忍者刘米米

9. Enumerations

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Enumerations

  • An enumeration defines a common type for a group of related values
  • The value for each case is optional, the value can be a string, a character, or a value of any integer or floating-point type.Unlike C and Objective-C, Swift enumeration cases are not assigned a default integer value when they are created.
  • cases can specify associated values of any type to be stored along with each different case value

Enumeration Syntax

Each enumeration definition defines a brand new type, which name should be with capital and singular rather than plural.

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enum SomeEnumeration {
    // enumeration definition goes here
}

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

// Multiple cases
enum Planet {
    case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune
}

var directionToHead = CompassPoint.west

directionToHead = .east

Matching Enumeration Values with a Switch Statement

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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"

// a switch statement must be exhaustive or provide default case to cover others
let somePlanet = Planet.earth
switch somePlanet {
case .earth:
    print("Mostly harmless")
default:
    print("Not a safe place for humans")
}
// Prints "Mostly harmless"

Associated Values

  • store additional custom information
  • store associated values of any given type, and the value types can be different for each case of the enumeration if needed
  • Extract value from associated value by prefix let orvar
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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."

// If all of the associated values for an enumeration case are extracted as constants
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."

Raw Value vs. Associate Value

Item Raw value Associate value
When to set value first define the enumeration set when you create a new instance
value each case is always the same can be different between each case
value type string,character,integer,float any given type

Raw Values

  • default values (called raw values), which are all of the same type.
  • Raw values can be strings, characters, or any of the integer or floating-point number types. Each raw value must be unique within its enumeration declaration.
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// stores raw ASCII values alongside named enumeration cases:
enum ASCIIControlCharacter: Character {
    case tab = "\t"
    case lineFeed = "\n"
    case carriageReturn = "\r"
}

Implicitly Assigned Raw Values

  • if type is Int,default value is 0, then increment.
  • if type is String,the implicit value for each case is the text of that case’s name.
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enum Planet: Int {
    case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune
}

enum CompassPoint: String {
    case north, south, east, west
}

let earthsOrder = Planet.earth.rawValue
// earthsOrder is 3
 
let sunsetDirection = CompassPoint.west.rawValue
// sunsetDirection is "west"

Initializing from a Raw Value

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let possiblePlanet = Planet(rawValue: 7)
// possiblePlanet is of type Planet? and equals Planet.uranus

let positionToFind = 11

// failable initializer
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"

Recursive Enumerations

????

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

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

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"