Enum methods and computed properties in Swift

Introduction

Enums group related values together. Methods and computed properties add useful actions and information to each enum case.

When you want to add a function that works differently for each enum case.

When you want to calculate a value based on the enum case without storing it.

When you want to keep related code organized inside the enum.

When you want to describe or format enum cases in a custom way.

When you want to add helper features to enum cases for easier use.

Syntax

Swift

enum EnumName {
    case case1
    case case2

    func methodName() -> ReturnType {
        // code using self to act based on case
    }

    var computedPropertyName: ReturnType {
        // code returning a value based on self
    }
}

Use func inside enums to add methods.

Use var with a computed getter to add computed properties.

Examples

A method description() returns a string for each direction.

Swift

enum Direction {
    case north, south, east, west

    func description() -> String {
        switch self {
        case .north: return "Up"
        case .south: return "Down"
        case .east: return "Right"
        case .west: return "Left"
        }
    }
}

A computed property isSafeToGo returns true only for green light.

Swift

enum Light {
    case red, yellow, green

    var isSafeToGo: Bool {
        return self == .green
    }
}

Computed property converts any temperature to Celsius.

Swift

enum Temperature {
    case celsius(Double)
    case fahrenheit(Double)

    var inCelsius: Double {
        switch self {
        case .celsius(let value): return value
        case .fahrenheit(let value): return (value - 32) * 5 / 9
        }
    }
}

Sample Program

This program defines an enum Mood with a method emoji() and a computed property description. It prints the emoji and description for the current mood.

Swift

enum Mood {
    case happy, sad, angry

    func emoji() -> String {
        switch self {
        case .happy: return "😊"
        case .sad: return "😢"
        case .angry: return "😠"
        }
    }

    var description: String {
        switch self {
        case .happy: return "Feeling good"
        case .sad: return "Feeling down"
        case .angry: return "Feeling mad"
        }
    }
}

let currentMood = Mood.happy
print("Mood emoji: \(currentMood.emoji())")
print("Mood description: \(currentMood.description)")

Important Notes

Use self inside methods and computed properties to refer to the current enum case.

Computed properties do not store values; they calculate and return them when accessed.

Methods can take parameters and return values, just like functions outside enums.

Summary

Enums can have methods to perform actions based on cases.

Computed properties provide calculated values related to enum cases.

These features help keep related code organized and clear.

Practice

1. What is the purpose of adding methods to an enum in Swift?

easy

A. To inherit properties from other enums

B. To store additional data inside enum cases

C. To perform actions or calculations related to enum cases

D. To convert enums into classes automatically

Solution

1. Step 1: Understand enum methods

   Enum methods allow you to add functions that perform tasks related to the enum's cases.

2. Step 2: Differentiate from other enum features

   Enums do not store extra data like classes; methods help organize related actions.

3. Final Answer:

   To perform actions or calculations related to enum cases -> Option C

4. Quick Check:

   Enum methods = perform actions [OK]

Hint: Methods in enums perform actions tied to cases [OK]

Common Mistakes:

• Thinking enums can inherit from other enums
• Confusing methods with stored properties
• Assuming enums convert to classes automatically

2. Which of the following is the correct syntax to declare a computed property inside an enum in Swift?

easy

A. static var description: String = "Example"

B. func description() -> String { return "Example" }

C. let description = "Example"

D. var description: String { return "Example" }

Solution

1. Step 1: Identify computed property syntax

   Computed properties use var with a code block returning a value, like var name: Type { ... }.

2. Step 2: Check options

   var description: String { return "Example" } matches computed property syntax; B is a method, C is a stored property, D is a static stored property.

3. Final Answer:

   var description: String { return "Example" } -> Option D

4. Quick Check:

   Computed property syntax = var with braces [OK]

Hint: Computed properties use var with braces returning a value [OK]

Common Mistakes:

• Using func instead of var for computed properties
• Declaring computed properties as let constants
• Confusing static stored properties with computed ones

3. Consider this Swift enum with a computed property and method:

enum Direction {
    case north, south, east, west

    var isVertical: Bool {
        return self == .north || self == .south
    }

    func description() -> String {
        switch self {
        case .north: return "Up"
        case .south: return "Down"
        case .east: return "Right"
        case .west: return "Left"
        }
    }
}

let dir = Direction.east
print(dir.isVertical)
print(dir.description())

What is the output?

medium

A. false
Right

B. true
Right

C. false
East

D. true
East

Solution

1. Step 1: Evaluate computed property isVertical

   For Direction.east, isVertical checks if east == north or south, which is false.

2. Step 2: Evaluate description() method

   Switch on east returns "Right" string.

3. Final Answer:

   false Right -> Option A

4. Quick Check:

   east isVertical = false, description = Right [OK]

Hint: Check enum case matches in computed property and switch [OK]

Common Mistakes:

• Assuming east is vertical
• Confusing method return values with case names
• Forgetting to call method with parentheses

4. This Swift enum code has an error:

enum Status {
    case success, failure

    var message: String {
        switch self {
        case .success:
            return "Success"
        case .failure
            return "Failure"
        }
    }
}

What is the error and how to fix it?

medium

A. Computed property must be a function; change to func message() -> String

B. Missing colon after case .failure; add ':' after .failure

C. Enum cases must have raw values; add raw values to cases

D. Return statements are not allowed in computed properties; remove returns

Solution

1. Step 1: Identify syntax error in switch cases

   Case .failure is missing a colon ':' after it, causing a syntax error.

2. Step 2: Fix the syntax

   Add ':' after case .failure to correct the switch syntax.

3. Final Answer:

   Missing colon after case .failure; add ':' after .failure -> Option B

4. Quick Check:

   Switch case syntax requires ':' [OK]

Hint: Switch cases must end with a colon ':' [OK]

Common Mistakes:

• Thinking computed properties need func keyword
• Adding raw values unnecessarily
• Removing return statements inside computed properties

5. You want to create an enum Light with cases red, yellow, and green. Add a computed property time that returns the duration in seconds for each light: red = 30, yellow = 5, green = 25. Also add a method next() that returns the next light in the traffic cycle (red -> green -> yellow -> red). Which implementation is correct?

hard

A. enum Light {
case red, yellow, green

var time: Int {
switch self {
case .red: return 30
case .yellow: return 5
case .green: return 25
}
}

func next() -> Light {
switch self {
case .red: return .green
case .green: return .yellow
case .yellow: return .red
}
}
}

B. enum Light {
case red, yellow, green

func time() -> Int {
if self == .red { return 30 }
else if self == .yellow { return 5 }
else { return 25 }
}

func next() -> Light {
if self == .red { return .yellow }
else if self == .yellow { return .green }
else { return .red }
}
}

C. enum Light {
case red, yellow, green

var time: Int {
if self == .red { return 30 }
else if self == .yellow { return 5 }
else { return 25 }
}

func next() -> Light {
switch self {
case .red: return .yellow
case .yellow: return .green
case .green: return .red
}
}
}

D. enum Light {
case red, yellow, green

var time: Int {
switch self {
case .red: return 25
case .yellow: return 30
case .green: return 5
}
}

func next() -> Light {
switch self {
case .red: return .green
case .green: return .yellow
case .yellow: return .red
}
}
}

Solution

1. Step 1: Check computed property time values

   enum Light { case red, yellow, green var time: Int { switch self { case .red: return 30 case .yellow: return 5 case .green: return 25 } } func next() -> Light { switch self { case .red: return .green case .green: return .yellow case .yellow: return .red } } } correctly returns 30 for red, 5 for yellow, and 25 for green as required.

2. Step 2: Check next() method cycle order

   enum Light { case red, yellow, green var time: Int { switch self { case .red: return 30 case .yellow: return 5 case .green: return 25 } } func next() -> Light { switch self { case .red: return .green case .green: return .yellow case .yellow: return .red } } } cycles red -> green -> yellow -> red, matching the problem statement.

3. Final Answer:

   enum Light { case red, yellow, green var time: Int { switch self { case .red: return 30 case .yellow: return 5 case .green: return 25 } } func next() -> Light { switch self { case .red: return .green case .green: return .yellow case .yellow: return .red } } } -> Option A

4. Quick Check:

   Correct time values and cycle order = enum Light { case red, yellow, green var time: Int { switch self { case .red: return 30 case .yellow: return 5 case .green: return 25 } } func next() -> Light { switch self { case .red: return .green case .green: return .yellow case .yellow: return .red } } } [OK]

Hint: Match switch cases exactly to problem requirements [OK]

Common Mistakes:

• Mixing up time values for colors
• Incorrect next() cycle order
• Using func instead of computed property for time