Why Swift has no implicit fallthrough - Visual Breakdown

Concept Flow - Why Swift has no implicit fallthrough

Start switch statement

↓

Match case?

No→Check next case

Yes↓

Execute case body

↓

Fallthrough?

No→Exit switch

Yes↓

Execute next case body

↓

Repeat fallthrough check

Swift switch checks each case and executes only the matched one unless 'fallthrough' is explicitly used to continue to the next case.

Execution Sample

Swift

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

This code prints "Two" and then "Three" because of the explicit fallthrough after case 2.

Execution Table

Step	Matched Case	Action	Output
1	case 1	Check if number == 1	No
2	case 2	Check if number == 2	Yes
3	case 2	Print "Two"	Two
4	case 2	fallthrough to next case
5	case 3	Execute case 3 body	Three
6	Exit switch	No more fallthrough

💡 No implicit fallthrough, switch exits after case 3 because no fallthrough keyword is used.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
number	2	2	2	2

Key Moments - 2 Insights

Why doesn't Swift automatically continue to the next case after a match?

Swift requires explicit 'fallthrough' to continue to the next case, preventing accidental execution of multiple cases as shown in steps 4 and 5 of the execution_table.

What happens if 'fallthrough' is omitted after a matched case?

The switch statement exits immediately after executing the matched case, as seen after step 3 where no fallthrough means no further cases run.

Visual Quiz - 3 Questions

Look at the execution table, what is printed at step 3?

A "Two"

B "Three"

C "One"

D Nothing

💡 Hint

Check the Output column at step 3 in the execution_table.

At which step does the switch statement use fallthrough?

A Step 2

B Step 4

C Step 6

D Step 7

💡 Hint

Look for the 'fallthrough' action in the Action column of the execution_table.

If the 'fallthrough' keyword was removed, what would be the output?

A "Two" and "Three"

B "Three" only

C "Two" only

D "One"

💡 Hint

Refer to the key_moments explanation about what happens without fallthrough.

Concept Snapshot

Swift switch statements do NOT fall through cases by default.
You must use the 'fallthrough' keyword to continue to the next case.
This prevents accidental execution of multiple cases.
Each case is exclusive unless fallthrough is explicit.
This makes code safer and clearer.

Full Transcript

In Swift, switch statements check each case one by one. When a case matches, Swift runs only that case's code and then exits the switch. Unlike some languages, Swift does not automatically continue to the next case. To run the next case, you must write the 'fallthrough' keyword explicitly. This behavior helps avoid bugs where multiple cases run by accident. For example, if number is 2, Swift runs case 2 and prints "Two". If 'fallthrough' is used, it also runs case 3 and prints "Three". Without 'fallthrough', it stops after printing "Two". This clear control flow makes Swift code safer and easier to understand.