent: (Self.Element, PossiblePrefix.Element) throws > Bool) rethrows > Bool where PossiblePrefix : Sequence
Declared In
Sequence
Where 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.
Declaration
func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence) > Bool where OtherSequence : Sequence, Self.Element == OtherSequence.Element
Declared In
Sequence
Returns the maximum element in the sequence.
This example finds the largest value in an array of height measurements.
let heights = [67.5, 65.7, 64.3, 61.1, 58.5, 60.3, 64.9]
let greatestHeight = heights.max()
print(greatestHeight)
// Prints "Optional(67.5)"
Returns: The sequence's maximum element. If the sequence has no
elements, returns nil
.
Declaration
@warn_unqualified_access
func max() > Self.Element?
Declared In
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
.
Declaration
@warn_unqualified_access
func min() > Self.Element?
Declared In
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.
Declaration
func sorted() > [Self.Element]
Declared In
Sequence
Where 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.Element) > Bool
Declared In
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.
Declaration
func elementsEqual<OtherSequence>(_ other: OtherSequence) > Bool where OtherSequence : Sequence, Self.Element == OtherSequence.Element
Declared In
Sequence
Returns the longest possible subsequences of the sequence, 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 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 at each
space character (" "). The first use of split
returns each word that
was originally separated by one or more spaces.
let line = "BLANCHE: I don't want realism. I want magic!"
print(line.split(separator: " ")
.map(String.init))
// Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
The second example passes 1
for the maxSplits
parameter, so the
original string is split just once, into two new strings.
print(line.split(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.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 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 consecutive pair of separator
elements in the sequence and for each instance of separator
at the
start or end of the sequence. If true
, only nonempty subsequences
are returned. The default value is true
.
Returns: An array of subsequences, split from this sequence's elements.
Declaration
func split(separator: Self.Element, maxSplits: Int = default, omittingEmptySubsequences: Bool = default) > [Self.SubSequence]
Declared In
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
.
Declaration
func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix) > Bool where PossiblePrefix : Sequence, Self.Element == PossiblePrefix.Element
Declared In
Sequence
Where Element : Sequence
Returns a lazy sequence that concatenates the elements of this sequence of sequences.
Declaration
func joined() > LazySequence<FlattenSequence<Self.Elements>>
Declared In
LazySequenceProtocol
, 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.
Declaration
func joined<Separator>(separator: Separator) > JoinedSequence<Self> where Separator : Sequence, Separator.Element == Self.Element.Element
Declared In
Sequence
Where Element : StringProtocol
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
Sequence
Where Elements : LazySequenceProtocol
Declaration
var lazy: Self.Elements { get }
Where Self == Elements
Identical to self
.
Declaration
var elements: Self { get }
Where SubSequence == AnySequence
Returns a subsequence by skipping the initial, consecutive elements that satisfy the given predicate.
The following example uses the drop(while:)
method to skip over the
positive numbers at the beginning of the numbers
array. The result
begins with the first element of numbers
that does not satisfy
predicate
.
let numbers = [3, 7, 4, 2, 9, 6, 10, 1]
let startingWithNegative = numbers.drop(while: { $0 > 0 })
// startingWithNegative == [2, 9, 6, 10, 1]
If predicate
matches every element in the sequence, the result is an
empty sequence.
predicate
: A closure that takes an element of the sequence as
its argument and returns a Boolean value indicating whether the
element should be included in the result.
Returns: A subsequence starting after the initial, consecutive elements
that satisfy predicate
.
Complexity: O(n), where n is the length of the collection.
Declaration
func drop(while predicate: (Self.Element) throws > Bool) rethrows > AnySequence<Self.Element>
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(1).
Declaration
func dropFirst(_ n: Int) > AnySequence<Self.Element>
Declared In
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.Element>
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. 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.Element>
Declared In
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.
Declaration
func prefix(while predicate: (Self.Element) throws > Bool) rethrows > AnySequence<Self.Element>
Declared In
Sequence
Returns the longest possible subsequences of the sequence, in order, that don't contain elements satisfying the given predicate. Elements that are used to split the sequence are not returned as part of any subsequence.
The following examples show the effects of the maxSplits
and
omittingEmptySubsequences
parameters when splitting a string using a
closure that matches spaces. The first use of split
returns each word
that was originally separated by one or more spaces.
let line = "BLANCHE: I don't want realism. I want magic!"
print(line.split(whereSeparator: { $0 == " " })
.map(String.init))
// Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
The second example passes 1
for the maxSplits
parameter, so the
original string is split just once, into two new strings.
print(
line.split(maxSplits: 1, whereSeparator: { $0 == " " })
.map(String.init))
// Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
The final example passes true
for the allowEmptySlices
parameter, so
the returned array contains empty strings where spaces were repeated.
print(
line.split(
omittingEmptySubsequences: false,
whereSeparator: { $0 == " " }
).map(String.init))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
Parameters:
maxSplits: The maximum number of times to split the sequence, or one
less than the number of subsequences to return. If maxSplits + 1
subsequences are returned, the last one is a suffix of the original
sequence containing the remaining elements. maxSplits
must be
greater than or equal to zero. The default value is Int.max
.
omittingEmptySubsequences: If false
, an empty subsequence is
returned in the result for each pair of consecutive elements
satisfying the isSeparator
predicate and for each element at the
start or end of the sequence satisfying the isSeparator
predicate.
If true
, only nonempty subsequences are returned. The default
value is true
.
isSeparator: A closure that returns true
if its argument should be
used to split the sequence; otherwise, false
.
Returns: An array of subsequences, split from this sequence's elements.
Declaration
func split(maxSplits: Int = default, omittingEmptySubsequences: Bool = default, whereSeparator isSeparator: (Self.Element) throws > Bool) rethrows > [AnySequence<Self.Element>]
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.
Complexity: O(n), where n is the length of the sequence.
Declaration
func suffix(_ maxLength: Int) > AnySequence<Self.Element>
Declared In
Sequence
A sequence on which normallyeager operations such as
map
andfilter
are implemented lazily.Lazy sequences can be used to avoid needless storage allocation and computation, because they use an underlying sequence for storage and compute their elements on demand. For example,
is a sequence containing {
2
,4
,6
}. Each time an element of the lazy sequence is accessed, an element of the underlying array is accessed and transformed by the closure.Sequence operations taking closure arguments, such as
map
andfilter
, are normally eager: they use the closure immediately and return a new array. Using thelazy
property gives the standard library explicit permission to store the closure and the sequence in the result, and defer computation until it is needed.To add new lazy sequence operations, extend this protocol with methods that return lazy wrappers that are themselves
LazySequenceProtocol
s. For example, given an eagerscan
method defined as followswe can build a sequence that lazily computes the elements in the result of
scan
:and finally, we can give all lazy sequences a lazy
scan
method:See Also:
LazySequence
,LazyCollectionProtocol
,LazyCollection
Note: The explicit permission to implement further operations lazily applies only in contexts where the sequence is statically known to conform to
LazySequenceProtocol
. Thus, sideeffects such as the accumulation ofresult
below are never unexpectedly dropped or deferred:[We don't recommend that you use
map
this way, because it creates and discards an array.sum
would be better implemented usingreduce
].