Switch must be exhaustive in Swift - Step-by-Step Execution

Concept Flow - Switch must be exhaustive

Start switch with value

↓

Check each case

↓

Execute case body

↓

All cases covered?

No→Error: Must be exhaustive

Yes↓

End

The switch checks each case for a match and must cover all possible values, otherwise it causes an error.

Execution Sample

Swift

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

let dir = Direction.north

switch dir {
case .north: print("Up")
case .south: print("Down")
case .east: print("Right")
case .west: print("Left")
}

This code switches over all Direction enum cases and prints a direction word.

Execution Table

Step	Switch Value	Case Checked	Match?	Action	Output
1	Direction.north	.north	Yes	Execute print("Up")	Up
2	Direction.north	.south	No	Skip
3	Direction.north	.east	No	Skip
4	Direction.north	.west	No	Skip
5	-	All cases checked	Yes	Switch ends

💡 All enum cases are covered, so switch is exhaustive and ends normally.

Variable Tracker

Variable	Start	After Step 1	After Step 5
dir	Direction.north	Direction.north	Direction.north

Key Moments - 3 Insights

Why does Swift require all enum cases to be handled in a switch?

Swift wants to make sure you don't forget any case, so the switch must be exhaustive as shown in execution_table row 5.

What if I leave out one case in the switch?

The code will not compile because the switch is not exhaustive, meaning not all enum cases are covered.

Can I use a default case to make the switch exhaustive?

Yes, a default case covers all remaining cases and makes the switch exhaustive if you don't want to list all explicitly.

Visual Quiz - 3 Questions

Look at the execution table, which case matches the switch value at step 1?

A .south

B .north

C .east

D .west

💡 Hint

Check the 'Case Checked' and 'Match?' columns at step 1 in the execution_table.

At which step does the switch confirm it is exhaustive?

A Step 1

B Step 3

C Step 5

D Step 4

💡 Hint

Look at the 'All cases checked' row in the execution_table.

If the case .west was missing, what would happen?

A The switch would not compile due to missing case.

B The switch would cause a runtime error.

C The switch would still compile and run.

D The switch would print nothing.

💡 Hint

Refer to key_moments about missing cases causing compile errors.

Concept Snapshot

Switch statements in Swift must cover all possible values of the switched variable.
For enums, this means all cases must be handled.
If not all cases are listed, the compiler gives an error.
You can use a default case to cover any missing cases.
This ensures safe and predictable code execution.

Full Transcript

In Swift, when you use a switch statement with an enum, you must cover every possible case. The switch checks each case one by one. If the value matches a case, it runs that case's code. If you forget a case, the compiler will stop you because the switch is not exhaustive. You can add a default case to handle any cases you don't list explicitly. This rule helps prevent bugs by making sure you think about every possible value.

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