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How to Break From Nested Loops in Rust

Loops in Rust aren’t the same as standard C-style languages. The syntax is different and there are some powerful options that make looping easier. First, let’s go over some looping basics, then we will cover how to handle breaking and continuing in nested loops in Rust.

Standard For-Loop

fn main() { for x in 0..10 { println!("{}", x); } }
Code language: Rust (rust)

Which prints:

0 1 2 3 4 5 6 7 8 9

0..10 is an iterator where the lower-bound is inclusive and the upper bound is exclusive.

More generically:

for var in iterator { // do stuff }
Code language: Rust (rust)

In my opinion, all languages should move to a single syntax with for-loops based on iterators. The simplicity makes Rust’s loops easy to read, while the ability to create custom iterators makes it more powerful than even more verbose formats like Go’s:

for i := 0; i < 10; i++ { fmt.Println(i) }
Code language: Go (go)

Rust’s for-loop doesn’t specify what happens after each iteration (i++) or what condition is required to continue the loop (i < 10), an iterator is simply supplied.

Continue and Break

for x in 0..10 { if x > 5 && x < 7 { continue } println!("{}", x); }
Code language: Rust (rust)

The continue keyword works in a familiar manner. In this example when x > 5 AND x < 7 the loop continues to the next iteration without printing. This results in:

0 1 2 3 4 5 7 8 9

The break keyword is also familiar:

for x in 0..10 { if x > 5{ break } println!("{}", x); }
Code language: Rust (rust)

which prints:

0 1 2 3 4 5

Working With Nested Loops

Nested loops can get tricky in a lot of languages. What if I want to continue through an outer loop when a condition within an inner loop occurs? We can do the following:

'outer: for x in 0..5 { for y in 0..5 { if y > 2{ break 'outer } println!("x: {}, y: {}", x, y); } }
Code language: Rust (rust)


x: 0, y: 0 x: 0, y: 1 x: 0, y: 2
Code language: HTTP (http)

By using the label ‘outer we are able to control explicitly which loop is broken. The default would have been to just break from the inner loop. The same labeling system works with the continue keyword as well.

Have questions or feedback?

Follow and hit me up on Twitter @q_vault if you have any questions or comments. If I’ve made a mistake in the article, please let me know so I can get it corrected!

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