3、【Swift】字符串

2020-12-09  本文已影响0人  Sunday_David

字符串和字符

Swift 的 String 类型与 Foundation NSString 类进行了无缝桥接。调用 NSString 的方法，无需进行类型转换。

更多关于在 Foundation 和 Cocoa 中使用 String 的信息请查看 Bridging Between String and NSString。

字符串字面量

let someString = "Some string literal value"

多行字符串字面量

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."
"""

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."
"""
image

字符串字面量的特殊字符

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

let threeDoubleQuotes = """
Escaping the first quote \"""
Escaping all three quotes \"\"\"
"""

扩展字符串分隔符

#"Line 1\nLine 2"#

#"Line 1\#nLine 2"#

###"Line1\###nLine2"###

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

初始化空字符串

var emptyString = ""               // 空字符串字面量
var anotherEmptyString = String()  // 初始化方法
// 两个字符串均为空并等价。

if emptyString.isEmpty {
    print("Nothing to see here")
}
// 打印输出：“Nothing to see here”

字符串可变性

var variableString = "Horse"
variableString += " and carriage"
// variableString 现在为 "Horse and carriage"

let constantString = "Highlander"
constantString += " and another Highlander"
// 这会报告一个编译错误（compile-time error） - 常量字符串不可以被修改。

字符串是值类型

使用字符

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

let exclamationMark: Character = "!"

let catCharacters: [Character] = ["C", "a", "t", "!", "🐱"]
let catString = String(catCharacters)
print(catString)
// 打印输出：“Cat!🐱”

连接字符串和字符

let string1 = "hello"
let string2 = " there"
var welcome = string1 + string2
// welcome 现在等于 "hello there"

var instruction = "look over"
instruction += string2
// instruction 现在等于 "look over there"

你不能把 String或者 Character追加到已经存在的 Character变量当中，因为 Character值能且只能包含一个字符。

字符串插值

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

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

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

Unicode

Unicode 标量

print("\u{0061}")// a
print("--------------------")
print("\u{1F425}")// 🐥
print("--------------------")

Unicode 标量码位位于 U+0000到 U+D7FF或者 U+E000到 U+10FFFF之间。Unicode 标量码位不包括从 U+D800到 U+DFFF的16位码元码位。

可扩展的字形群集

let eAcute: Character = "\u{E9}"                         // é
let combinedEAcute: Character = "\u{65}\u{301}"          // e 后面加上  ́
// eAcute 是 é, combinedEAcute 是 é

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

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

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

计算字符数量/字符统计

let unusualMenagerie = "Koala 🐨, Snail 🐌, Penguin 🐧, Dromedary 🐪"
print("unusualMenagerie has \(unusualMenagerie.count) characters")
// 打印输出“unusualMenagerie has 40 characters”

注意: 使用可拓展的字符群集作为 Character 值来连接或改变字符串时，并不一定会更改字符串的字符数量。

var word = "cafe"
print("the number of characters in \(word) is \(word.count)")
// 打印输出“the number of characters in cafe is 4”

word += "\u{301}"    // 拼接一个重音，U+0301

print("the number of characters in \(word) is \(word.count)")
// 打印输出“the number of characters in café is 4”
  • 扩展字形集群能够组合一个或者多个 Unicode 标量。这意味着不同的字符——以及相同字符的不同表示——能够获得不同大小的内存来储存. 特殊的长字符串值，要注意 count属性为了确定字符串中的字符要遍历整个字符串的 Unicode 标量。
  • count 属性返回的字符数量并不总是与包含相同字符的 NSString 的 length 属性相同
  • NSString 的 length 属性是利用 UTF-16 表示的十六位代码单元数字，而不是 Unicode 可扩展的字符群集

访问和修改字符串

字符串索引

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

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

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

可以在任何遵循了 Collection 协议的类型中使用 startIndex 和 endIndex 属性以及 index(before:) ，index(after:) 和 index(_:offsetBy:) 方法。这包括这里使用的 String ，还有集合类型比如 Array ，Dictionary 和 Set

插入和删除

var welcome = "hello"
welcome.insert("!", at: welcome.endIndex)
// welcome 变量现在等于 "hello!"

welcome.insert(contentsOf:" there", at: welcome.index(before: welcome.endIndex))
// welcome 变量现在等于 "hello there!"

welcome.remove(at: welcome.index(before: welcome.endIndex))
// welcome 现在等于 "hello there"

let range = welcome.index(welcome.endIndex, offsetBy: -6)..<welcome.endIndex
welcome.removeSubrange(range)
// welcome 现在等于 "hello"

任意一个确认的并遵循 RangeReplaceableCollection 协议的类型里面，可使用 insert(_:at:)、insert(contentsOf:at:)、remove(at:) 和 removeSubrange(_:) 方法, 在如上文用在 String 中，也用在 Array、Dictionary 和 Set

子字符串

let greeting = "Hello, world!"
let index = greeting.index(of: ",") ?? greeting.endIndex
let beginning = greeting[..<index]
// beginning is "Hello"
 
// Convert the result to a String for long-term storage.
let newString = String(beginning)
img

String 和 Substring 都遵循 StringProtocol 协议，这意味着操作字符串的函数使用 StringProtocol 会更加方便。你可以传入 String 或 Substring 去调用函数

比较字符串

三种方式比较文本值：字符串字符相等、前缀相等和后缀相等。

字符串/字符相等

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")
}
// 打印输出“These two strings are considered equal”

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

// "Voulez-vous un café?" 使用 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")
}
// 打印输出“These two strings are considered equal”

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

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

if latinCapitalLetterA != cyrillicCapitalLetterA {
    print("These two characters are not equivalent")
}
// 打印“These two characters are not equivalent”

前缀/后缀相等

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"
]

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

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")
// 打印输出“6 mansion scenes; 2 cell scenes”

字符串的 Unicode 表示形式

let dogString = "Dog‼🐶"

UTF-8 表示

UTF-16 表示

Unicode 标量表示

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