常量和变量的声明
import UIKit
// 声明常量和变量
let maximumNumberOfLoginAttempts = 10
var currentLoginAttempt = 0
多个变量同时声明
// 一次声明多个常量或者变量，用,分割
var x = 0.0, y = 0.0, z = 0.0
显示指定变量类型
// 类型指定
var welcomeMessage: String
welcomeMessage = "Hello"
// 一次指定多个变量类型(变量间用,分割，只需要在最后一个变量指定类型)
var red, green, blue: Double
字符串插值打印法
// 字符串插值打印法
var friendlyWelcome = "Hello"
friendlyWelcome = "Boujour!"
print("The current value of friendlyWelcom is \(friendlyWelcome)")
类型转换的使用
// 类型转换
let twoThousand: UInt16 = 2_000
let one: UInt8 = 1
// 直接相加会出错，需要做类型转换
// twoThousand + one
let twoThousandAndOne = twoThousand + UInt16(one)
print(twoThousandAndOne)
别名的使用
// 别名
typealias AudioSample = UInt16
var maxAmplitudeFound = AudioSample.max
print(maxAmplitudeFound)
Bool类型的使用
// Bool 类型
let turnipsAreDelicious = false
if turnipsAreDelicious {
print("Mmm, tasty turnips!")
} else {
print("Eww, turnips ar horrible.")
}
let i = 1
// if i {} 这种判断会出错, i不是Bool 类型
if i == 1{
print("i 的值是1哦")
}
元组的使用
// 元组
let http404Error = (404, "Not Found")
// 通过下标访问
print("The status code is \(http404Error.0)")
print("The status message is \(http404Error.1)")
// 通过分解元组访问
let (statusCode, statusMessage) = http404Error
print("The status code is \(statusCode)")
print("The status message is \(statusMessage)")
// 通过分解元组访问时，不关心的元素可以用_代替
let (justStatusCode, _) = http404Error
print("The status code is \(justStatusCode)")
// 通过定义元组时命名访问
let http200Status = (statusCode: 200, description: "OK")
print("The status code is \(http200Status.statusCode)")
print("The status message is \(http200Status.description)")
可选类型的使用
// 可选类型
var serverResponseCode: Int? = 404
serverResponseCode = nil
let possibleNumber = "123"
let convertedNumber = Int(possibleNumber)
if convertedNumber != nil {
//print("convertedNumber has an interger value of \(convertedNumber)")
// 如果不做强制拆包输出为convertedNumber has an interger value of Optional(123),不是想要的结果, 加一个!做强制拆包
print("convertedNumber has an interger value of \(convertedNumber!)")
}
// 可选绑定
if let otherConvertedNumber = convertedNumber {
print("convertedNumber has an interger value of \(otherConvertedNumber)")
}
if let firstNumber = Int("4"), let secondNumber = Int("42") where firstNumber < secondNumber && secondNumber < 100 {
print("\(firstNumber) < \(secondNumber) < 100")
}
if let firstNumber = Int("4")
{
if let secondNumber = Int("42")
{
if firstNumber < secondNumber && secondNumber < 100
{
print("\(firstNumber) < \(secondNumber) < 100")
}
}
}
断言的使用
// assert 断言的使用
let age = 10
assert(age > 0, "A person's age cannot be less than zero")
