Switch must be exhaustive in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean for a switch statement to be exhaustive in Swift?

It means the switch must cover all possible cases of the value being checked, so no case is left unhandled.

beginner

Why does Swift require switch statements to be exhaustive?

To ensure safety by forcing the programmer to handle every possible value, preventing unexpected behavior at runtime.

beginner

How can you make a switch statement exhaustive when you don't want to handle all cases explicitly?

You can add a default case to catch all remaining values.

intermediate

Given an enum with 3 cases, what happens if you write a switch statement that only handles 2 cases without a default?

The code will not compile because the switch is not exhaustive; the missing case must be handled or a default added.

intermediate

Can switch statements on non-enum types be exhaustive in Swift?

Yes, but only by using a default case because non-enum types can have many values.

What keyword can you use to make a switch statement exhaustive without listing all cases?

A default

B else

C catch

D final

The default case catches all values not explicitly handled, making the switch exhaustive.

If you have an enum with 4 cases, how many cases must your switch statement handle to be exhaustive?

A All 4 or use default

B None, switch is always exhaustive

C Only 1

D At least 3

You must handle all 4 cases explicitly or add a default case to cover any unhandled cases.

What happens if a switch statement on an enum is not exhaustive in Swift?

A It compiles but crashes at runtime

B It compiles with a warning

C It does not compile

D It runs but skips missing cases

Swift requires exhaustive switches on enums, so missing cases cause a compile-time error.

Can you omit the default case if your switch covers all enum cases?

A No, default is always required

B Yes, if all cases are covered

C Only if enum has less than 3 cases

D Only for integer types

If all enum cases are explicitly handled, the switch is exhaustive without a default.

Why might you prefer to list all enum cases explicitly instead of using default?

A To avoid runtime errors

B To make code shorter

C To improve performance

D To get compiler errors if new cases are added later

Explicit cases help catch missing cases when enums change, improving code safety.

Explain what it means for a switch statement to be exhaustive in Swift and why it is important.

Think about how Swift prevents missing cases in switch.

Describe how you can make a switch statement exhaustive when working with enums and when working with other types.

Consider differences between enums and other types.

Practice

1. What does it mean when we say a Swift switch statement must be exhaustive?

easy

A. It must handle all possible values of the variable being switched on.

B. It can skip some cases if they are unlikely to happen.

C. It only needs to handle the first case it matches.

D. It must have at least two cases.

Solution

1. Step 1: Understand the meaning of exhaustive switch

   In Swift, a switch must cover every possible value of the variable it checks to avoid missing cases.

2. Step 2: Compare options to definition

   Only It must handle all possible values of the variable being switched on. states that all possible values must be handled, which matches the requirement.

3. Final Answer:

   It must handle all possible values of the variable being switched on. -> Option A

4. Quick Check:

   Exhaustive means all cases covered = A [OK]

Hint: Remember: switch covers all cases or uses default [OK]

Common Mistakes:

• Thinking switch can skip cases
• Believing only first match matters
• Assuming minimum number of cases

2. Which of the following Swift switch statements is syntactically correct and exhaustive for an enum Direction with cases north, south, east, and west?

easy

A. switch dir { case .north: print("North") case .south: print("South") }

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

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

D. switch (dir) { case .north: print("North") case .south: print("South") default: print("Other") }

Solution

1. Step 1: Check all enum cases covered

   The enum has four cases: north, south, east, west. The switch must cover all or use default.

2. Step 2: Analyze each option

   switch dir { case .north: print("North") case .south: print("South") case .east: print("East") case .west: print("West") } covers all four cases explicitly, making it exhaustive and syntactically correct.

3. Final Answer:

   switch with all four cases covered explicitly -> Option B

4. Quick Check:

   All enum cases covered = D [OK]

Hint: Cover all enum cases or use default to be exhaustive [OK]

Common Mistakes:

• Missing some enum cases
• Not using default when cases are incomplete
• Assuming partial cases are enough

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

enum Mood { case happy, sad, neutral }
let todayMood = Mood.sad
switch todayMood {
 case .happy: print("Feeling great!")
 case .sad: print("Feeling down.")
 case .neutral: print("Just okay.")
}

medium

A. Feeling great!

B. Just okay.

C. Feeling down.

D. Compile error: switch not exhaustive

Solution

1. Step 1: Identify the value of todayMood

   todayMood is set to Mood.sad.

2. Step 2: Match the switch case

   The switch has a case for .sad that prints "Feeling down." so this case runs.

3. Final Answer:

   Feeling down. -> Option C

4. Quick Check:

   Value sad matches case sad = B [OK]

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

Common Mistakes:

• Confusing cases and printing wrong output
• Thinking switch is not exhaustive here
• Ignoring the actual value assigned

4. Identify the error in this Swift switch statement for enum Color with cases red, green, and blue:

switch color {
 case .red: print("Red")
 case .green: print("Green")
}

medium

A. Cannot use enum cases without raw values.

B. Syntax error: missing parentheses after switch.

C. No error, this switch is correct.

D. Missing case for .blue or default case, so switch is not exhaustive.

Solution

1. Step 1: Check all enum cases handled

   The enum has three cases: red, green, blue. The switch only handles red and green.

2. Step 2: Identify missing coverage

   Since .blue is missing and no default case is provided, the switch is not exhaustive, causing a compile error.

3. Final Answer:

   Missing case for .blue or default case, so switch is not exhaustive. -> Option D

4. Quick Check:

   All cases must be covered or default = C [OK]

Hint: Always cover all enum cases or add default [OK]

Common Mistakes:

• Ignoring missing enum cases
• Thinking parentheses needed after switch
• Confusing raw values with case coverage

5. Given this enum and switch, how can you make the switch exhaustive without listing all cases explicitly?

enum Status { case success, failure, unknown }
let currentStatus = Status.unknown
switch currentStatus {
 case .success: print("Success")
 case .failure: print("Failure")
}

hard

A. Add a default case to handle all other values.

B. Remove the switch and use if-else instead.

C. Add a case for .unknown explicitly.

D. Change enum to have only two cases.

Solution

1. Step 1: Understand the problem with current switch

   The switch handles .success and .failure but misses .unknown, so it's not exhaustive.

2. Step 2: Find a way to cover missing cases without listing all explicitly

   Adding a default case covers any cases not explicitly handled, making the switch exhaustive.

3. Final Answer:

   Add a default case to handle all other values. -> Option A

4. Quick Check:

   Use default to cover missing cases = A [OK]

Hint: Use default to cover all unlisted cases quickly [OK]

Common Mistakes:

• Thinking you must list every case explicitly
• Removing switch instead of fixing it
• Changing enum just to fix switch