protocol Sequence

A type that provides sequential, iterated access to its elements.

Conforming Types AnySequence, Collection, Dictionary, DropFirstSequence, DropWhileSequence, EmptyCollection, EmptyCollection.Iterator, EnumeratedSequence, EnumeratedSequence.Iterator, FlattenSequence, IndexingIterator, IteratorSequence, JoinedSequence, LazyDropWhileSequence, LazyFilterSequence, LazyFilterSequence.Iterator, LazyMapSequence.Iterator, LazyPrefixWhileSequence, LazyPrefixWhileSequence.Iterator, LazySequence, LazySequenceProtocol, PrefixSequence, ReversedCollection, ReversedCollection.Iterator, Set, StrideThrough, StrideTo, UnfoldSequence, UnsafeBufferPointer, UnsafeMutableBufferPointer, UnsafeMutableRawBufferPointer, UnsafeRawBufferPointer, UnsafeRawBufferPointer.Iterator, Zip2Sequence
Associated Types
associatedtype Element
associatedtype Iterator

A sequence is a list of values that you can step through one at a time. The most common way to iterate over the elements of a sequence is to use a for-in loop:

let oneTwoThree = 1...3
for number in oneTwoThree {
// Prints "1"
// Prints "2"
// Prints "3"

While seemingly simple, this capability gives you access to a large number of operations that you can perform on any sequence. As an example, to check whether a sequence includes a particular value, you can test each value sequentially until you've found a match or reached the end of the sequence. This example checks to see whether a particular insect is in an array.

let bugs = ["Aphid", "Bumblebee", "Cicada", "Damselfly", "Earwig"]
var hasMosquito = false
for bug in bugs {
    if bug == "Mosquito" {
        hasMosquito = true
print("'bugs' has a mosquito: \(hasMosquito)")
// Prints "'bugs' has a mosquito: false"

The Sequence protocol provides default implementations for many common operations that depend on sequential access to a sequence's values. For clearer, more concise code, the example above could use the array's contains(_:) method, which every sequence inherits from Sequence, instead of iterating manually:

if bugs.contains("Mosquito") {
    print("Break out the bug spray.")
} else {
    print("Whew, no mosquitos!")
// Prints "Whew, no mosquitos!"

Repeated Access

The Sequence protocol makes no requirement on conforming types regarding whether they will be destructively consumed by iteration. As a consequence, don't assume that multiple for-in loops on a sequence will either resume iteration or restart from the beginning:

for element in sequence {
    if ... some condition { break }

for element in sequence {
    // No defined behavior

In this case, you cannot assume either that a sequence will be consumable and will resume iteration, or that a sequence is a collection and will restart iteration from the first element. A conforming sequence that is not a collection is allowed to produce an arbitrary sequence of elements in the second for-in loop.

To establish that a type you've created supports nondestructive iteration, add conformance to the Collection protocol.

Conforming to the Sequence Protocol

Making your own custom types conform to Sequence enables many useful operations, like for-in looping and the contains method, without much effort. To add Sequence conformance to your own custom type, add a makeIterator() method that returns an iterator.

Alternatively, if your type can act as its own iterator, implementing the requirements of the IteratorProtocol protocol and declaring conformance to both Sequence and IteratorProtocol are sufficient.

Here's a definition of a Countdown sequence that serves as its own iterator. The makeIterator() method is provided as a default implementation.

struct Countdown: Sequence, IteratorProtocol {
    var count: Int

    mutating func next() -> Int? {
        if count == 0 {
            return nil
        } else {
            defer { count -= 1 }
            return count

let threeToGo = Countdown(count: 3)
for i in threeToGo {
// Prints "3"
// Prints "2"
// Prints "1"

Expected Performance

A sequence should provide its iterator in O(1). The Sequence protocol makes no other requirements about element access, so routines that traverse a sequence should be considered O(n) unless documented otherwise.

Instance Variables

var underestimatedCount Required

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.


var underestimatedCount: Int

Instance Methods

func makeIterator() -> Self.Iterator Required

Returns an iterator over the elements of this sequence.


func makeIterator() -> Self.Iterator
func withContiguousStorageIfAvailable(_ body: (UnsafeBufferPointer<Self.Element>) throws -> R) rethrows -> R? Required

Call body(p), where p is a pointer to the collection's contiguous storage. If no such storage exists, it is first created. If the collection does not support an internal representation in a form of contiguous storage, body is not called and nil is returned.

A Collection that provides its own implementation of this method must also guarantee that an equivalent buffer of its SubSequence can be generated by advancing the pointer by the distance to the slice's startIndex.


func withContiguousStorageIfAvailable<R>(_ body: (UnsafeBufferPointer<Self.Element>) throws -> R) rethrows -> R?