Structure

String.UTF8View

A view of a string’s contents as a collection of UTF-8 code units.

Overview

You can access a string’s view of UTF-8 code units by using its utf8 property. A string’s UTF-8 view encodes the string’s Unicode scalar values as 8-bit integers.

let flowers = "Flowers 💐"
for v in flowers.utf8 {
    print(v)
}
// 70
// 108
// 111
// 119
// 101
// 114
// 115
// 32
// 240
// 159
// 146
// 144

A string’s Unicode scalar values can be up to 21 bits in length. To represent those scalar values using 8-bit integers, more than one UTF-8 code unit is often required.

let flowermoji = "💐"
for v in flowermoji.unicodeScalars {
    print(v, v.value)
}
// 💐 128144

for v in flowermoji.utf8 {
    print(v)
}
// 240
// 159
// 146
// 144

In the encoded representation of a Unicode scalar value, each UTF-8 code unit after the first is called a continuation byte.

UTF8View Elements Match Encoded C Strings

Swift streamlines interoperation with C string APIs by letting you pass a String instance to a function as an Int8 or UInt8 pointer. When you call a C function using a String, Swift automatically creates a buffer of UTF-8 code units and passes a pointer to that buffer. The code units of that buffer match the code units in the string’s utf8 view.

The following example uses the C strncmp function to compare the beginning of two Swift strings. The strncmp function takes two const char* pointers and an integer specifying the number of characters to compare. Because the strings are identical up to the 14th character, comparing only those characters results in a return value of 0.

let s1 = "They call me 'Bell'"
let s2 = "They call me 'Stacey'"

print(strncmp(s1, s2, 14))
// Prints "0"
print(String(s1.utf8.prefix(14)))
// Prints "They call me '"

Extending the compared character count to 15 includes the differing characters, so a nonzero result is returned.

print(strncmp(s1, s2, 15))
// Prints "-17"
print(String(s1.utf8.prefix(15)))
// Prints "They call me 'B"

Topics

Instance Properties

var count: Int

The number of elements in the collection.

var customMirror: Mirror

Returns a mirror that reflects the UTF-8 view of a string.

var endIndex: String.UTF8View.Index

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

var first: UInt8?

The first element of the collection.

var indices: DefaultIndices<String.UTF8View>

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: UInt8?

The last element of the collection.

var lazy: LazySequence<String.UTF8View>

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<String.UTF8View>

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

var startIndex: String.UTF8View.Index

The position of the first code unit if the UTF-8 view is nonempty.

var underestimatedCount: Int

A value less than or equal to the number of elements in the sequence, calculated nondestructively.

var underestimatedCount: Int

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

Instance Methods

func allSatisfy((UInt8) -> Bool) -> Bool

Returns a Boolean value indicating whether every element of a sequence satisfies a given predicate.

Beta
func compactMap<ElementOfResult>((UInt8) -> ElementOfResult?) -> [ElementOfResult]

Returns an array containing the non-nil results of calling the given transformation with each element of this sequence.

func contains(UInt8) -> Bool

Returns a Boolean value indicating whether the sequence contains the given element.

func contains(where: (UInt8) -> Bool) -> Bool

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

func drop(while: (UInt8) -> Bool) -> Substring.UTF8View

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

func dropFirst() -> Substring.UTF8View

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

func dropFirst(Int) -> Substring.UTF8View

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

func dropLast() -> Substring.UTF8View

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

func dropLast(Int) -> Substring.UTF8View

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

func elementsEqual<OtherSequence>(OtherSequence) -> Bool

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

func elementsEqual<OtherSequence>(OtherSequence, by: (UInt8, OtherSequence.Element) -> Bool) -> 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() -> EnumeratedSequence<String.UTF8View>

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 filter((UInt8) -> Bool) -> [UInt8]

Returns an array containing, in order, the elements of the sequence that satisfy the given predicate.

func first(where: (UInt8) -> Bool) -> UInt8?

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

func firstIndex(of: UInt8) -> String.Index?

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

Beta
func firstIndex(where: (UInt8) -> Bool) -> String.Index?

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

Beta
func forEach((UInt8) -> Void)

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

func formIndex(inout String.Index, offsetBy: Int)

Offsets the given index by the specified distance.

func formIndex(inout String.Index, offsetBy: Int, limitedBy: String.Index) -> Bool

Offsets the given index by the specified distance, or so that it equals the given limiting index.

func formIndex(after: inout String.Index)

Replaces the given index with its successor.

func formIndex(before: inout String.Index)

Replaces the given index with its predecessor.

func index(String.Index, offsetBy: Int) -> String.Index

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

func index(String.Index, offsetBy: Int, limitedBy: String.Index) -> String.Index?

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

func index(after: String.UTF8View.Index) -> String.UTF8View.Index

Returns the next consecutive position after i.

func last(where: (UInt8) -> Bool) -> UInt8?

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

Beta
func lastIndex(of: UInt8) -> String.Index?

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

Beta
func lastIndex(where: (UInt8) -> Bool) -> String.Index?

Returns the index of the last element in the collection that matches the given predicate.

Beta
func lexicographicallyPrecedes<OtherSequence>(OtherSequence) -> Bool

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: (UInt8, UInt8) -> Bool) -> 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 map<T>((UInt8) -> T) -> [T]

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

Beta
func map<T>((UInt8) -> T) -> [T]

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

Beta
func max() -> UInt8?

Returns the maximum element in the sequence.

func max(by: (UInt8, UInt8) -> Bool) -> UInt8?

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

func min() -> UInt8?

Returns the minimum element in the sequence.

func min(by: (UInt8, UInt8) -> Bool) -> UInt8?

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

func prefix(Int) -> Substring.UTF8View

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

func prefix(through: String.Index) -> Substring.UTF8View

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

func prefix(upTo: String.Index) -> Substring.UTF8View

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

func prefix(while: (UInt8) -> Bool) -> Substring.UTF8View

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

func randomElement() -> UInt8?

Returns a random element of the collection.

Beta
func randomElement<T>(using: inout T) -> UInt8?

Returns a random element of the collection, using the given generator as a source for randomness.

Beta
func reduce<Result>(Result, (Result, UInt8) -> Result) -> Result

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

func reduce<Result>(into: Result, (inout Result, UInt8) -> ()) -> Result

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

func reversed() -> ReversedCollection<String.UTF8View>

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

func shuffled() -> [UInt8]

Returns the elements of the sequence, shuffled.

Beta
func shuffled<T>(using: inout T) -> [UInt8]

Returns the elements of the sequence, shuffled using the given generator as a source for randomness.

Beta
func sorted() -> [UInt8]

Returns the elements of the sequence, sorted.

func sorted(by: (UInt8, UInt8) -> Bool) -> [UInt8]

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

func split(maxSplits: Int, omittingEmptySubsequences: Bool, whereSeparator: (UInt8) -> Bool) -> [Substring.UTF8View]

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

func split(separator: UInt8, maxSplits: Int, omittingEmptySubsequences: Bool) -> [Substring.UTF8View]

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

func starts<PossiblePrefix>(with: PossiblePrefix) -> Bool

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: (UInt8, PossiblePrefix.Element) -> Bool) -> 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.

Beta
func suffix(Int) -> Substring.UTF8View

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

func suffix(from: String.Index) -> Substring.UTF8View

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

Subscripts

Relationships

From Protocol

See Also

Related String Types

struct Substring

A slice of a string.

protocol StringProtocol

A type that can represent a string as a collection of characters.

struct String.Index

A position of a character or code unit in a string.

struct String.UnicodeScalarView

A view of a string’s contents as a collection of Unicode scalar values.

struct String.UTF16View

A view of a string’s contents as a collection of UTF-16 code units.

Beta Software

This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software.

Learn more about using Apple's beta software