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.
Note
Swift’s String type is bridged seamlessly to Foundation’s NSString class. If you are working with the Foundation framework in Cocoa or Cocoa Touch, the entire NSString API is available to call on any String value you create, in addition to the String features described in this chapter. You can also use a String value with any API that requires an NSString instance.
For more information about using String with Foundation and Cocoa, see Using Swift with Cocoa and Objective-C.
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:
Note that Swift infers a type of String for the someString constant, because it is initialized with a string literal value.
Note
For information about using special characters in string literals, see .
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:
Find out whether a String value is empty by checking its Boolean isEmpty property:
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):
Note
This approach is different from string mutation in Objective-C and Cocoa, where you choose between two classes (NSString and NSMutableString) to indicate whether a string can be mutated.
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.
Note
This behavior differs from that of NSString in Cocoa. When you create an NSString instance in Cocoa, and pass it to a function or method or assign it to a variable, you are always passing or assigning a reference to the same single NSString. No copying of the string takes place, unless you specifically request it.
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
Swift’s String type represents a collection of Character values in a specified order. You can access the individual Character values in a string by iterating over that string with a for-in loop:
The for-in loop is described in For Loops.
Alternatively, create a stand-alone Character constant or variable from a single-character string literal by providing a Character type annotation:
Concatenating Strings and Characters
String values can be added together (or concatenated) with the addition operator (+) to create a new String value:
You can also append a String value to an existing String variable with the addition assignment operator (+=):
You can append a Character value to a String variable with the String type’s append method:
Note
You can’t append a String or Character to an existing Character variable, because a Character value must contain a single character only.
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:
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.
Note
The expressions you write inside parentheses within an interpolated string cannot contain an unescaped double quote (") or backslash (), and cannot contain a carriage return or line feed.
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
A Unicode scalar is any Unicode code point in the range U+0000 to U+D7FF inclusive or U+E000 to U+10FFFF inclusive. Unicode scalars do not include the Unicode surrogate pair code points, which are the code points in the range U+D800 to U+DFFF inclusive.
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 Unicode Characters in String Literals
String literals can include the following special Unicode 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 between one and eight hexadecimal digits
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:
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:
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:
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:
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):
Counting Characters
To retrieve a count of the Character values in a string, call the global countElements function and pass in a string as the function’s sole parameter:
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:
Note
Extended grapheme clusters can be composed of one or more 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 do not each take up the same amount of memory within a string’s representation. As a result, the number of characters in a string cannot 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 countElements function must iterate over the Unicode scalars in the entire string in order to calculate an accurate character count for that string.
Note also that the character count returned by countElements is not 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. To reflect this fact, the length property from NSString is called utf16Count when it is accessed on a Swift String 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:
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:
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:
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:
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:
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 .
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 .
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):
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:
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:
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:
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:
Basic Operators
Collection Types
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