Loops

Syntax#

loop { block } // infinite loop
while condition { block }
while let pattern = expr { block }
for pattern in expr { block } // expr must implement IntoIterator
continue // jump to the end of the loop body, starting a new iteration if necessary
break // stop the loop
’label: loop { block }
’label: while condition { block }
’label: while let pattern = expr { block }
’label: for pattern in expr { block }
continue ’label // jump to the end of the loop body labelled label, starting a new iteration if necessary
break ’label // stop the loop labelled label

Basics

There are 4 looping constructs in Rust. All examples below produce the same output.

Infinite Loops

let mut x = 0;
loop {
    if x > 3 { break; }
    println!("{}", x);
    x += 1;
}

While Loops

let mut x = 0;
while x <= 3 {
    println!("{}", x);
    x += 1;
}

Also see: What is the difference between loop and while true?

Pattern-matched While Loops

These are sometimes known as while let loops for brevity.

let mut x = Some(0);
while let Some(v) = x {
    println!("{}", v);
    x = if v < 3 { Some(v + 1) }
        else     { None };
}

This is equivalent to a match inside a loop block:

let mut x = Some(0);
loop {
    match x {
        Some(v) => {
            println!("{}", v);
            x = if v < 3 { Some(v + 1) }
                else     { None };
        }
        _       => break,
    }
}

For Loops

In Rust, for loop can only be used with an “iterable” object (i.e. it should implement IntoIterator).

for x in 0..4 {
    println!("{}", x);
}

This is equivalent to the following snippet involving while let:

let mut iter = (0..4).into_iter();
while let Some(v) = iter.next() {
    println!("{}", v);
}

Note: 0..4 returns a Range object which already implements the Iterator trait. Therefore into_iter() is unnecessary, but is kept just to illustrate what for does. For an in-depth look, see the official docs on for Loops and IntoIterator.

Also see: https://stackoverflow.com/documentation/rust/4657/iterators

More About For Loops

As mentioned in Basics, we can use anything which implements IntoIterator with the for loop:

let vector = vec!["foo", "bar", "baz"]; // vectors implement IntoIterator
for val in vector {
    println!("{}", val);
}

Expected output:

foo  
bar  
baz

Note that iterating over vector in this way consumes it (after the for loop, vector can not be used again). This is because IntoIterator::into_iter moves self.

IntoIterator is also implemented by &Vec<T> and &mut Vec<T> (yielding values with types &T and &mut T respectively) so you can prevent the move of vector by simply passing it by reference:

let vector = vec!["foo", "bar", "baz"];
for val in &vector {
    println!("{}", val);
}
println!("{:?}", vector);

Note that val is of type &&str, since vector is of type Vec<&str>.

Loop Control

All looping constructs allow the use of break and continue statements. They affect the immediately surrounding (innermost) loop.

Basic Loop Control

break terminates the loop:

for x in 0..5 {
    if x > 2 { break; }
    println!("{}", x);
}

Output

0
1
2

continue finishes the current iteration early

for x in 0..5 {
    if x < 2 { continue; }
    println!("{}", x);
}

Output

2
3
4

Advanced Loop Control

Now, suppose we have nested loops and want to break out to the outer loop. Then, we can use loop labels to specify which loop a break or continue applies to. In the following example, 'outer is the label given to the outer loop.

'outer: for i in 0..4 {
    for j in i..i+2 {
        println!("{} {}", i, j);
        if i > 1 {
            continue 'outer;
        }
    }
    println!("--");
}

Output

For i > 1, the inner loop was iterated only once and -- was not printed.

Note: Do not confuse a loop label with a lifetime variable. Lifetime variables only occurs beside an & or as a generic parameter within <>.