1. 字符串字面量

let someString = "Some string literal value"
// someString 常量被推斷類型為 String

2. 空字符串的初始化

兩種方式，舉個(gè)栗子?? :

var emptyString = ""               // 方式一 
var anotherEmptyString = String()  // 方式二
// 兩種方式都初始化為一個(gè)空字符串断盛，即 “”弯予。

通過(guò)檢查字符串常量或變量的isEmpty屬性來(lái)確認(rèn)一個(gè)String值是否為空:

if emptyString.isEmpty {
    print("Nothing to see here")
}
// 結(jié)果為: "Nothing to see here"

3. 字符串可變性

String常量不可修改值；
String變量 可以修改值刊棕。

4. 字符串是值類型

值類型意味著炭晒，每一次賦值，拿到的都是String對(duì)象的拷貝甥角，而非對(duì)象本身网严。 (淺拷貝)

5. 字符類型 (Character)

• 用字符串字面量初始化一個(gè)Character類型的變量:

let exclamationMark: Character = "!"

• 用for...in語(yǔ)句遍歷String類型對(duì)象，打印所有字符嗤无，舉個(gè)?? :

for character in "Dog!?".characters {
    print(character)
}
// D
// o
// g
// !
// ?

• 用字符數(shù)組初始化一個(gè)字符串:

let catCharacters: [Character] = ["C", "a", "t", "!", "??"] // 初始化一個(gè)字符數(shù)組
let catString = String(catCharacters)                       // 將數(shù)組轉(zhuǎn)化為字符串
print(catString)                                            // 結(jié)果為: "Cat!??"

6. 連接字符串或字符

舉個(gè)?? :

let string1 = "hello"
let string2 = " there"
var welcome = string1 + string2
// welcome 現(xiàn)在為 "hello there"

上面的?? 也可以簡(jiǎn)寫為下面的 ?? :

var instruction = "look over"
instruction += string2
// instruction 現(xiàn)在為 "look over there"

字符串后追加字符, 使用append(_)函數(shù):

let exclamationMark: Character = "!"
welcome.append(exclamationMark)
// welcome 現(xiàn)在為 "hello there!"

7. 字符串插值

一個(gè)反斜杠 \ 加上一枚小括號(hào)() 等于 \(), 舉個(gè)?? :

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

8. 字符串的字符統(tǒng)計(jì)

let unusualMenagerie = "Koala ??, Snail ??, Penguin ??, Dromedary ?? "
print("unusualMenagerie has \(unusualMenagerie.characters.count) characters")
// 'unusualMenagerie.characters' 為字符串的字符數(shù)組震束，然后取其count值，即此字符串的字符數(shù)量当犯。
// 結(jié)果為: "unusualMenagerie has 40 characters"

9. 訪問(wèn)和修改字符串

字符串索引

每一個(gè)String值都有索引類型垢村，String.Index，它相當(dāng)于每個(gè)Character在字符串中的位置嚎卫。使用startIndex屬性來(lái)訪問(wèn)String中第一個(gè)Character的位置:

let greeting = "Guten Tag!"
greeting[greeting.startIndex]  // G

endIndex屬性是 String中最后一個(gè)字符再往后一位的位置肝断。因此，endIndex屬性并不是字符串下標(biāo)腳本的合法實(shí)際參數(shù)。如果String為空胸懈，則 startIndex與 endIndex相等担扑。

使用 index(before:) 和 index(after:) 方法來(lái)訪問(wèn)給定索引的前或后一個(gè)位置:

greeting[greeting.index(before: greeting.endIndex)]
// !
greeting[greeting.index(after: greeting.startIndex)]
// u

使用index(_:offsetBy:)方法訪問(wèn)偏離第一個(gè)參數(shù)某個(gè)長(zhǎng)度的位置:

let index = greeting.index(greeting.startIndex, offsetBy: 7)
greeting[index]
// a

嘗試訪問(wèn)索引位置在字符串范圍之外，就會(huì)觸發(fā)運(yùn)行時(shí)錯(cuò)誤:

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

字符串characters屬性是一個(gè)字符數(shù)組趣钱，數(shù)組的indices(index的復(fù)數(shù)形式)屬性涌献，代表字符串的索引數(shù)組:

for index in greeting.characters.indices {
    print("\(greeting[index]) ", terminator: " ")
}
// 結(jié)果為: "G u t e n   T a g ! "

插入和刪除

插入字符，使用insert(_:at:)方法首有，
插入字符串燕垃，使用insert(contentsOf:at:)方法:

var welcome = "hello"
welcome.insert("!", at: welcome.endIndex)
// welcome 現(xiàn)在為 "hello!"
 
welcome.insert(contentsOf:" there".characters, at: welcome.index(before: welcome.endIndex))
// welcome 現(xiàn)在為 "hello there!"

移除某個(gè)位置字符，使用remove(at:)方法井联，
移除某段位置字符卜壕，使用removeSubrange(_:)方法：

welcome.remove(at: welcome.index(before: welcome.endIndex))
// welcome 現(xiàn)在為 "hello there"
 
let range = welcome.index(welcome.endIndex, offsetBy: -6)..<welcome.endIndex
welcome.removeSubrange(range)
// welcome 現(xiàn)在為 "hello"

任何遵循RangeReplaceableIndexable協(xié)議的類型都可使用insert(_:at:)，insert(contentsOf:at:)烙常，remove(at:)方法轴捎，包括 String類型，還有集合類型如 : Array蚕脏，Dictionary 和 Set侦副。

10. 字符串比較

字符串和字符相等性

字符串比較使用“等于”運(yùn)算符 (==) 和“不等”運(yùn)算符 (!=)進(jìn)行檢查:

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")
}
// 結(jié)果為: "These two strings are considered equal"

前綴和后綴相等性

要檢查一個(gè)字符串是否擁有特定的字符串前綴或者后綴，調(diào)用字符串的hasPrefix(_:)和 hasSuffix(_:)方法驼鞭，返回一個(gè)布爾值秦驯。舉個(gè)羅密歐與朱麗葉的?? :

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"
]
// 初始化數(shù)組

使用hasPrefix(_:)方法計(jì)算數(shù)組中前綴為"Act 1"的數(shù)量:

var act1SceneCount = 0
for scene in romeoAndJuliet {
    if scene.hasPrefix("Act 1 ") {
        ++act1SceneCount
    }
}
print("There are \(act1SceneCount) scenes in Act 1")
// 結(jié)果為: "There are 5 scenes in Act 1"

使用hasSuffix(_:)方法計(jì)算數(shù)組中后綴為"Capulet’s mansion"和"Friar Lawrence’s cell"的數(shù)量:

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
    }
}
print("\(mansionCount) mansion scenes; \(cellCount) cell scenes")
// 結(jié)果為: "6 mansion scenes; 2 cell scenes"