Generic Structure

ArraySlice

A slice of an Array, ContiguousArray, or ArraySlice instance.

Overview

The ArraySlice type makes it fast and efficient for you to perform operations on sections of a larger array. Instead of copying over the elements of a slice to new storage, an ArraySlice instance presents a view onto the storage of a larger array. And because ArraySlice presents the same interface as Array, you can generally perform the same operations on a slice as you could on the original array.

For more information about using arrays, see Array and ContiguousArray, with which ArraySlice shares most properties and methods.

Slices Are Views onto Arrays

For example, suppose you have an array holding the number of absences from each class during a session.

let absences = [0, 2, 0, 4, 0, 3, 1, 0]

You want to compare the absences in the first half of the session with those in the second half. To do so, start by creating two slices of the absences array.

let midpoint = absences.count / 2

let firstHalf = absences[..<midpoint]
let secondHalf = absences[midpoint...]

Neither the firstHalf nor secondHalf slices allocate any new storage of their own. Instead, each presents a view onto the storage of the absences array.

You can call any method on the slices that you might have called on the absences array. To learn which half had more absences, use the reduce(_:_:) method to calculate each sum.

let firstHalfSum = firstHalf.reduce(0, +)
let secondHalfSum = secondHalf.reduce(0, +)

if firstHalfSum > secondHalfSum {
    print("More absences in the first half.")
} else {
    print("More absences in the second half.")
}
// Prints "More absences in the first half."

Slices Maintain Indices

Unlike Array and ContiguousArray, the starting index for an ArraySlice instance isn’t always zero. Slices maintain the same indices of the larger array for the same elements, so the starting index of a slice depends on how it was created, letting you perform index-based operations on either a full array or a slice.

Sharing indices between collections and their subsequences is an important part of the design of Swift’s collection algorithms. Suppose you are tasked with finding the first two days with absences in the session. To find the indices of the two days in question, follow these steps:

  1. Call index(where:) to find the index of the first element in the absences array that is greater than zero.

  2. Create a slice of the absences array starting after the index found in step 1.

  3. Call index(where:) again, this time on the slice created in step 2. Where in some languages you might pass a starting index into an indexOf method to find the second day, in Swift you perform the same operation on a slice of the original array.

  4. Print the results using the indices found in steps 1 and 3 on the original absences array.

Here’s an implementation of those steps:

if let i = absences.index(where: { $0 > 0 }) {                      // 1
    let absencesAfterFirst = absences[(i + 1)...]                   // 2
    if let j = absencesAfterFirst.index(where: { $0 > 0 }) {        // 3
        print("The first day with absences had \(absences[i]).")    // 4
        print("The second day with absences had \(absences[j]).")
    }
}
// Prints "The first day with absences had 2."
// Prints "The second day with absences had 4."

In particular, note that j, the index of the second day with absences, was found in a slice of the original array and then used to access a value in the original absences array itself.

Topics

Comparing Array Slices

func ==<Element>(ArraySlice<Element>, ArraySlice<Element>)

Returns true if these arrays contain the same elements.

func !=<Element>(ArraySlice<Element>, ArraySlice<Element>)

Returns true if the arrays do not contain the same elements.

Initializers

init()

Creates a new, empty array.

init<S>(S)

Creates a new instance of a collection containing the elements of a sequence.

init<S>(S)

Creates an array containing the elements of a sequence.

init(arrayLiteral: ArraySlice.Element...)

Creates an array from the given array literal.

init(repeating: Element, count: Int)

Creates a new collection containing the specified number of a single, repeated value.

init(repeating: ArraySlice.Element, count: Int)

Creates a new array containing the specified number of a single, repeated value.

Instance Properties

var capacity: Int

The total number of elements that the array can contain without allocating new storage.

var count: Int

The number of elements in the collection.

var count: Int

The number of elements in the array.

var customMirror: Mirror

A mirror that reflects the array.

var debugDescription: String

A textual representation of the array and its elements, suitable for debugging.

var description: String

A textual representation of the array and its elements.

var endIndex: Int

The array’s “past the end” position—that is, the position one greater than the last valid subscript argument.

var first: Element?

The first element of the collection.

var indices: DefaultRandomAccessIndices<ArraySlice<Element>>

The indices that are valid for subscripting the collection, in ascending order.

var indices: CountableRange<Int>

The indices that are valid for subscripting the collection, in ascending order.

var isEmpty: Bool

A Boolean value indicating whether the collection is empty.

var last: Element?

The last element of the collection.

var lazy: LazySequence<ArraySlice<Element>>

A sequence containing the same elements as this sequence, but on which some operations, such as map and filter, are implemented lazily.

var lazy: LazyCollection<ArraySlice<Element>>

A view onto this collection that provides lazy implementations of normally eager operations, such as map and filter.

var lazy: LazyBidirectionalCollection<ArraySlice<Element>>

A view onto this collection that provides lazy implementations of normally eager operations, such as map and filter.

var lazy: LazyRandomAccessCollection<ArraySlice<Element>>

A view onto this collection that provides lazy implementations of normally eager operations, such as map and filter.

var startIndex: Int

The position of the first element in a nonempty array.

var underestimatedCount: Int

Returns a value less than or equal to the number of elements in the sequence, nondestructively.

var underestimatedCount: Int

A value less than or equal to the number of elements in the collection.

Instance Methods

func append(Element)

Adds an element to the end of the collection.

func append(ArraySlice.Element)

Adds a new element at the end of the array.

func append<S>(contentsOf: S)

Adds the elements of a sequence or collection to the end of this collection.

func contains(where: (Element) -> Bool)

Returns a Boolean value indicating whether the sequence contains an element that satisfies the given predicate.

func distance(from: Int, to: Int)

Returns the distance between two indices.

func drop(while: (Element) -> Bool)

Returns a subsequence by skipping elements while predicate returns true and returning the remaining elements.

func dropFirst()

Returns a subsequence containing all but the first element of the sequence.

func dropFirst(Int)

Returns a subsequence containing all but the given number of initial elements.

func dropLast()

Returns a subsequence containing all but the last element of the sequence.

func dropLast(Int)

Returns a subsequence containing all but the specified number of final elements.

func elementsEqual<OtherSequence>(OtherSequence)

Returns a Boolean value indicating whether this sequence and another sequence contain the same elements in the same order.

func elementsEqual<OtherSequence>(OtherSequence, by: (Element, Element) -> Bool)

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.

func enumerated()

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

func first(where: (Element) -> Bool)

Returns the first element of the sequence that satisfies the given predicate.

func forEach((Element) -> Void)

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

func index(Int, offsetBy: Int)

Returns an index that is the specified distance from the given index.

func index(Int, offsetBy: Int, limitedBy: Int)

Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index.

func index(of: Element)

Returns the first index where the specified value appears in the collection.

func index(where: (Element) -> Bool)

Returns the first index in which an element of the collection satisfies the given predicate.

func insert(Element, at: Int)

Inserts a new element into the collection at the specified position.

func insert(ArraySlice.Element, at: Int)

Inserts a new element at the specified position.

func insert<C>(contentsOf: C, at: Int)

Inserts the elements of a sequence into the collection at the specified position.

func joined()

Returns the elements of this collection of collections, concatenated.

func joined()

Returns the elements of this collection of collections, concatenated.

func joined()

Returns the elements of this sequence of sequences, concatenated.

func joined<Separator>(separator: Separator)

Returns the concatenated elements of this sequence of sequences, inserting the given separator between each element.

func joined(separator: String)

Returns a new string by concatenating the elements of the sequence, adding the given separator between each element.

func lexicographicallyPrecedes<OtherSequence>(OtherSequence)

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the less-than operator (<) to compare elements.

func lexicographicallyPrecedes<OtherSequence>(OtherSequence, by: (Element, Element) -> Bool)

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the given predicate to compare elements.

func makeIterator()

Returns an iterator over the elements of the collection.

func map<T>((Element) -> T)

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

func map<T>((Element) -> T)

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

func max()

Returns the maximum element in the sequence.

func max(by: (Element, Element) -> Bool)

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

func min()

Returns the minimum element in the sequence.

func min(by: (Element, Element) -> Bool)

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

func popFirst()

Removes and returns the first element of the collection.

func popLast()

Removes and returns the last element of the collection.

func prefix(Int)

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

func prefix(through: Int)

Returns a subsequence from the start of the collection through the specified position.

func prefix(upTo: Int)

Returns a subsequence from the start of the collection up to, but not including, the specified position.

func prefix(while: (Element) -> Bool)

Returns a subsequence containing the initial elements until predicate returns false and skipping the remaining elements.

func reduce<Result>(Result, (Result, Element) -> Result)

Returns the result of combining the elements of the sequence using the given closure.

func reduce<Result>(into: Result, (inout Result, Element) -> ())

Returns the result of combining the elements of the sequence using the given closure.

func remove(at: Int)

Removes and returns the element at the specified position.

func remove(at: Int)

Removes and returns the element at the specified position.

func removeAll(keepingCapacity: Bool)

Removes all elements from the collection.

func removeAll(keepingCapacity: Bool)

Removes all elements from the array.

func removeFirst()

Removes and returns the first element of the collection.

func removeFirst()

Removes and returns the first element of the collection.

func removeFirst()

Removes and returns the first element of the collection.

func removeFirst(Int)

Removes the specified number of elements from the beginning of the collection.

func removeFirst(Int)

Removes the specified number of elements from the beginning of the collection.

func removeFirst(Int)

Removes the specified number of elements from the beginning of the collection.

func removeLast()

Removes and returns the last element of the collection.

func removeLast()

Removes and returns the last element of the collection.

func removeLast()

Removes and returns the last element of the collection.

func removeLast(Int)

Removes the specified number of elements from the end of the collection.

func removeLast(Int)

Removes the given number of elements from the end of the collection.

func removeLast(Int)

Removes the specified number of elements from the end of the collection.

func removeSubrange(Range<Int>)

Removes the elements in the specified subrange from the collection.

func removeSubrange<R>(R)

Removes the elements in the specified subrange from the collection.

func replaceSubrange<C>(Range<Int>, with: C)

Replaces a range of elements with the elements in the specified collection.

func replaceSubrange<C, R>(R, with: C)

Replaces the specified subrange of elements with the given collection.

func reserveCapacity(Int)

Reserves enough space to store the specified number of elements.

func reserveCapacity(Int)

Prepares the collection to store the specified number of elements, when doing so is appropriate for the underlying type.

func reversed()

Returns a view presenting the elements of the collection in reverse order.

func sort()

Sorts the collection in place.

func sort(by: (Element, Element) -> Bool)

Sorts the collection in place, using the given predicate as the comparison between elements.

func sorted()

Returns the elements of the sequence, sorted.

func sorted()

Returns the elements of the collection, sorted.

func sorted(by: (Element, Element) -> Bool)

Returns the elements of the collection, sorted using the given predicate as the comparison between elements.

func split(maxSplits: Int, omittingEmptySubsequences: Bool, whereSeparator: (Element) -> Bool)

Returns the longest possible subsequences of the collection, in order, that don’t contain elements satisfying the given predicate.

func split(separator: Element, maxSplits: Int, omittingEmptySubsequences: Bool)

Returns the longest possible subsequences of the sequence, in order, around elements equal to the given element.

func split(separator: Element, maxSplits: Int, omittingEmptySubsequences: Bool)

Returns the longest possible subsequences of the collection, in order, around elements equal to the given element.

func starts<PossiblePrefix>(with: PossiblePrefix)

Returns a Boolean value indicating whether the initial elements of the sequence are the same as the elements in another sequence.

func starts<PossiblePrefix>(with: PossiblePrefix, by: (Element, Element) -> Bool)

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.

func suffix(Int)

Returns a subsequence, up to the given maximum length, containing the final elements of the collection.

func suffix(from: Int)

Returns a subsequence from the specified position to the end of the collection.

func swapAt(Int, Int)

Exchanges the values at the specified indices of the collection.

func withUnsafeBufferPointer<R>((UnsafeBufferPointer<ArraySlice.Element>) -> R)

Calls a closure with a pointer to the array’s contiguous storage.

func withUnsafeBytes<R>((UnsafeRawBufferPointer) -> R)

Calls the given closure with a pointer to the underlying bytes of the array’s contiguous storage.

func withUnsafeMutableBufferPointer<R>((inout UnsafeMutableBufferPointer<ArraySlice.Element>) -> R)

Calls the given closure with a pointer to the array’s mutable contiguous storage.

func withUnsafeMutableBytes<R>((UnsafeMutableRawBufferPointer) -> R)

Calls the given closure with a pointer to the underlying bytes of the array’s mutable contiguous storage.

Subscripts

subscript(Int)

Accesses the element at the specified position.

subscript(Range<Int>)

Accesses a contiguous subrange of the collection’s elements.

subscript(Range<Int>)

Accesses a contiguous subrange of the array’s elements.

See Also

Related Array Types

struct ContiguousArray

A contiguously stored array.