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Strings e Caracteres

Uma string é uma sequência de caracteres, como por exemplo: "hello, world" ou "albatross". Strings no Swift são representadas pelo tipo String. Os conteúdos de uma String podem ser acessados de várias maneiras, inclusive como uma lista de valores do tipo Character.

Os tipos String e Character do Swift fornecem uma maneira rápida, compatível com Unicode de trabalhar com textos no seu código. A sintaxe para criação e manipulação de uma string é descomplicada e legível, com uma sintaxe literal de string que é similar à do C. Concatenação de strings é tão simples quanto combinar duas strings com o operador +, e a mutabilidade de uma string é gerenciada pela escolha entre uma constante ou uma variável, assim como qualquer outro valor em Swift. Você também pode usar strings para a inserção de constantes, variáveis, literais e expressões formando strings maiores, em um processo conhecido como interpolação de strings. Isso facilita a criação de valores de string personalizados para exibição, armazenamento e prints.

Apesar da simplicidade da sintaxe, o tipo String do Swift é uma implementação rápida e moderna de string. Toda string é composta por caracteres Unicode de codificação independente e fornece suporte para acessar esses caracteres em várias representações Unicode.

Nota

O tipo String do Swift é ligado com a classe NSString do Foundation. Foundation também estende String para expor métodos definidos pelo NSString. Isso significa, se você importar o Foundation, poderá acessar esses métodos de NSString com String sem "casting". Para mais informações sobre o uso de String com Foundation e Cocoa, veja Bridging Between String and NSString.

Literais de String

Você pode incluir valores String predefinidos em seu código como literais de string. Um literal de string é uma sequência de caracteres entre aspas duplas (").

Use um literal de string como valor inicial para uma constante ou variável:

let someString = "Some string literal value"

Note que Swift infere um tipo de String para a constante someString porque ela é inicializada com um valor literal de string.

Literais de String Multilinha

Se você precisar de uma string que se estenda por várias linhas, use um literal de string multilinha—uma sequência de caracteres entre três aspas duplas:

let quotation = """
The White Rabbit put on his spectacles. "Where shall I
      begin,
please your Majesty?" he asked.

"Begin at the beginning," the King said gravely, "and go on
till you come to the end; then stop."
"""

Um literal de string multilinha inclui todas as linhas entre as aspas de abertura e fechamento. A string começa na primeira linha após as aspas de abertura (""") e termina na linha antes das aspas de fechamento, o que significa que nenhuma das strings abaixo começa ou termina com uma quebra de linha:

let singleLineString = "These are the same."
let multilineString = """
These are the same.
"""

Quando seu código-fonte inclui uma quebra de linha dentro de um literal de string multilinha, essa quebra de linha também aparece no valor da string. Se você quer usar quebras de linha para tornar seu código-fonte mais fácil de ler, mas não quer que elas façam parte do valor da string, escreva uma barra invertida (\) no final dessas linhas:

let softWrappedQuotation = """
The White Rabbit put on his spectacles.  "Where shall I
      begin, \
please your Majesty?" he asked.

"Begin at the beginning," the King
      said gravely, "and go on \
till you come to the end; then
      stop."
"""

Para fazer uma string literal multilinha que começa ou termina com uma quebra de linha, escreva uma linha em branco como a primeira ou a última linha. Por exemplo:

let lineBreaks = """

This string starts with a line break.
It also ends with a line break.

"""

Uma string multilinha pode ser identada para corresponder ao código ao redor. O espaço em branco antes das aspas de fechamento (""") informa qual espaço deve ser ignorado antes de todas as outras linhas. No entanto, se você escrever um espaço em branco no início de uma linha além do que está antes das aspas de fechamento, esse espaço em branco é incluído.

../_images/multilineStringWhitespace_2x.png

No exemplo acima, embora todo o literal de string multilinha esteja identado, a primeira e a última linhas não começam com espaço em branco. A linha do meio tem mais identação do que as aspas de fechamento, portanto, ela começa com esse recuo extra de quatro espaços.

Caracteres Especiais em Literais de String

Literais de string podem incluir os seguintes caracteres especiais:

  • Os caracteres especiais de escape \0 (caractere nulo), \\ (barra invertida), \t (tab horizontal), \n (quebra de linha), \r (carriage return), \" (aspas duplas) e \' (aspas simples)

  • Um valor escalar Unicode arbitrário, escrito como \u{n}, onde n é um número hexadecimal de 1–8 dígitos (Unicode é discutido em Unicode abaixo)

O código a seguir mostra quatro exemplos desses caracteres especiais. A constante wiseWords contém duas aspas duplas escapadas. As constantes dollarSign, blackHeart e sparklingHeart demonstram o formato escalar Unicode:

let wiseWords = "\"Imagination is more important than knowledge\" - Einstein"
// "Imagination is more important than knowledge" - Einstein
let dollarSign = "\u{24}" // $, Unicode scalar U+0024
let blackHeart = "\u{2665}" // ♥, Unicode scalar U+2665
let sparklingHeart = "\u{1F496}" // 💖, Unicode scalar U+1F496

Como os literais de string multilinha usam três aspas duplas em vez de apenas uma, você pode inserir aspa dupla (") dentro da string sem escapá-la. Para incluir o texto """ em uma string multilinha, escape pelo menos uma das aspas. Por exemplo:

let threeDoubleQuotationMarks = """
Escaping the first quotation mark \"""
Escaping all three quotation marks \"\"\"
"""

Delimitadores Estendidos de String

Você pode colocar um literal de string dentro de delimitadores estendidos para incluir caracteres especiais em uma string sem invocar seu efeito. Você coloca sua string entre aspas (") e envolve-a com sinais numéricos (#). Por exemplo, imprimir a string literal #"Linha 1\nLinha 2"# imprime a sequência de caracteres da quebra de linha (\n) em vez de imprimir a string em duas linhas.

Se você precisar dos efeitos especiais de um caractere em uma literal de string, adicione sinais numéricos depois do caractere de escape (\), de forma que a quantidade seja a mesma que a de dentro da string. Por exemplo, se sua string é #"Linha 1\nLinha 2"# e você deseja quebrar a linha, você pode usar #"Linha 1\#nLinha 2"# em seu lugar. Da mesma forma, ###"Line1\###nLine2"### também quebra a linha.

Literais de string criadas usando delimitadores estendidos também podem ser literais de string multilinha. Você pode usar delimitadores estendidos para incluir o texto """ em uma string multilinha, substituindo o comportamento padrão que termina o literal. Por exemplo:

let threeMoreDoubleQuotationMarks = #"""
Here are three more double quotes: """
"""#

Inicializando uma String Vazia

Para criar uma String vazia como ponto de partida para construir uma string mais longa, atribua um literal de string vazia a uma variável ou inicialize uma nova instância de String com a sintaxe do inicializador:

var emptyString = ""        // literal de string vazia
var anotherEmptyString = String()     // sintaxe do inicializador
// essas duas strings estão vazias e são equivalentes uma à outra

Descubra se um valor String está vazio verificando sua propriedade booleana isEmpty:

if emptyString.isEmpty {
  print("Nada para ver aqui")
}
// Prints "Nada para ver aqui"

Mutabilidade da String

Você indica se uma determinada String pode ser modificada (ou mutada) atribuindo-a a uma variável (nesse caso, ela pode ser modificada) ou a uma constante (neste caso, não pode ser modificada):

var variableString = "Horse"
variableString += " and carriage"
// variableString agora é "Horse and carriage"

let constantString = "Highlander"
constantString += " and another Highlander"
// isso reporta um erro em tempo de compilação - uma string constante não pode ser modificada

Nota

Esta abordagem é diferente da mutação de string em Objective-C e Cocoa, onde você escolhe entre duas classes (NSString e NSMutableString) para indicar se uma string pode sofrer mutação.

Strings São Value Types

O tipo String é um value type. Se você criar um novo valor String, esse valor String será copiado quando for passado para uma função ou método, ou quando for atribuído a uma constante ou variável. Em cada caso, uma nova cópia do valor String existente é criada e a nova cópia é passada ou atribuída, não a versão original. Os value types são descritos em Struturas e Enumerações são Value Types.

O comportamento copy-by-default da String no Swift garante que quando uma função ou método passa um valor String, fica claro que você possui aquele valor String exato, independentemente de onde ele veio. Você pode ter certeza de que a string passada não será modificada, a menos que você mesmo a modifique.

Nos bastidores, o compilador do Swift otimiza o uso de string para que a cópia real ocorra apenas quando for absolutamente necessário. Isso significa que você sempre obtém um ótimo desempenho ao trabalhar com strings como value types.

Working with Characters

You can access the individual Character values for a String by iterating over the string with a for-in loop:

for character in "Dog!🐶" {
print(character)
}
// D
// o
// g
// !
// 🐶

The for-in loop is described in For-In Loops.

Alternatively, you can create a stand-alone Character constant or variable from a single-character string literal by providing a Character type annotation:

let exclamationMark: Character = "!"

String values can be constructed by passing an array of Character values as an argument to its initializer:

let catCharacters: [Character] = \["C", "a", "t", "!", "🐱"]
let catString = String(catCharacters)
print(catString)
// Prints "Cat!🐱"

Concatenating Strings and Characters

String values can be added together (or concatenated) with the addition operator (+) to create a new String value:

let string1 = "hello"
let string2 = " there"
var welcome = string1 + string2
// welcome now equals "hello there"

You can also append a String value to an existing String variable with the addition assignment operator (+=):

var instruction = "look over"
instruction += string2
// instruction now equals "look over there"

You can append a Character value to a String variable with the String type’s append() method:

let exclamationMark: Character = "!"
welcome.append(exclamationMark)
// welcome now equals "hello there!"

Note

You can’t append a String or Character to an existing Character variable, because a Character value must contain a single character only.

If you’re using multiline string literals to build up the lines of a longer string, you want every line in the string to end with a line break, including the last line. For example:

let badStart = """
one
two
"""
let end = """
three
"""
print(badStart + end)
// Prints two lines:
// one
// twothree

let goodStart = """
one
two

"""
print(goodStart + end)
// Prints three lines:
// one
// two
// three

In the code above, concatenating badStart with end produces a two-line string, which isn’t the desired result. Because the last line of badStart doesn’t end with a line break, that line gets combined with the first line of end. In contrast, both lines of goodStart end with a line break, so when it’s combined with end the result has three lines, as expected.

String Interpolation

String interpolation is a way to construct a new String value from a mix of constants, variables, literals, and expressions by including their values inside a string literal. You can use string interpolation in both single-line and multiline string literals. Each item that you insert into the string literal is wrapped in a pair of parentheses, prefixed by a backslash (\):

let multiplier = 3
let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"
// message is "3 times 2.5 is 7.5"

In the example above, the value of multiplier is inserted into a string literal as \(multiplier). This placeholder is replaced with the actual value of multiplier when the string interpolation is evaluated to create an actual string.

The value of multiplier is also part of a larger expression later in the string. This expression calculates the value of Double(multiplier) * 2.5 and inserts the result (7.5) into the string. In this case, the expression is written as \(Double(multiplier) * 2.5) when it’s included inside the string literal.

You can use extended string delimiters to create strings containing characters that would otherwise be treated as a string interpolation. For example:

print(#"Write an interpolated string in Swift using \(multiplier)."#)
// Prints "Write an interpolated string in Swift using \(multiplier)."

To use string interpolation inside a string that uses extended delimiters, match the number of number signs before the backslash to the number of number signs at the beginning and end of the string. For example:

print(#"6 times 7 is \#(6 * 7)."#)
// Prints "6 times 7 is 42."

Note

The expressions you write inside parentheses within an interpolated string can’t contain an unescaped backslash (\), a carriage return, or a line feed. However, they can contain other string literals.

Unicode

Unicode is an international standard for encoding, representing, and processing text in different writing systems. It enables you to represent almost any character from any language in a standardized form, and to read and write those characters to and from an external source such as a text file or web page. Swift’s String and Character types are fully Unicode-compliant, as described in this section.

Unicode Scalar Values

Behind the scenes, Swift’s native String type is built from Unicode scalar values. A Unicode scalar value is a unique 21-bit number for a character or modifier, such as U+0061 for LATIN SMALL LETTER A ("a"), or U+1F425 for FRONT-FACING BABY CHICK ("🐥").

Note that not all 21-bit Unicode scalar values are assigned to a character—some scalars are reserved for future assignment or for use in UTF-16 encoding. Scalar values that have been assigned to a character typically also have a name, such as LATIN SMALL LETTER A and FRONT-FACING BABY CHICK in the examples above.

Extended Grapheme Clusters

Every instance of Swift’s Character type represents a single extended grapheme cluster. An extended grapheme cluster is a sequence of one or more Unicode scalars that (when combined) produce a single human-readable character.

Here’s an example. The letter é can be represented as the single Unicode scalar é (LATIN SMALL LETTER E WITH ACUTE, or U+00E9). However, the same letter can also be represented as a pair of scalars—a standard letter e (LATIN SMALL LETTER E, or U+0065), followed by the COMBINING ACUTE ACCENT scalar (U+0301). The COMBINING ACUTE ACCENT scalar is graphically applied to the scalar that precedes it, turning an e into an é when it’s rendered by a Unicode-aware text-rendering system.

In both cases, the letter é is represented as a single Swift Character value that represents an extended grapheme cluster. In the first case, the cluster contains a single scalar; in the second case, it’s a cluster of two scalars:

let eAcute: Character = "\u{E9}"    // é
let combinedEAcute: Character = "\u{65}\u{301}"      // e followed by ́
// eAcute is é, combinedEAcute is é

Extended grapheme clusters are a flexible way to represent many complex script characters as a single Character value. For example, Hangul syllables from the Korean alphabet can be represented as either a precomposed or decomposed sequence. Both of these representations qualify as a single Character value in Swift:

let precomposed: Character = "\u{D55C}"   // 한
let decomposed: Character = "\u{1112}\u{1161}\u{11AB}"    // ᄒ, ᅡ, ᆫ
// precomposed is 한, decomposed is 한

Extended grapheme clusters enable scalars for enclosing marks (such as COMBINING ENCLOSING CIRCLE, or U+20DD) to enclose other Unicode scalars as part of a single Character value:

let enclosedEAcute: Character = "\u{E9}\u{20DD}"
// enclosedEAcute is é⃝

Unicode scalars for regional indicator symbols can be combined in pairs to make a single Character value, such as this combination of REGIONAL INDICATOR SYMBOL LETTER U (U+1F1FA) and REGIONAL INDICATOR SYMBOL LETTER S (U+1F1F8):

let regionalIndicatorForUS: Character = "\u{1F1FA}\u{1F1F8}"
// regionalIndicatorForUS is 🇺🇸

Counting Characters

To retrieve a count of the Character values in a string, use the count property of the string:

let unusualMenagerie = "Koala 🐨, Snail 🐌, Penguin 🐧, Dromedary 🐪"
print("unusualMenagerie has \(unusualMenagerie.count) characters")
// Prints "unusualMenagerie has 40 characters"

Note that Swift’s use of extended grapheme clusters for Character values means that string concatenation and modification may not always affect a string’s character count.

For example, if you initialize a new string with the four-character word cafe, and then append a COMBINING ACUTE ACCENT (U+0301) to the end of the string, the resulting string will still have a character count of 4, with a fourth character of é, not e:

var word = "cafe"
print("the number of characters in \(word) is \(word.count)")
// Prints "the number of characters in cafe is 4"

word += "\u{301}" // COMBINING ACUTE ACCENT, U+0301

print("the number of characters in \(word) is \(word.count)")
// Prints "the number of characters in café is 4"

Note

Extended grapheme clusters can be composed of multiple Unicode scalars. This means that different characters—and different representations of the same character—can require different amounts of memory to store. Because of this, characters in Swift don’t each take up the same amount of memory within a string’s representation. As a result, the number of characters in a string can’t be calculated without iterating through the string to determine its extended grapheme cluster boundaries. If you are working with particularly long string values, be aware that the count property must iterate over the Unicode scalars in the entire string in order to determine the characters for that string.
The count of the characters returned by the count property isn’t always the same as the length property of an NSString that contains the same characters. The length of an NSString is based on the number of 16-bit code units within the string’s UTF-16 representation and not the number of Unicode extended grapheme clusters within the string.

Accessing and Modifying a String

You access and modify a string through its methods and properties, or by using subscript syntax.

String Indices

Each String value has an associated index type, String.Index, which corresponds to the position of each Character in the string.

As mentioned above, different characters can require different amounts of memory to store, so in order to determine which Character is at a particular position, you must iterate over each Unicode scalar from the start or end of that String. For this reason, Swift strings can’t be indexed by integer values.

Use the startIndex property to access the position of the first Character of a String. The endIndex property is the position after the last character in a String. As a result, the endIndex property isn’t a valid argument to a string’s subscript. If a String is empty, startIndex and endIndex are equal.

You access the indices before and after a given index using the index(before:) and index(after:) methods of String. To access an index farther away from the given index, you can use the index(_:offsetBy:) method instead of calling one of these methods multiple times.

You can use subscript syntax to access the Character at a particular String index.

let greeting = "Guten Tag!"
greeting[greeting.startIndex]
// G
greeting[greeting.index(before: greeting.endIndex)]
// !
greeting[greeting.index(after: greeting.startIndex)]
// u
let index = greeting.index(greeting.startIndex, offsetBy: 7)
greeting[index]
// a

Attempting to access an index outside of a string’s range or a Character at an index outside of a string’s range will trigger a runtime error.

greeting[greeting.endIndex]    // Error
greeting.index(after: greeting.endIndex)     // Error

Use the indices property to access all of the indices of individual characters in a string.

for index in greeting.indices {
  print("\(greeting[index]) ", terminator: "")
}
// Prints "G u t e n T a g ! "

Note

You can use the startIndex and endIndex properties and the index(before:), index(after:), and index(_:offsetBy:) methods on any type that conforms to the Collection protocol. This includes String, as shown here, as well as collection types such as Array, Dictionary, and Set.

Inserting and Removing

To insert a single character into a string at a specified index, use the insert(_:at:) method, and to insert the contents of another string at a specified index, use the insert(contentsOf:at:) method.

var welcome = "hello"
welcome.insert("!", at: welcome.endIndex)
// welcome now equals "hello!"

welcome.insert(contentsOf: " there", at: welcome.index(before: welcome.endIndex))
// welcome now equals "hello there!"

To remove a single character from a string at a specified index, use the remove(at:) method, and to remove a substring at a specified range, use the removeSubrange(_:) method:

welcome.remove(at: welcome.index(before: welcome.endIndex))
// welcome now equals "hello there"

let range = welcome.index(welcome.endIndex, offsetBy: \-6)..<welcome.endIndex
welcome.removeSubrange(range)
// welcome now equals "hello"

Note

You can use the insert(_:at:), insert(contentsOf:at:), remove(at:), and removeSubrange(_:) methods on any type that conforms to the RangeReplaceableCollection protocol. This includes String, as shown here, as well as collection types such as Array, Dictionary, and Set.

Substrings

When you get a substring from a string—for example, using a subscript or a method like prefix(_:)—the result is an instance of Substring, not another string. Substrings in Swift have most of the same methods as strings, which means you can work with substrings the same way you work with strings. However, unlike strings, you use substrings for only a short amount of time while performing actions on a string. When you’re ready to store the result for a longer time, you convert the substring to an instance of String. For example:

let greeting = "Hello, world!"
let index = greeting.firstIndex(of: ",") ?? greeting.endIndex
let beginning = greeting[..<index]
// beginning is "Hello"

// Convert the result to a String for long-term storage.
let newString = String(beginning)

Like strings, each substring has a region of memory where the characters that make up the substring are stored. The difference between strings and substrings is that, as a performance optimization, a substring can reuse part of the memory that’s used to store the original string, or part of the memory that’s used to store another substring. (Strings have a similar optimization, but if two strings share memory, they are equal.) This performance optimization means you don’t have to pay the performance cost of copying memory until you modify either the string or substring. As mentioned above, substrings aren’t suitable for long-term storage—because they reuse the storage of the original string, the entire original string must be kept in memory as long as any of its substrings are being used.

In the example above, greeting is a string, which means it has a region of memory where the characters that make up the string are stored. Because beginning is a substring of greeting, it reuses the memory that greeting uses. In contrast, newString is a string—when it’s created from the substring, it has its own storage. The figure below shows these relationships:

../_images/stringSubstring_2x.png

Note

Both String and Substring conform to the StringProtocol protocol, which means it’s often convenient for string-manipulation functions to accept a StringProtocol value. You can call such functions with either a String or Substring value.

Comparing Strings

Swift provides three ways to compare textual values: string and character equality, prefix equality, and suffix equality.

String and Character Equality

String and character equality is checked with the “equal to” operator (==) and the “not equal to” operator (!=), as described in Comparison Operators:

let quotation = "We're a lot alike, you and I."
let sameQuotation = "We're a lot alike, you and I."
if quotation == sameQuotation {
print("These two strings are considered equal")
}
// Prints "These two strings are considered equal"

Two String values (or two Character values) are considered equal if their extended grapheme clusters are canonically equivalent. Extended grapheme clusters are canonically equivalent if they have the same linguistic meaning and appearance, even if they’re composed from different Unicode scalars behind the scenes.

For example, LATIN SMALL LETTER E WITH ACUTE (U+00E9) is canonically equivalent to LATIN SMALL LETTER E (U+0065) followed by COMBINING ACUTE ACCENT (U+0301). Both of these extended grapheme clusters are valid ways to represent the character é, and so they’re considered to be canonically equivalent:

// "Voulez-vous un café?" using LATIN SMALL LETTER E WITH ACUTE
let eAcuteQuestion = "Voulez-vous un caf\u{E9}?"

// "Voulez-vous un café?" using LATIN SMALL LETTER E and COMBINING ACUTE ACCENT
let combinedEAcuteQuestion = "Voulez-vous un caf\u{65}\u{301}?"

if eAcuteQuestion == combinedEAcuteQuestion {
print("These two strings are considered equal")
}
// Prints "These two strings are considered equal"

Conversely, LATIN CAPITAL LETTER A (U+0041, or "A"), as used in English, is not equivalent to CYRILLIC CAPITAL LETTER A (U+0410, or "А"), as used in Russian. The characters are visually similar, but don’t have the same linguistic meaning:

let latinCapitalLetterA: Character = "\u{41}"

let cyrillicCapitalLetterA: Character = "\u{0410}"

if latinCapitalLetterA != cyrillicCapitalLetterA {
print("These two characters are not equivalent.")
}
// Prints "These two characters are not equivalent."

Note

String and character comparisons in Swift are not locale-sensitive.

Prefix and Suffix Equality

To check whether a string has a particular string prefix or suffix, call the string’s hasPrefix(_:) and hasSuffix(_:) methods, both of which take a single argument of type String and return a Boolean value.

The examples below consider an array of strings representing the scene locations from the first two acts of Shakespeare’s Romeo and Juliet:

let romeoAndJuliet = [
  "Act 1 Scene 1: Verona, A public place",
  "Act 1 Scene 2: Capulet's mansion",
  "Act 1 Scene 3: A room in Capulet's mansion",
  "Act 1 Scene 4: A street outside Capulet's mansion",
  "Act 1 Scene 5: The Great Hall in Capulet's mansion",
  "Act 2 Scene 1: Outside Capulet's mansion",
  "Act 2 Scene 2: Capulet's orchard",
  "Act 2 Scene 3: Outside Friar Lawrence's cell",
  "Act 2 Scene 4: A street in Verona",
  "Act 2 Scene 5: Capulet's mansion",
  "Act 2 Scene 6: Friar Lawrence's cell"
]

You can use the hasPrefix(_:) method with the romeoAndJuliet array to count the number of scenes in Act 1 of the play:

var act1SceneCount = 0
for scene in romeoAndJuliet {
  if scene.hasPrefix("Act 1 ") {
      act1SceneCount += 1
    }
}
print("There are \(act1SceneCount) scenes in Act 1")
// Prints "There are 5 scenes in Act 1"

Similarly, use the hasSuffix(_:) method to count the number of scenes that take place in or around Capulet’s mansion and Friar Lawrence’s cell:

var mansionCount = 0
var cellCount = 0
for scene in romeoAndJuliet {
  if scene.hasSuffix("Capulet's mansion") {
    mansionCount += 1
  } else if scene.hasSuffix("Friar Lawrence's cell") {
    cellCount += 1
  }
}
print("\(mansionCount) mansion scenes; \(cellCount) cell scenes")
// Prints "6 mansion scenes; 2 cell scenes"

Note

The hasPrefix(_:) and hasSuffix(_:) methods perform a character-by-character canonical equivalence comparison between the extended grapheme clusters in each string, as described in String and Character Equality.

Unicode Representations of Strings

When a Unicode string is written to a text file or some other storage, the Unicode scalars in that string are encoded in one of several Unicode-defined encoding forms. Each form encodes the string in small chunks known as code units. These include the UTF-8 encoding form (which encodes a string as 8-bit code units), the UTF-16 encoding form (which encodes a string as 16-bit code units), and the UTF-32 encoding form (which encodes a string as 32-bit code units).

Swift provides several different ways to access Unicode representations of strings. You can iterate over the string with a for-in statement, to access its individual Character values as Unicode extended grapheme clusters. This process is described in Working with Characters.

Alternatively, access a String value in one of three other Unicode-compliant representations:

  • A collection of UTF-8 code units (accessed with the string’s utf8 property)

  • A collection of UTF-16 code units (accessed with the string’s utf16 property)

  • A collection of 21-bit Unicode scalar values, equivalent to the string’s UTF-32 encoding form (accessed with the string’s unicodeScalars property)

Each example below shows a different representation of the following string, which is made up of the characters D, o, g, ‼ (DOUBLE EXCLAMATION MARK, or Unicode scalar U+203C), and the 🐶 character (DOG FACE, or Unicode scalar U+1F436):

let dogString = "Dog‼🐶"

UTF-8 Representation

You can access a UTF-8 representation of a String by iterating over its utf8 property. This property is of type String.UTF8View, which is a collection of unsigned 8-bit (UInt8) values, one for each byte in the string’s UTF-8 representation:

../_images/UTF8_2x.png

for codeUnit in dogString.utf8 {
  print("\(codeUnit) ", terminator: "")
}
print("")
// Prints "68 111 103 226 128 188 240 159 144 182 "

In the example above, the first three decimal codeUnit values (68, 111, 103) represent the characters D, o, and g, whose UTF-8 representation is the same as their ASCII representation. The next three decimal codeUnit values (226, 128, 188) are a three-byte UTF-8 representation of the DOUBLE EXCLAMATION MARK character. The last four codeUnit values (240, 159, 144, 182) are a four-byte UTF-8 representation of the DOG FACE character.

UTF-16 Representation

You can access a UTF-16 representation of a String by iterating over its utf16 property. This property is of type String.UTF16View, which is a collection of unsigned 16-bit (UInt16) values, one for each 16-bit code unit in the string’s UTF-16 representation:

../_images/UTF16_2x.png

for codeUnit in dogString.utf16 {
  print("\(codeUnit) ", terminator: "")
}
print("")
// Prints "68 111 103 8252 55357 56374 "

Again, the first three codeUnit values (68, 111, 103) represent the characters D, o, and g, whose UTF-16 code units have the same values as in the string’s UTF-8 representation (because these Unicode scalars represent ASCII characters).

The fourth codeUnit value (8252) is a decimal equivalent of the hexadecimal value 203C, which represents the Unicode scalar U+203C for the DOUBLE EXCLAMATION MARK character. This character can be represented as a single code unit in UTF-16.

The fifth and sixth codeUnit values (55357 and 56374) are a UTF-16 surrogate pair representation of the DOG FACE character. These values are a high-surrogate value of U+D83D (decimal value 55357) and a low-surrogate value of U+DC36 (decimal value 56374).

Unicode Scalar Representation

You can access a Unicode scalar representation of a String value by iterating over its unicodeScalars property. This property is of type UnicodeScalarView, which is a collection of values of type UnicodeScalar.

Each UnicodeScalar has a value property that returns the scalar’s 21-bit value, represented within a UInt32 value:

../_images/UnicodeScalar_2x.png

for scalar in dogString.unicodeScalars {
  print("\(scalar.value) ", terminator: "")
}
print("")
// Prints "68 111 103 8252 128054 "

The value properties for the first three UnicodeScalar values (68, 111, 103) once again represent the characters D, o, and g.

The fourth codeUnit value (8252) is again a decimal equivalent of the hexadecimal value 203C, which represents the Unicode scalar U+203C for the DOUBLE EXCLAMATION MARK character.

The value property of the fifth and final UnicodeScalar, 128054, is a decimal equivalent of the hexadecimal value 1F436, which represents the Unicode scalar U+1F436 for the DOG FACE character.

As an alternative to querying their value properties, each UnicodeScalar value can also be used to construct a new String value, such as with string interpolation:

for scalar in dogString.unicodeScalars {
  print("\(scalar) ")
}
// D
// o
// g
// ‼
// 🐶