Enum with switch pattern matching in Swift - Step-by-Step Execution

Concept Flow - Enum with switch pattern matching

Define Enum with cases

↓

Create Enum instance

↓

Switch on Enum instance

↓

Match case 1?

No→Match case 2?

↓

Execute case 1

↓

End switch

We define an enum, create an instance, then use a switch to check which case it matches and run code accordingly.

Execution Sample

Swift

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

let dir = Direction.east

switch dir {
case .north:
    print("Going north")
case .south:
    print("Going south")
case .east:
    print("Going east")
case .west:
    print("Going west")
}

This code defines directions, sets one, then prints a message based on the direction using switch.

Execution Table

Step	Action	Enum Instance	Switch Case Checked	Match?	Output
1	Create enum instance	Direction.east	-	-	-
2	Switch start	Direction.east	case .north	No	-
3	Switch continue	Direction.east	case .south	No	-
4	Switch continue	Direction.east	case .east	Yes	Going east
5	Switch ends	Direction.east	-	-	-

💡 Matched case .east, switch ends after executing its block.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
dir	undefined	Direction.east	Direction.east	Direction.east	Direction.east	Direction.east

Key Moments - 3 Insights

Why does the switch check each case one by one?

The switch tries cases in order until it finds a match, as shown in rows 2-4 of the execution table.

What happens if no case matches?

If no case matches, the switch would run a default case if provided, but here it matches at step 4 and stops.

Why do we write cases as .north instead of Direction.north inside switch?

Inside switch on an enum instance, Swift lets us use shorthand .caseName to avoid repetition, as seen in the code sample.

Visual Quiz - 3 Questions

Look at the execution table, what is the enum instance value at step 3?

A Direction.north

B Direction.east

C Direction.south

D Direction.west

💡 Hint

Check the 'Enum Instance' column at step 3 in the execution table.

At which step does the switch find a matching case?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Match?' column to see where it says 'Yes'.

If the enum instance was Direction.west, which case would match?

A case .west

B case .south

C case .north

D case .east

💡 Hint

Refer to the enum cases and how switch matches the instance value.

Concept Snapshot

enum Direction { case north, south, east, west }
let dir = Direction.east
switch dir {
 case .north: ...
 case .south: ...
 case .east: ...
 case .west: ...
}
Use switch to run code based on enum case, matching one case exactly.

Full Transcript

We start by defining an enum called Direction with four cases: north, south, east, and west. Then we create a variable dir and set it to Direction.east. Next, we use a switch statement on dir. The switch checks each case in order: first .north, then .south, then .east. When it reaches .east, it matches and prints 'Going east'. The switch then ends. If no case matched, a default case would run if provided. Inside the switch, we use shorthand .caseName instead of full enum names. This example shows how to use enums with switch pattern matching in Swift.

Practice

1. What is the main purpose of using an enum with associated values in Swift?

easy

A. To group related values and store extra data in each case

B. To create a class with multiple methods

C. To define a function that returns multiple values

D. To declare a variable that can only hold integers

Solution

1. Step 1: Understand enum basics

   An enum groups related values under one type, making code organized.

2. Step 2: Recognize associated values

   Enums with associated values can store extra data for each case, like a label or number.

3. Final Answer:

   To group related values and store extra data in each case -> Option A

4. Quick Check:

   Enum with associated values = group + extra data [OK]

Hint: Enums group related data with extra info easily [OK]

Common Mistakes:

• Confusing enums with classes
• Thinking enums only hold fixed values
• Assuming enums are functions

2. Which of the following is the correct syntax to match an enum case with associated values using switch in Swift?

easy

A. case .someCase(let value):

B. case someCase = value:

C. case .someCase => value:

D. case someCase(value) {

Solution

1. Step 1: Recall switch case syntax for enums

   In Swift, to match an enum case with associated values, use case .caseName(let variable):.

2. Step 2: Identify correct pattern

   case .someCase(let value): uses the correct dot syntax and binding with let.

3. Final Answer:

   case .someCase(let value): -> Option A

4. Quick Check:

   Switch enum case with let binding = case .someCase(let value): [OK]

Hint: Use dot and let to bind enum associated values [OK]

Common Mistakes:

• Missing dot before case name
• Using = instead of :
• Wrong arrow or braces syntax

3. What will be the output of this Swift code?

enum Direction {
    case north, south, east, west
}

let dir = Direction.east

switch dir {
case .north:
    print("Going North")
case .south:
    print("Going South")
case .east:
    print("Going East")
case .west:
    print("Going West")
}

medium

A. No output

B. Going North

C. Going West

D. Going East

Solution

1. Step 1: Identify enum value assigned

   The variable dir is assigned Direction.east.

2. Step 2: Match switch case

   The switch matches .east case and prints "Going East".

3. Final Answer:

   Going East -> Option D

4. Quick Check:

   dir = east prints "Going East" [OK]

Hint: Match enum value to switch case to find output [OK]

Common Mistakes:

• Confusing enum case names
• Forgetting to include all cases
• Assuming default case runs

4. Identify the error in this Swift code using enum and switch:

enum Result {
    case success(Int)
    case failure(String)
}

let res = Result.success(10)

switch res {
case .success(let value):
    print("Value: \(value)")
case failure(let message):
    print("Error: \(message)")
}

medium

A. Switch statement missing default case

B. Wrong associated value type for 'success'

C. Missing dot before 'failure' in switch case

D. Enum cases cannot have associated values

Solution

1. Step 1: Check switch case syntax

   In switch, enum cases must be prefixed with a dot, like .failure.

2. Step 2: Identify missing dot

   The second case uses failure without a dot, causing a syntax error.

3. Final Answer:

   Missing dot before 'failure' in switch case -> Option C

4. Quick Check:

   Enum case in switch needs dot prefix [OK]

Hint: Always prefix enum cases with dot in switch [OK]

Common Mistakes:

• Omitting dot before enum case in switch
• Confusing associated value types
• Forgetting to handle all cases

5. Given this enum and function, what will describeShape(.rectangle(width: 5, height: 10)) print?

enum Shape {
    case circle(radius: Double)
    case rectangle(width: Double, height: Double)
    case point
}

func describeShape(_ shape: Shape) {
    switch shape {
    case .circle(let r):
        print("Circle with radius \(r)")
    case .rectangle(let w, let h):
        print("Rectangle \(w) x \(h)")
    case .point:
        print("Just a point")
    }
}

hard

A. Circle with radius 5.0

B. Rectangle 5.0 x 10.0

C. Just a point

D. Syntax error in switch

Solution

1. Step 1: Identify enum case and associated values

   The input is .rectangle(width: 5, height: 10), so width = 5 and height = 10.

2. Step 2: Match switch case and print

   The switch matches .rectangle(let w, let h) and prints "Rectangle 5.0 x 10.0".

3. Final Answer:

   Rectangle 5.0 x 10.0 -> Option B

4. Quick Check:

   Rectangle case prints dimensions [OK]

Hint: Match enum case and extract all associated values [OK]

Common Mistakes:

• Using wrong variable names in case
• Missing multiple associated values in pattern
• Assuming default case runs