Swift 的 String类型桥接到了基础库中的 NSString类。Foundation 同时也扩展了所有 NSString 定义的方法给 String 。也就是说，如果你导入 Foundation ，就可以在 String 中访问所有的 NSString 方法，无需转换格式

1、 字符串字面量

let someString = "Some string literal value"

2、初始化一个空字符串

var emptyString = ""
var anotherEmptyString = String()

3、判断字符串是否为空

if emptyString.isEmpty {
    print("Nothing to see here")
}

4、 操作字符

for character in "Dog!?" {
    print(character)
}

let catCharacters: [Character] = ["C", "a", "t", "!", "?"]
let catString = String(catCharacters)
print(catString)

5、 字符串的拼接

let string1 = "hello"
let string2 = " there"
var welcome = string1 + string2
    welcome += string2
print(welcome)

6、 字符串的穿插值

let name = "zhang"
let age = 19
let height = 1.87
let message = "my name is\(name) my age is \(age) my height \(height)"
let personStr = String(format: "my name is %@ my age is %d my height %f", name,age,height)
print(message,personStr)

7、字符串的长度

let unusualMenagerie = "Koala ?, Snail ?, Penguin ?, Dromedary ?"
print(unusualMenagerie.count)

8、字符串的索引

let greeting = "Guten Tag!"
print(greeting[greeting.startIndex])  //greeting.startIndex 这是个数值
print(greeting[greeting.index(before: greeting.endIndex)])
print(greeting[greeting.index(after: greeting.startIndex)])
let index = greeting.index(greeting.startIndex, offsetBy: 7)
print(greeting[index])

9、 使用 indices属性来访问字符串中每个字符的索引

for index in greeting.indices {
    print("\(greeting[index]) ", terminator: "")
}

10、 插入

var insertWelcome = "hello"
insertWelcome.insert("!", at: insertWelcome.endIndex)
insertWelcome.insert(contentsOf: " there", at: insertWelcome.index(before: insertWelcome.endIndex))
print(insertWelcome)

11、删除

let reoveOne = insertWelcome.remove(at: insertWelcome.index(before: insertWelcome.endIndex))
print(reoveOne)

let reoveTwo = insertWelcome.index(insertWelcome.endIndex, offsetBy: -6)..<insertWelcome.endIndex
print(insertWelcome.removeSubrange(reoveTwo))

12、 字符串的截取

let timeString = "http://www.baidu.com"
let header = (timeString as NSString).substring(to: 3)
print(header)
let middle = (timeString as NSString).substring(with: NSMakeRange(7, 8))
print(middle)
let footer = (timeString as NSString).substring(from: 7)
print(footer)

13、字符串的比较

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")
}