Swift4.2基础学习笔记(十一)

参考资料与链接https://www.cnswift.org

枚举

枚举语法

• 用 enum关键字来定义一个枚举

enum CompassPoint {
    case north
    case south
    case east
    case west
}

//多个成员值可以出现在同一行中,要用逗号隔开
enum Planet {
    case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune
}

Swift 的枚举成员在被创建时不会分配一个默认的整数值

var directionToHead = CompassPoint.west

//一旦 directionToHead以 CompassPoint类型被声明，你就可以用一个点语法把它设定成不同的 CompassPoint值
directionToHead = .east

使用Switch语句来匹配枚举值

directionToHead = .south
switch directionToHead {
case .north:
    print("Lots of planets have a north")
case .south:
    print("Watch out for penguins")
case .east:
    print("Where the sun rises")
case .west:
    print("Where the skies are blue")
}
// prints "Watch out for penguins"

遍历枚举情况(case)

• 可以通过在枚举名字后面写 : CaseIterable 来允许枚举被遍历

enum Beverage: CaseIterable {
    case coffee, tea, juice
}
let numberOfChoices = Beverage.allCases.count
print("\(numberOfChoices) beverages available")

for beverage in Beverage.allCases {
    print(beverage)
}
// coffee
// tea
// juice

关联值

enum Barcode {
    case upc(Int, Int, Int, Int)
    case qrCode(String)
}

var productBarcode = Barcode.upc(8, 85909, 51226, 3)

productBarcode = .qrCode("ABCDEFGHIJKLMNOP")

switch productBarcode {
case .upc(let numberSystem, let manufacturer, let product, let check):
    print("UPC: \(numberSystem), \(manufacturer), \(product), \(check).")
case .qrCode(let productCode):
    print("QR code: \(productCode).")
}
// Prints "QR code: ABCDEFGHIJKLMNOP."


switch productBarcode {
case let .upc(numberSystem, manufacturer, product, check):
    print("UPC : \(numberSystem), \(manufacturer), \(product), \(check).")
case let .qrCode(productCode):
    print("QR code: \(productCode).")
}
// Prints "QR code: ABCDEFGHIJKLMNOP."

原始值

enum ASCIIControlCharacter: Character {
    case tab = "\t"
    case lineFeed = "\n"
    case carriageReturn = "\r"
}

隐式指定的原始值

• 当整数值被用于作为原始值时，每成员的隐式值都比前一个大一。如果第一个成员没有值，那么它的值是 0

enum Planet: Int {
    case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune
}

• 当字符串被用于原始值，那么每一个成员的隐式原始值则是那个成员的名称。

enum CompassPoint: String {
    case north, south, east, west
}
//CompassPoint.south有一个隐式原始值 "south"

//可以用 rawValue属性来访问一个枚举成员的原始值：
let earthsOrder = Planet.Earth.rawValue
// earthsOrder is 3
 
let sunsetDirection = CompassPoint.west.rawValue
// sunsetDirection is "west"

从原始值初始化

//从原始值来判断枚举成员,返回一个枚举成员或者nil
let possiblePlanet = Planet(rawValue: 7)
// possiblePlanet is of type Planet? and equals Planet.Uranus

//当以不存在的原始值来初始化时,可选项Planet值将会是nil
let positionToFind = 11
if let somePlanet = Planet(rawValue: positionToFind) {
    switch somePlanet {
    case .earth:
        print("Mostly harmless")
    default:
        print("Not a safe place for humans")
    }
} else {
    print("There isn't a planet at position \(positionToFind)")
}
// Prints "There isn't a planet at position 11"

递归枚举

enum ArithmeticExpression {
    case number(Int)
    indirect case addition(ArithmeticExpression, ArithmeticExpression)
    indirect case multiplication(ArithmeticExpression, ArithmeticExpression)
}
或
indirect enum ArithmeticExpression {
    case number(Int)
    case addition(ArithmeticExpression, ArithmeticExpression)
    case multiplication(ArithmeticExpression, ArithmeticExpression)
}

//(5 + 4) * 2
let five = ArithmeticExpression.number(5)
let four = ArithmeticExpression.number(4)
let sum = ArithmeticExpression.addition(five, four)
let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2))

func evaluate(_ expression: ArithmeticExpression) -> Int {
    switch expression {
    case let .number(value):
        return value
    case let .addition(left, right):
        return evaluate(left) + evaluate(right)
    case let .multiplication(left, right):
        return evaluate(left) * evaluate(right)
    }
}
 
print(evaluate(product))
// Prints "18"

//这个函数通过直接返回关联值来判断普通数字。它通过衡量表达式左手侧和右手侧判断是加法还是乘法，然后对它们加或者乘。