No implicit fallthrough in switch in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'No implicit fallthrough in switch' mean in Swift?

In Swift, each case in a switch statement must explicitly end with a break or return. Unlike some languages, Swift does not automatically continue to the next case unless you explicitly tell it to with the 'fallthrough' keyword.

beginner

How do you make a switch case continue to the next case in Swift?

You use the 'fallthrough' keyword at the end of a case to continue execution to the next case. Without 'fallthrough', Swift stops after the current case.

intermediate

Why does Swift require explicit fallthrough instead of implicit?

Swift's design avoids bugs by preventing accidental fallthrough. This makes code clearer and safer because you must clearly state when you want to continue to the next case.

beginner

What happens if you forget to add 'break' or 'fallthrough' in a Swift switch case?

Swift automatically ends the case after its code runs. You don't need to add 'break' explicitly because Swift does not fall through by default.

beginner

Show a simple Swift switch example that uses 'fallthrough'.

Example:
let number = 2
switch number {
 case 1:
  print("One")
 case 2:
  print("Two")
  fallthrough
 case 3:
  print("Three")
 default:
  print("Other")
}
Output:
Two
Three

In Swift, what happens after a switch case finishes executing if there is no 'fallthrough'?

A It automatically continues to the next case

B It throws an error

C The switch exits and does not continue to the next case

D It repeats the current case

Swift switch cases do not fall through by default. Execution stops after the current case unless 'fallthrough' is used.

Which keyword allows a Swift switch case to continue to the next case?

A next

B continue

C break

D fallthrough

The 'fallthrough' keyword explicitly tells Swift to continue to the next case.

Why does Swift avoid implicit fallthrough in switch statements?

A To make code run faster

B To prevent accidental bugs and improve clarity

C Because it is harder to implement

D To allow multiple cases to run automatically

Explicit fallthrough prevents accidental execution of multiple cases, making code safer and clearer.

What will this Swift code print? let x = 1 switch x { case 1: print("One") case 2: print("Two") fallthrough case 3: print("Three") default: print("Other") }

A One

B One Two Three

C Two Three

D One Three

Since x is 1, only the first case runs and stops. No fallthrough is used, so it prints 'One' only.

Is the 'break' keyword required at the end of each Swift switch case?

A No, Swift does not require it

B Only if you want to stop execution

C Yes, always

D Only in default case

Swift automatically ends each case without needing 'break'.

Explain how Swift handles switch case execution and how it differs from languages with implicit fallthrough.

Think about what happens after a case runs in Swift.

Write a Swift switch statement that prints 'Low', 'Medium', or 'High' for values 1, 2, and 3, and uses 'fallthrough' to print 'Check again' after 'Medium'.

Use 'fallthrough' only where you want to continue to the next case.

Practice

1. What happens if you do not use fallthrough in a Swift switch statement?

easy

A. Only the matched case runs; no automatic fallthrough to the next case.

B. All cases after the matched one run automatically.

C. The program throws a runtime error.

D. The switch statement ignores the matched case.

Solution

1. Step 1: Understand Swift switch behavior

   Swift switch statements do not automatically continue to the next case after a match.

2. Step 2: Role of fallthrough keyword

   To continue to the next case, you must explicitly use the fallthrough keyword.

3. Final Answer:

   Only the matched case runs; no automatic fallthrough to the next case. -> Option A

4. Quick Check:

   No implicit fallthrough = matched case only [OK]

Hint: Remember: Swift switch stops after matched case unless fallthrough used [OK]

Common Mistakes:

• Assuming all cases run after a match like in C
• Forgetting to add fallthrough when needed
• Expecting runtime errors without fallthrough
• Thinking switch ignores matched cases

2. Which of the following is the correct syntax to explicitly continue to the next case in a Swift switch?

easy

A. Use the keyword continue inside the case.

B. No syntax needed; Swift does it automatically.

C. Use the keyword fallthrough inside the case.

D. Use a comma , between cases.

Solution

1. Step 1: Identify the keyword for fallthrough

   Swift uses the keyword fallthrough to continue to the next case.

2. Step 2: Eliminate incorrect options

   continue is for loops, commas separate cases but don't cause fallthrough, and Swift does not do it automatically.

3. Final Answer:

   Use the keyword fallthrough inside the case. -> Option C

4. Quick Check:

   Explicit fallthrough keyword = fallthrough [OK]

Hint: Use fallthrough keyword to continue to next case in Swift switch [OK]

Common Mistakes:

• Confusing fallthrough with continue
• Using commas to join cases expecting fallthrough
• Assuming automatic fallthrough without keyword
• Using break to continue to next case

3. What is the output of the following Swift code?

let number = 2
switch number {
 case 1:
   print("One")
 case 2:
   print("Two")
   fallthrough
 case 3:
   print("Three")
 default:
   print("Other")
}

medium

A. Three

B. Two
Three

C. Two

D. Two
Three
Other

Solution

1. Step 1: Identify matched case

   The value 2 matches case 2, so it prints "Two".

2. Step 2: Check for fallthrough

   Since fallthrough is used, execution continues to case 3, printing "Three".

3. Final Answer:

   Two Three -> Option B

4. Quick Check:

   fallthrough causes next case to run = Two and Three [OK]

Hint: Look for fallthrough keyword to know if next case runs [OK]

Common Mistakes:

• Ignoring fallthrough and printing only "Two"
• Printing "Three" only without "Two"
• Expecting default case to run
• Assuming all cases print

4. Identify the error in this Swift switch statement:

let value = 3
switch value {
 case 1:
   print("One")
 case 2:
   print("Two")
 case 3:
   print("Three")
   fallthrough
 case 4:
   print("Four")
   fallthrough
 case 5:
   print("Five")
}

medium

A. Missing default case causes a compile error.

B. Multiple fallthroughs cause a runtime error.

C. No error; code runs and prints Three, Four, Five.

D. Fallthrough cannot be used in the last case.

Solution

1. Step 1: Exhaustiveness requirement

   Swift switch on Int must be exhaustive: cover all possible values or add default. Cases 1-5 don't cover all Ints, no default: compile error.

2. Step 2: Fallthrough usage is valid

   Fallthroughs in cases 3 and 4 are allowed; no fallthrough in case 5 (correct); multiples OK, no runtime issues.

3. Final Answer:

   Missing default case causes a compile error. -> Option A

4. Quick Check:

   Non-exhaustive switch on Int = compile error [OK]

Hint: Swift switch on Int needs default unless all cases covered [OK]

Common Mistakes:

• Picking A: no fallthrough in last case
• Picking B: multiple fallthroughs allowed
• Picking C: ignores compile-time exhaustiveness error
• Thinking value=3 makes it exhaustive

5. You want to print all numbers from a matched case up to 5 in a Swift switch. Which code correctly uses fallthrough to achieve this?

let num = 3
switch num {
 case 1:
   print(1)
   fallthrough
 case 2:
   print(2)
   fallthrough
 case 3:
   print(3)
   fallthrough
 case 4:
   print(4)
   fallthrough
 case 5:
   print(5)
 default:
   print("Done")
}

hard

A. This code prints 1 to 5 regardless of num.

B. This code prints only 3 when num is 3.

C. This code causes a compile error due to fallthrough in default.

D. This code prints 3, 4, 5 when num is 3.

Solution

1. Step 1: Identify matched case and fallthrough chain

   When num is 3, case 3 matches and prints 3, then fallthrough continues to cases 4 and 5 printing 4 and 5.

2. Step 2: Confirm output and default usage

   Default is not reached because cases 3 to 5 cover the fallthrough chain; no error occurs.

3. Final Answer:

   This code prints 3, 4, 5 when num is 3. -> Option D

4. Quick Check:

   Fallthrough chains print from matched case to end [OK]

Hint: Use fallthrough in each case to print all following cases [OK]

Common Mistakes:

• Expecting only matched case to print
• Thinking default runs after fallthrough chain
• Assuming compile error from fallthrough in default
• Believing all cases print regardless of match