Loop labels in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a loop label in Rust?

A loop label is a name given to a loop so you can refer to it explicitly, especially useful to control nested loops by breaking or continuing a specific loop.

beginner

How do you define a loop label in Rust?

You define a loop label by writing an apostrophe followed by the label name before the loop keyword, like 'label_name: loop { ... }.

beginner

How do you break out of a specific labeled loop in Rust?

Use break 'label_name; to exit the loop with that label, even if it is nested inside other loops.

intermediate

What happens if you use continue 'label_name; inside nested loops?

It skips the rest of the current iteration of the labeled loop and starts the next iteration of that loop, not just the innermost loop.

beginner

Why are loop labels useful in nested loops?

They let you control which loop to break or continue, avoiding confusion and making your code clearer when dealing with multiple loops inside each other.

How do you start a loop with a label named 'outer' in Rust?

A label outer: loop { ... }

B 'outer: loop { ... }

C loop 'outer { ... }

D loop label 'outer { ... }

The correct syntax is an apostrophe followed by the label name before the loop keyword, like 'outer: loop { ... }.

What does break 'outer; do inside nested loops?

A Breaks out of the loop labeled 'outer'

B Breaks out of the innermost loop only

C Continues the loop labeled 'outer'

D Causes a syntax error

It breaks out of the loop that has the label 'outer', regardless of nesting.

Which keyword is used with a loop label to skip to the next iteration of that loop?

A break

B skip

C continue

D next

The keyword 'continue' is used with a loop label to skip the rest of the current iteration and start the next iteration of the labeled loop.

Can you use loop labels with all kinds of loops in Rust?

A No, only with infinite loops

B Only with for loops

C Only with the 'loop' keyword loops

D Yes, with any loop including for, while, and loop

Loop labels can be used with any loop type: for, while, and loop.

What is the main benefit of using loop labels in nested loops?

A To control which loop to break or continue

B To create infinite loops

C To avoid using break statements

D To make loops run faster

Loop labels help you specify exactly which loop to break or continue, which is very helpful in nested loops.

Explain how loop labels work in Rust and why they are useful.

Think about how you control loops inside loops.

Describe the difference between using break and continue with loop labels.

Consider what happens to the loop after each keyword.

Practice

1. What is the main purpose of using loop labels in Rust?

easy

A. To create infinite loops without conditions.

B. To declare variables inside loops.

C. To name loops so you can break or continue specific loops inside nested loops.

D. To automatically parallelize loops for performance.

Solution

1. Step 1: Understand loop labels concept

   Loop labels let you assign a name to a loop, especially useful when loops are nested.

2. Step 2: Identify the use of labels

   Labels allow you to break or continue a specific loop by referring to its name, avoiding ambiguity.

3. Final Answer:

   To name loops so you can break or continue specific loops inside nested loops. -> Option C

4. Quick Check:

   Loop labels = control nested loops [OK]

Hint: Labels name loops for clear break/continue control [OK]

Common Mistakes:

• Thinking labels create infinite loops
• Confusing labels with variable declarations
• Assuming labels optimize performance automatically

2. Which of the following is the correct syntax to label a loop in Rust?

easy

A. label: loop { /* code */ }

B. loop label { /* code */ }

C. loop: label { /* code */ }

D. loop { label: /* code */ }

Solution

1. Step 1: Recall Rust loop label syntax

   In Rust, a loop label is written as 'label: before the loop keyword.

2. Step 2: Match syntax options

   label: loop { /* code */ } correctly places the label before the loop with a colon, e.g., 'label: loop {}.

3. Final Answer:

   label: loop { /* code */ } -> Option A

4. Quick Check:

   Label syntax = 'label: loop' [OK]

Hint: Labels come before loop with a colon [OK]

Common Mistakes:

• Placing label after loop keyword
• Using colon after loop instead of label
• Putting label inside loop body

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

fn main() {
    let mut count = 0;
    'outer: loop {
        println!("Outer loop: {}", count);
        let mut inner_count = 10;
        loop {
            println!("  Inner loop: {}", inner_count);
            if inner_count == 9 {
                break 'outer;
            }
            inner_count -= 1;
        }
        count += 1;
    }
    println!("Done");
}

medium

A. Outer loop: 0
Inner loop: 10
Inner loop: 9
Inner loop: 8
Done

B. Outer loop: 0
Inner loop: 10
Done

C. Infinite loop with no output

D. Outer loop: 0
Inner loop: 10
Inner loop: 9
Done

Solution

1. Step 1: Trace outer loop first iteration

   count starts at 0, prints "Outer loop: 0". Then inner loop starts with inner_count = 10.

2. Step 2: Trace inner loop and break condition

   Inner loop prints 10, then 9. When inner_count == 9, it breaks the 'outer loop, exiting both loops.

3. Final Answer:

   Outer loop: 0 Inner loop: 10 Inner loop: 9 Done -> Option D

4. Quick Check:

   Break 'outer stops both loops at inner_count 9 [OK]

Hint: Break 'label exits named loop immediately [OK]

Common Mistakes:

• Thinking break only exits inner loop
• Missing that break 'outer exits both loops
• Assuming loops continue infinitely

4. Identify the error in this Rust code using loop labels:

fn main() {
    'start: loop {
        loop {
            break start;
        }
    }
}

medium

A. Missing apostrophe before label name in break statement.

B. Label 'start' is not defined before use.

C. Cannot break a labeled loop from an inner loop.

D. Syntax error: break statements cannot have labels.

Solution

1. Step 1: Check break statement syntax

   In Rust, breaking a labeled loop requires the label with an apostrophe, e.g., break 'start;.

2. Step 2: Identify the missing apostrophe

   The code uses break start; missing the apostrophe before the label name, causing a syntax error.

3. Final Answer:

   Missing apostrophe before label name in break statement. -> Option A

4. Quick Check:

   Label break syntax = break 'label [OK]

Hint: Always prefix label with apostrophe in break/continue [OK]

Common Mistakes:

• Omitting apostrophe in break label
• Assuming labels are variables
• Thinking break can't target outer loops

5. Given nested loops labeled 'outer and 'inner, how can you skip the current iteration of the outer loop from inside the inner loop?

hard

A. Use break 'inner; inside the inner loop.

B. Use continue 'outer; inside the inner loop.

C. Use continue; without label inside the inner loop.

D. Use break 'outer; inside the inner loop.

Solution

1. Step 1: Understand continue with labels

   Using continue 'outer; skips the current iteration of the loop labeled 'outer, jumping to its next iteration.

2. Step 2: Differentiate break and continue

   break exits the loop, while continue skips to the next iteration. To skip outer loop iteration from inner loop, use continue 'outer;.

3. Final Answer:

   Use continue 'outer; inside the inner loop. -> Option B

4. Quick Check:

   continue 'label skips that loop iteration [OK]

Hint: Use continue 'label to skip outer loop iteration [OK]

Common Mistakes:

• Using break instead of continue to skip iteration
• Omitting label in continue statement
• Confusing break and continue effects