• 用 + 拼接字符串

var variableString = "Horse"variableString += " and carriage"

• append() 在为字符串添加字符（Character）

let exclamationMark: Character = "!"variableString.append(exclamationMark)

• \ 利用\在字符串中插入常量/变量 8

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

• characters 属性 ，用于获取字符串中的字符

let varString:String = "Jaesun is 25"        for char  in varString.characters {    print(char)}

• index() 获取某字符串字符下标

let greeting = "Guten Tag!"    // starindex 起始下标 endIndex 结尾下标    greeting[greeting.startIndex]    //  某下标之前的下标 如：! 的下标    greeting.index(before: greeting.endIndex)    // 某下标之后的下标： 如 u 的下标    greeting[greeting.index(after: greeting.startIndex)]    // 从某下标开始向后偏移后的下标 如：a的下标    let index = greeting.index(greeting.startIndex, offsetBy: 7)

• .characters.indices