Loop labels in Rust

Introduction

Loop labels help you control which loop to stop or continue when you have loops inside loops.

When you have a loop inside another loop and want to stop the outer loop from inside the inner loop.

When you want to skip to the next round of an outer loop from inside a nested loop.

When you want to make your code clearer by naming loops instead of guessing which loop breaks.

When you want to avoid confusing or complicated logic with flags or extra variables.

Syntax

Rust

'label_name: loop {
    // code
    break 'label_name;
}

Loop labels start with a single quote followed by a name, like 'outer.

You use break 'label_name; or continue 'label_name; to control that loop.

Examples

This breaks the labeled loop named 'outer immediately.

Rust

'outer: loop {
    println!("Outer loop");
    break 'outer;
}

This breaks the outer loop from inside the inner loop.

Rust

'outer: loop {
    'inner: loop {
        break 'outer;
    }
}

This skips the rest of the outer loop when i is 3.

Rust

'counting_up: loop {
    for i in 1..=5 {
        if i == 3 {
            continue 'counting_up;
        }
        println!("i = {}", i);
    }
    break;
}

Sample Program

This program shows how to use loop labels to break or continue the outer loop from inside the inner loop.

Rust

fn main() {
    let mut count = 0;
    'outer: loop {
        println!("Start outer loop: count = {}", count);
        let mut inner_count = 10;
        'inner: loop {
            println!("  Inner loop: inner_count = {}", inner_count);
            if inner_count == 8 {
                println!("  Breaking outer loop from inner loop");
                break 'outer;
            }
            if inner_count == 9 {
                println!("  Continuing outer loop from inner loop");
                inner_count -= 1;
                continue 'outer;
            }
            inner_count -= 1;
        }
        count += 1;
    }
    println!("Exited outer loop");
}

Important Notes

Loop labels only work with loop, while, and for loops.

Labels make nested loops easier to manage and understand.

Summary

Loop labels let you name loops to control them directly.

You can break or continue a specific loop by its label.

This helps when you have loops inside loops and want clear control.

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