swift学习笔记（3）--- 字符串和字符

2019-10-19  本文已影响0人  Rui_ai

字符串是一系列字符的集合，例如 "hello, world"，"albatross"。Swift 的字符串通过 String 类型来表示

1、字符串字面量

字符串字面量是由一对双引号包裹着的具有固定顺序的字符集。可以用于为常量和变量提供初始值。

let someString = "Some string literal value"
（1）多行字符串字面量

多行字符串字面量是由一对三个双引号（"""）包裹的具有固定顺序的文本字符集

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."
"""
（2）字符串字面量的特殊字符

字符串字面量可以包含以下特殊字符：

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

2、初始化字符串

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”

3、字符串可变性

通过将一个特定字符串分配给一个变量来对其进行修改，或者分配给一个常量来保证其不会被修改：

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。

4、字符串是值类型

5、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!🐱”

6、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"

let exclamationMark: Character = "!"
welcome.append(exclamationMark)
// welcome 现在等于 "hello there!"

注意：不能将一个字符串或者字符添加到一个已经存在的字符变量上，因为字符变量只能包含一个字符。

7、字符串插值

字符串插值是一种构建新字符串的方式，可以在其中包含常量、变量、字面量和表达式。字符串字面量和多行字符串字面量都可以使用字符串插值。

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

8、计算字符数量

可以使用 count 属性来获得一个字符串中 Character 值的数量。

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

9、访问和修改字符串

（1）字符串索引

每一个 String 值都有一个关联的索引（index）类型，String.Index，它对应着字符串中的每一个 Character 的位置。

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

for index in greeting.indices {
   print("\(greeting[index]) ", terminator: "")
}
// 打印输出“G u t e n   T a g ! ”
（2）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"

10、子字符串

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

// 把结果转化为 String 以便长期存储。
let newString = String(beginning)

11、比较字符串

（1）字符串/字符相等

字符串/字符可以用等于操作符（==）和不等于操作符（!=）

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”
（2）前缀/后缀相等

通过调用字符串的 hasPrefix(_:)/hasSuffix(_:) 方法来检查字符串是否拥有特定前缀/后缀，两个方法均接收一个 String 类型的参数，并返回一个布尔值。

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”
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