iOS Developer Library — Prerelease

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The Swift Programming Language

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Strings and Characters

A string is an ordered collection of characters, such as "hello, world" or "albatross". Swift strings are represented by the String type, which in turn represents a collection of values of Character type.

Swift’s String and Character types provide a fast, Unicode-compliant way to work with text in your code. The syntax for string creation and manipulation is lightweight and readable, with a string literal syntax that is similar to C. String concatenation is as simple as adding together two strings with the + operator, and string mutability is managed by choosing between a constant or a variable, just like any other value in Swift.

Despite this simplicity of syntax, Swift’s String type is a fast, modern string implementation. Every string is composed of encoding-independent Unicode characters, and provides support for accessing those characters in various Unicode representations.

You can also use strings to insert constants, variables, literals, and expressions into longer strings, in a process known as string interpolation. This makes it easy to create custom string values for display, storage, and printing.

String Literals

You can include predefined String values within your code as string literals. A string literal is a fixed sequence of textual characters surrounded by a pair of double quotes ("").

Use a string literal as an initial value for a constant or variable:

  1. let someString = "Some string literal value"

Note that Swift infers a type of String for the someString constant, because it is initialized with a string literal value.

Initializing an Empty String

To create an empty String value as the starting point for building a longer string, either assign an empty string literal to a variable, or initialize a new String instance with initializer syntax:

  1. var emptyString = "" // empty string literal
  2. var anotherEmptyString = String() // initializer syntax
  3. // these two strings are both empty, and are equivalent to each other

Find out whether a String value is empty by checking its Boolean isEmpty property:

  1. if emptyString.isEmpty {
  2. print("Nothing to see here")
  3. }
  4. // prints "Nothing to see here"

String Mutability

You indicate whether a particular String can be modified (or mutated) by assigning it to a variable (in which case it can be modified), or to a constant (in which case it cannot be modified):

  1. var variableString = "Horse"
  2. variableString += " and carriage"
  3. // variableString is now "Horse and carriage"
  4. let constantString = "Highlander"
  5. constantString += " and another Highlander"
  6. // this reports a compile-time error - a constant string cannot be modified

Strings Are Value Types

Swift’s String type is a value type. If you create a new String value, that String value is copied when it is passed to a function or method, or when it is assigned to a constant or variable. In each case, a new copy of the existing String value is created, and the new copy is passed or assigned, not the original version. Value types are described in Structures and Enumerations Are Value Types.

Swift’s copy-by-default String behavior ensures that when a function or method passes you a String value, it is clear that you own that exact String value, regardless of where it came from. You can be confident that the string you are passed will not be modified unless you modify it yourself.

Behind the scenes, Swift’s compiler optimizes string usage so that actual copying takes place only when absolutely necessary. This means you always get great performance when working with strings as value types.

Working with Characters

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

  1. for character in "Dog!🐶".characters {
  2. print(character)
  3. }
  4. // D
  5. // o
  6. // g
  7. // !
  8. // 🐶

The for-in loop is described in For Loops.

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

  1. let exclamationMark: Character = "!"

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

  1. let catCharacters: [Character] = ["C", "a", "t", "!", "🐱"]
  2. let catString = String(catCharacters)
  3. print(catString)
  4. // prints "Cat!🐱"

Concatenating Strings and Characters

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

  1. let string1 = "hello"
  2. let string2 = " there"
  3. var welcome = string1 + string2
  4. // welcome now equals "hello there"

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

  1. var instruction = "look over"
  2. instruction += string2
  3. // instruction now equals "look over there"

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

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

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. Each item that you insert into the string literal is wrapped in a pair of parentheses, prefixed by a backslash:

  1. let multiplier = 3
  2. let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"
  3. // 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 is included inside the string literal.

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 Scalars

Behind the scenes, Swift’s native String type is built from Unicode scalar values. A Unicode scalar 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 scalars are assigned to a character—some scalars are reserved for future assignment. Scalars 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.

Special Characters in String Literals

String literals can include the following special characters:

  • The escaped special characters \0 (null character), \\ (backslash), \t (horizontal tab), \n (line feed), \r (carriage return), \" (double quote) and \' (single quote)

  • An arbitrary Unicode scalar, written as \u{n}, where n is a 1–8 digit hexadecimal number with a value equal to a valid Unicode code point

The code below shows four examples of these special characters. The wiseWords constant contains two escaped double quote characters. The dollarSign, blackHeart, and sparklingHeart constants demonstrate the Unicode scalar format:

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

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 is 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 is a cluster of two scalars:

  1. let eAcute: Character = "\u{E9}" // é
  2. let combinedEAcute: Character = "\u{65}\u{301}" // e followed by ́
  3. // 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:

  1. let precomposed: Character = "\u{D55C}" // 한
  2. let decomposed: Character = "\u{1112}\u{1161}\u{11AB}" // ᄒ, ᅡ, ᆫ
  3. // 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:

  1. let enclosedEAcute: Character = "\u{E9}\u{20DD}"
  2. // 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):

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

Counting Characters

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

  1. let unusualMenagerie = "Koala 🐨, Snail 🐌, Penguin 🐧, Dromedary 🐪"
  2. print("unusualMenagerie has \(unusualMenagerie.characters.count) characters")
  3. // 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 , not e:

  1. var word = "cafe"
  2. print("the number of characters in \(word) is \(word.characters.count)")
  3. // prints "the number of characters in cafe is 4"
  4. word += "\u{301}" // COMBINING ACUTE ACCENT, U+0301
  5. print("the number of characters in \(word) is \(word.characters.count)")
  6. // prints "the number of characters in café is 4"

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 positions of each Character it contains.

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 cannot be indexed by integer values.

Use the startIndex property to access the position of the first Character of a String. The endIndex property returns the “past-the-end” position of the last character, If the String is empty, startIndex and endIndex are equal.

A String.Index value can access its immediately preceding index by calling the predecessor() method, and its immediately succeeding index by calling the successor() method. Any index in a String can be accessed from any other index by chaining these methods together, or by using the global advance(start:n:) function. Attempting to access an index outside of a string’s range will trigger a runtime error.

You can use subscript syntax to access the Character at a particular String index. Attempting to access a Character at an index outside of a string’s range will trigger a runtime error.

  1. let greeting = "Guten Tag"
  2. greeting[greeting.startIndex]
  3. // G
  4. greeting[greeting.endIndex.predecessor()]
  5. // g
  6. greeting[greeting.startIndex.successor()]
  7. // u
  8. let index = advance(greeting.startIndex, 7)
  9. greeting[index]
  10. // a
  11. greeting[greeting.endIndex] // error
  12. greeting.endIndex.successor() // error

Use the indices property of the characters property to create a Range of all of the indexes used to access individual characters in a string.

  1. for index in greeting.characters.indices {
  2. print("\(greeting[index]) ", appendNewline: false)
  3. }
  4. // prints "G u t e n T a g !"

Inserting and Removing

To insert a character into a string at a specified index, use the insert(_:atIndex:) method.

  1. var welcome = "hello"
  2. welcome.insert("!", atIndex: welcome.endIndex)
  3. // welcome now equals "hello!"

To insert another string at a specified index, use the splice(_:atIndex:) method.

  1. welcome.splice(" there".characters, atIndex: welcome.endIndex.predecessor())
  2. // welcome now equals "hello there!"

To remove a character from a string at a specified index, use the removeAtIndex(_:) method.

  1. welcome.removeAtIndex(welcome.endIndex.predecessor())
  2. // welcome now equals "hello there"

To remove a substring at a specified range, use the removeRange(_:) method:

  1. let range = advance(welcome.endIndex, -6)..<welcome.endIndex
  2. welcome.removeRange(range)
  3. // welcome now equals "hello"

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:

  1. let quotation = "We're a lot alike, you and I."
  2. let sameQuotation = "We're a lot alike, you and I."
  3. if quotation == sameQuotation {
  4. print("These two strings are considered equal")
  5. }
  6. // 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 are 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 are considered to be canonically equivalent:

  1. // "Voulez-vous un café?" using LATIN SMALL LETTER E WITH ACUTE
  2. let eAcuteQuestion = "Voulez-vous un caf\u{E9}?"
  3. // "Voulez-vous un café?" using LATIN SMALL LETTER E and COMBINING ACUTE ACCENT
  4. let combinedEAcuteQuestion = "Voulez-vous un caf\u{65}\u{301}?"
  5. if eAcuteQuestion == combinedEAcuteQuestion {
  6. print("These two strings are considered equal")
  7. }
  8. // 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 do not have the same linguistic meaning:

  1. let latinCapitalLetterA: Character = "\u{41}"
  2. let cyrillicCapitalLetterA: Character = "\u{0410}"
  3. if latinCapitalLetterA != cyrillicCapitalLetterA {
  4. print("These two characters are not equivalent")
  5. }
  6. // prints "These two characters are not equivalent"

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:

  1. let romeoAndJuliet = [
  2. "Act 1 Scene 1: Verona, A public place",
  3. "Act 1 Scene 2: Capulet's mansion",
  4. "Act 1 Scene 3: A room in Capulet's mansion",
  5. "Act 1 Scene 4: A street outside Capulet's mansion",
  6. "Act 1 Scene 5: The Great Hall in Capulet's mansion",
  7. "Act 2 Scene 1: Outside Capulet's mansion",
  8. "Act 2 Scene 2: Capulet's orchard",
  9. "Act 2 Scene 3: Outside Friar Lawrence's cell",
  10. "Act 2 Scene 4: A street in Verona",
  11. "Act 2 Scene 5: Capulet's mansion",
  12. "Act 2 Scene 6: Friar Lawrence's cell"
  13. ]

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

  1. var act1SceneCount = 0
  2. for scene in romeoAndJuliet {
  3. if scene.hasPrefix("Act 1 ") {
  4. ++act1SceneCount
  5. }
  6. }
  7. print("There are \(act1SceneCount) scenes in Act 1")
  8. // 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:

  1. var mansionCount = 0
  2. var cellCount = 0
  3. for scene in romeoAndJuliet {
  4. if scene.hasSuffix("Capulet's mansion") {
  5. ++mansionCount
  6. } else if scene.hasSuffix("Friar Lawrence's cell") {
  7. ++cellCount
  8. }
  9. }
  10. print("\(mansionCount) mansion scenes; \(cellCount) cell scenes")
  11. // prints "6 mansion scenes; 2 cell scenes"

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):

  1. 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:

image: ../Art/UTF8_2x.png
  1. for codeUnit in dogString.utf8 {
  2. print("\(codeUnit) ", appendNewline: false)
  3. }
  4. print("")
  5. // 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:

image: ../Art/UTF16_2x.png
  1. for codeUnit in dogString.utf16 {
  2. print("\(codeUnit) ", appendNewline: false)
  3. }
  4. print("")
  5. // 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:

image: ../Art/UnicodeScalar_2x.png
  1. for scalar in dogString.unicodeScalars {
  2. print("\(scalar.value) ", appendNewline: false)
  3. }
  4. print("")
  5. // 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:

  1. for scalar in dogString.unicodeScalars {
  2. print("\(scalar) ")
  3. }
  4. // D
  5. // o
  6. // g
  7. // ‼
  8. // 🐶