【学习】重学Swift5-基本数据类型

一、基本数据类型

1.声明常量和变量

let maxNum = 10
var currentNum = 0

let x = 10, y = 20, z = 30
let x1 = 10, y2 = 20, z = "30"

var welcomeMessage: String = "hello world"

print(maxNum)
print("x = \(x), y = \(y)")

2.整型、浮点型

Int8 Int16 Int32 Int64 UInt8 UInt16 UInt32 UInt64 
Int UInt 当前平台原生字相同的长度
Double 64位浮点数，至少15位的精度
Float 32位浮点数，至少6位的精度

3.Bool

Bool: true和false
Swift的类型安全机制会阻止你用一个非布尔量的值替换掉Bool

4.类型别名

类型别名是一个为已存在类型定义一个可选择的名字
typealias AudioSample = UInt8
let simple: AudioSample = 32

5.元祖（Tuple）

let error = (1, "没有权限")
print(error.0, error.1)
let error2 = (errorCode: 1, errorMessage: "没有权限")
print(error.errorCode, error.errorMessage)

let (errcode, errMsg) = error
print(errcode, errMsg)

let (_, errMsg) = error
print(errMsg)

6.Optional

var str: String? = nil

var str1: String? = "abc"
if str1 != nil {
  let count = str1!.count
  print(count)
}

// 可选项绑定
if let actualStr = str1 {
  let count = actualStr.count
  print(count)
}

// 隐式展开
var str2: String! = "abc"
let count = str2.count
print(count)

// 可选链
let count = str1?.count
if count != nil {
  let lastIndex = count! - 1
}

/*
原理
Optional是标准库里的一个enum类型
Optional.none 就是nil
Optional.some	则包装了实际的值
范型属性 unsafelyUnwrapped
*/
var str3: Optional<String> = "abc"
if let actualStr = str3 {
  let count = actualStr.cout
}
let count = str3.unsafelyUnwrapped.count

7.字符串

1. 初始化

   // 字面量
   var emptyString = ""
   // 初始化器
   var emptyStr = String()
   let catCharacters: [Character] = ["C","a","t"]
   let catStr = String(catCharacters)
   
   // 多行
   let softWrappedQuotation = """The White Rabbit put on his spectacles. "Where shall O begin, plaese your Majesty" he asked.
   """
   

2. 是否是空串

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

3. 常见操作

   // 可变不可变使用var、let
   var variableString = "Horse"
   variableString += " and carriage" 
   
   // 遍历
   for character in "Dog! Cat！" {
     print(character)
   }
   
   // 拼接
   var newVarStr = variableString + "new sreing"
   variableString += " and carriage" // 末尾追加，不会创建新的
   variableString.append("d")
   
   // 插值
   let multiplier = 3
   let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"
   
   // 字符串索引
   let greeting = "Guten tag!"
   greeting[greeting.startIndex]
   // 不能使用greeting[1]这样的
   greeting[greeting.index(after: greeting.startIndex)]
   greeting[greeting.index(before: greeting.endIndex)]
   let index = greeting.index(greeting.startIndex, offsetBy: 7)
   greeting[index]
   
   // 插入
   var welcomeMessage = "hello"
   welcomeMessage.insert("!", at: welcomeMessage.endIndex)
   welcomeMessage.insert(contentsOf: " world", at: welcomeMessage.index(before: welcomeMessage.endIndex))
   // 移除
   welcomeMessage.remove(at: welcomeMessage.index(before: welcomeMessage.endIndex))
   let range = welcomeMessage.index(welcomeMessage.endIndex, offsetBy: -6)..<welcomeMessage.endIndex
   welcomeMessage.removeSubrange(range)
   
   // 子字符串 
   let welcome = "hello, world"
   let index = welcome.firstIndex(of: ",") ?? welcome.endIndex
   let subString = welcome[..<index]
   let newString = String(subString)
   // 比较、前后缀
   let welcome1 = "hello"
   print(welcome == welcome1)
   print(welcome != welcome1)
   print(welcome.hasPrefix("hello"))
   print(welcome.hasSuffix("world"))