Nested Loops
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A Basic Nested Loop
Placing one for loop inside another lets you iterate over every combination of two ranges, which is the basis for processing grids, tables, and matrices.
Example: A Basic Nested Loop
fn main() {
for row in 1..=3 {
for col in 1..=3 {
print!("({},{}) ", row, col);
}
println!();
}
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Breaking the Inner Loop Only
By default, break only exits the innermost loop it is written in. The outer loop continues running its remaining iterations normally.
Example: Breaking the Inner Loop Only
fn main() {
for i in 1..=3 {
for j in 1..=5 {
if j == 2 {
break;
}
println!("i={}, j={}", i, j);
}
}
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Breaking the Outer Loop with a Label
To exit an outer loop from inside a nested inner loop, label the outer loop and use that label with break. This gives precise control over which loop level actually stops.
Example: Breaking the Outer Loop with a Label
fn main() {
'search: for i in 1..=3 {
for j in 1..=3 {
if i == 2 && j == 2 {
println!("Found match at i={}, j={}", i, j);
break 'search;
}
}
}
println!("Search complete");
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Building a Multiplication Table
Nested loops are commonly used to build a full table of computed values, iterating rows on the outer loop and columns on the inner loop.
Example: Building a Multiplication Table
fn main() {
for i in 1..=3 {
for j in 1..=3 {
print!("{}\t", i * j);
}
println!();
}
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- Using unlabeled
breakinside a nested loop when you meant to break out of the outer loop instead of just the inner one. - Writing deeply nested loops for a problem better solved with iterator combinators like
.flat_map()or.zip(). - Forgetting that variables declared in the outer loop are reset per outer iteration if declared inside the outer loop body but outside the inner loop.
- A loop written inside another loop's body is a nested loop, useful for grid-like or combinatorial problems.
- Labels like
'outer:letbreak/continuetarget a specific loop level instead of just the innermost one. - Nested loops multiply iteration counts, so a 10x10 nested loop runs the inner body 100 times.
- For simple transformations, iterator methods can often replace nested loops with clearer, equally fast code.
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