열거형

열거형 구문

enum SomeEnumeration {
    // enumeration definition goes here
}

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

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

사용

var directionToHead = CompassPoint.west

directionToHead = .east

Switch에서의 사용

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")
default:
    print("Not a safe place for humans")
}

반복문

열거형 이름 뒤에 CaseIterable을 붙여 활성화 한다.

enum Beverage: CaseIterable {
    case coffee, tea, juice
}
let numberOfChoices = Beverage.allCases.count
print("\(numberOfChoices) beverages available")
// Prints "3 beverages available"

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

## 연관된 값
> 어떤 시스템이 2가지 타입의 바코드로 제품을 추적해야할 때

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

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

원시값

초기화를 미리할 수 있음

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

암시적으로 할당된 원시값

타입과 미리 할당된 값으로부터 유추한 원시값을 각각 가지고 있다.

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"

원시값으로 초기화

rawValue를 이용해서 초기화를 할 수 있는데, 없는 경우도 있으므로 원시값 초기화는 항상 옵셔널로 반환된다.

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"

재귀 열거형

케이스 작성 전에 indirect를 작성

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)
}

사용 예시

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"