swift中文文档翻译之--字符串和字符

字符串和字符

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 similar syntax to C strings. 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.

字符串是一串有序的字符集合，如“hello, world”或“albatross”。Swift 字符串是由String类型表示，这又代表字符类型的值的集合。

Swift的String 和Character 类型提供了一种快速，兼容Unicode的方式来在你的代码中与文本工作。字符串的创建和操作语法是简便和易读的 ，与C语言的字符串有着类似的语法。字符串连接很简单的，只要使用+运算符把两个字符串相加即可，字符串的可变性由常量或变量之间进行选择来管理，就像在Swift中任何的其他值。

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.

Strings can also be used 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

字符串字面值

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 ("").

A string literal can be used to provide an initial value for a constant or variable:在你的代码中你可以在字符串字面值中包含预定义的字符串值。字符串面值是由一对双引号括（“”）起来的文本字符的固定顺序。

字符串面值可以被用来提供一个常量或变量的初始值：

•     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.

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)

    Single-byte Unicode scalars, written as \xnn, where nn is two hexadecimal digits

    Two-byte Unicode scalars, written as \unnnn, where nnnn is four hexadecimal digits

    Four-byte Unicode scalars, written as \Unnnnnnnn, where nnnnnnnn is eight hexadecimal digits

 

The code below shows an example of each kind of special character. The wiseWords constant contains two escaped double quote characters. The dollarSign, blackHeart, and sparklingHeart constants demonstrate the three different Unicode scalar character formats:

    let wiseWords = "\"Imagination is more important than knowledge\" - Einstein"
    // "Imagination is more important than knowledge" - Einstein
    let dollarSign = "\x24" // $, Unicode scalar U+0024
    let blackHeart = "\u2665" // ♥, Unicode scalar U+2665
    let sparklingHeart = "\U0001F496" // ?, Unicode scalar U+1F496

 

初始化一个空字符串

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:

创建一个空的字符串值作为构建一个更长的字符串的起点，给一个变量赋一个空字符串值作为字面值，或者用初始化语法初始化一个新的String实例都可以初始化一个空字符串，如下：

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

 

You can find out whether a String value is empty by checking its Boolean isEmpty property:

你可以通过IsEmpty检查其布尔属性找到一个字符串值是否为空：

if emptyString.isEmpty {
    println("Nothing to see here")
    }
    // 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):

• var variableString = "Horse"
• variableString += " and carriage"
• // variableString is now "Horse and carriage"
• let constantString = "Highlander"
• constantString += " and another Highlander"
• // this reports a compile-time error - a constant string 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. Each Character value represents a single Unicode character. You can access the individual Character values in a string by iterating over that string with a for-in loop:

• for character in "Dog!?" {
• println(character)
• }
• // D
• // o
• // g
• // !
• // ?

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:

• let yenSign: Character = "¥"

Counting Characters

To retrieve a count of the characters in a string, call the global countElements function and pass in a string as the function’s sole parameter:

• let unusualMenagerie = "Koala ?, Snail ?, Penguin ?, Dromedary ?"
• println("unusualMenagerie has \(countElements(unusualMenagerie)) characters")
• // prints "unusualMenagerie has 40 characters"

Note

Different Unicode characters and different representations of the same Unicode 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 length of a string cannot be calculated without iterating through the string to consider each of its characters in turn. If you are working with particularly long string values, be aware that the countElements function must iterate over the characters within a 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 characters within the string. To reflect this fact, the length property from NSString is called utf16count when it is accessed on a Swift String value.

Concatenating Strings and Characters

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

• let string1 = "hello"
• let string2 = " there"
• let character1: Character = "!"
• let character2: Character = "?"
• let stringPlusCharacter = string1 + character1 // equals "hello!"
• let stringPlusString = string1 + string2 // equals "hello there"
• let characterPlusString = character1 + string1 // equals "!hello"
• let characterPlusCharacter = character1 + character2 // equals "!?"

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

• var instruction = "look over"
• instruction += string2
• // instruction now equals "look over there"
• var welcome = "good morning"
• welcome += character1
• // welcome now equals "good morning!"

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:

• 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 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.

Comparing Strings

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

String Equality

Two String values are considered equal if they contain exactly the same characters in the same order:

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

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. Both methods perform a character-by-character comparison between the base string and the prefix or suffix string.

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
• }
• }
• println("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
• } else if scene.hasSuffix("Friar Lawrence's cell") {
• ++cellCount
• }
• }
• println("\(mansionCount) mansion scenes; \(cellCount) cell scenes")
• // prints "6 mansion scenes; 2 cell scenes"

Uppercase and Lowercase Strings

You can access an uppercase or lowercase version of a string with its uppercaseString and lowercaseString properties:

• let normal = "Could you help me, please?"
• let shouty = normal.uppercaseString
• // shouty is equal to "COULD YOU HELP ME, PLEASE?"
• let whispered = normal.lowercaseString
• // whispered is equal to "could you help me, please?"

Unicode

Unicode is an international standard for encoding and representing text. 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. They support a number of different Unicode encodings, as described below.

Unicode Terminology

Every character in Unicode can be represented by one or more unicode scalars. A unicode scalar is a unique 21-bit number (and name) for a character or modifier, such as U+0061 for LOWERCASE LATIN LETTER A ("a"), or U+1F425 for FRONT-FACING BABY CHICK ("?").

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

Unicode Representations of Strings

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 characters. 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 (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, !, and the ? character (DOG FACE, or Unicode scalar U+1F436):

• let dogString = "Dog!?"

UTF-8

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

• for codeUnit in dogString.utf8 {
• print("\(codeUnit) ")
• }
• print("\n")
• // 68 111 103 33 240 159 144 182

In the example above, the first four decimal codeUnit values (68, 111, 103, 33) represent the characters D, o, g, and !, whose UTF-8 representation is the same as their ASCII representation. The last four codeUnit values (240, 159, 144, 182) are a four-byte UTF-8 representation of the DOG FACE character.

UTF-16

You can access a UTF-16 representation of a String by iterating over its utf16 property. This property is of type 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:

• for codeUnit in dogString.utf16 {
• print("\(codeUnit) ")
• }
• print("\n")
• // 68 111 103 33 55357 56374

Again, the first four codeUnit values (68, 111, 103, 33) represent the characters D, o, g, and !, whose UTF-16 code units have the same values as in the string’s UTF-8 representation.

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 lead surrogate value of U+D83D (decimal value 55357) and a trail surrogate value of U+DC36 (decimal value 56374).

Unicode Scalars

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. A Unicode scalar is any 21-bit Unicode code point that is not a lead surrogate or trail surrogate code point.

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

• for scalar in dogString.unicodeScalars {
• print("\(scalar.value) ")
• }
• print("\n")
• // 68 111 103 33 128054

The value properties for the first four UnicodeScalar values (68, 111, 103, 33) once again represent the characters D, o, g, and !. The value property of the fifth and final UnicodeScalar, 128054, is a decimal equivalent of the hexadecimal value 1F436, which is equivalent to 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 {
• println("\(scalar) ")
• }
• // D
• // o
• // g
• // !
• // ?

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