Loop labels in Rust - Step-by-Step Execution

Concept Flow - Loop labels

Start Outer Loop 'outer'

↓

Start Inner Loop 'inner'

↓

Check Condition in Inner

↓

Break 'outer'

↓

Exit Outer

↓

Repeat Outer

The flow shows how labeled loops let you break or continue specific loops, even nested ones.

Execution Sample

Rust

fn main() {
    'outer: for i in 1..=3 {
        'inner: for j in 1..=3 {
            if i == 2 && j == 2 {
                break 'outer;
            }
            println!("i={}, j={}", i, j);
        }
    }
}

This code uses loop labels to break the outer loop from inside the inner loop when i and j both equal 2.

Execution Table

Step	i	j	Condition (i==2 && j==2)	Action	Output
1	1	1	false	Print i=1, j=1	i=1, j=1
2	1	2	false	Print i=1, j=2	i=1, j=2
3	1	3	false	Print i=1, j=3	i=1, j=3
4	2	1	false	Print i=2, j=1	i=2, j=1
5	2	2	true	Break 'outer' loop
6	-	-	-	Exit all loops

💡 At step 5, condition is true, so break 'outer' exits all loops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
i	-	1	1	1	2	2	-
j	-	1	2	3	1	2	-

Key Moments - 3 Insights

Why does the program stop printing after i=2, j=2?

Because at step 5 in the execution_table, the condition i==2 && j==2 is true, and break 'outer' exits the outer loop immediately, stopping all loops.

What happens if we use break without a label inside nested loops?

Without a label, break only exits the innermost loop. Here, break 'outer' is needed to exit the outer loop from inside the inner loop, as shown in step 5.

Can continue use labels like break?

Yes, continue can also use labels to skip to the next iteration of a specific loop, but this example only shows break with labels.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of j at step 4?

A 1

B 2

C 3

D -

💡 Hint

Check the 'j' column at step 4 in the execution_table.

At which step does the program break out of the outer loop?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look at the 'Action' column where break 'outer' is executed.

If we remove the label 'outer' from break, what would happen?

A Breaks the outer loop anyway

B Causes a compile error

C Breaks only the inner loop

D Continues the inner loop

💡 Hint

Recall that break without label exits only the innermost loop, as explained in key_moments.

Concept Snapshot

Loop labels in Rust let you name loops.
Use 'label: for ...' to name a loop.
Use break 'label or continue 'label to control specific loops.
This helps manage nested loops clearly.
Without labels, break/continue affect only innermost loop.

Full Transcript

This example shows how Rust loop labels work. We have two loops: outer and inner. The inner loop runs inside the outer loop. When i equals 2 and j equals 2, the program breaks the outer loop using break 'outer;. This stops all looping immediately. The execution table shows each step: printing values until the break condition is met. Variable tracker shows how i and j change. Key moments explain why the program stops and how labels affect break behavior. The quiz tests understanding of variable values and loop control. Loop labels help control nested loops precisely.