Lesson 09 of 20

8 mins read

Loops, Defer Execution & Labeled Statements

In Plain English

Master loops and execution transfers in Swift. Learn for-in loops with tuple enumeration (.enumerated()), while, repeat-while, labeled statements for breaking outer loops, and the defer keyword for guaranteed LIFO resource cleanup.

The Mental Model

"defer is like writing your return checklist right as you check into a hotel room ("put room key on desk"). Even if you leave in a rush, the checklist executes right as you step through the doorway."

Deep Dive: How It Works

for-in Loops: Iterates over arrays, dictionaries, sets, ranges, and characters.

Enumerated Traversal: for (index, item) in list.enumerated() yields both offset and value.

repeat-while: Executes loop body at least once before testing condition (analogous to do-while in C/Java).

The defer Keyword: Schedules a block of code to run when the current execution scope exits, regardless of how it exits.

Labeled Statements: Prefix loops with a label (outerLoop: for ...) to specify which loop break outerLoop terminates.

Syntax Blueprint

defer { print("Cleanup executed") }
for (idx, val) in items.enumerated() {
    print("\(idx): \(val)")
}

Use defer for guaranteed exit cleanup and .enumerated() for indexed traversal.

Core Rules to Remember

Guaranteed Defer Cleanup: defer executes even if an error is thrown or an early return occurs.

LIFO Defer Order: Multiple defer blocks inside the same scope execute in reverse order (Last-In, First-Out).

Labeled Control: Enables breaking or continuing specific outer loops cleanly without flag variables.

Common Beginner Traps & How to Fix Them

Mistake:Expecting defer inside a loop to run when the function returns.

Why it happens: defer executes at the end of the immediate enclosing scope (each loop iteration), not function scope.

How to fix: Place defer at the appropriate scope level where cleanup is intended.

Pro Tip: Use for _ in 1...5 when you need to repeat an action N times without using the loop counter variable.

Live Interactive Example

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Defer Cleanup Execution and Labeled Loops

Swift 5.10 / LLVM

Output Console

Click "Run Code" to view the rendered output.

How it works: Defers executed in reverse order (2 then 1), and break outerLoop exited nested loops cleanly.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Enumerate Array Elements

Declare let languages = ["Swift", "Go", "Rust"].
Use for (idx, lang) in languages.enumerated() to print "#\(idx + 1): \(lang)".

Sandbox Output

Click "Run & Check" to test your solution.

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