Loops

Examples working with standard loops in Rust. For a more functional approach see the std::iter::Iterator documentation.

For Loops

Use a range to create a loop over a set of numbers. The range values will be < the end value. So the following will print 0 to 9.

for x in 0..10 {
    println!("{}", x);
}

For Loop with Step

Use the .step_by() method to create a step, note this requires wrapping the range in parentheses to create it as an iterator.

for x in (0..20).step_by(2) {
    println!("{}", x);
}

Loop over Vector

Using a vector in a for loop will automatically iterate over the elements in the array.

let v = vec![ 1, 2, 3, 4, 5];
for e in v {
    println!("{}", e);
}

If you want to loop over a vector getting the index and value use .enumerate() off the vector’s .iter() like so:

let v = vec![ 'a', 'b', 'c', 'd' ];

for (i, ch) in v.iter().enumerate() {
    println!("{}: {}", i, ch);
}
// 0: a
// 1: b
// 2: c
// 3: d

Infinite Loop

Use loop { } to create an infinite loop. Rust recognizes the standard break to exit a loop, and continue to move to next iteration.

Here is an example summing the Fibonacci sequences values less than 1,000.

let mut f1 = 1;
let mut f2 = 1;
let mut sum = 2;

loop {
    let next = f1 + f2;
    if next > 1000 { break; }
    
   
    sum = sum + next;
    f1 = f2;
    f2 = next;    
}

println!("{}", sum);

While Let Loop

The above Fibonacci sum could also be done using a while let loop. Create a get_next() function that return an Option, this is a bit contrived but illustrates the loop and optional parameter.

fn main() {
    let mut f1 = 1;
    let mut f2 = 1;
    let mut sum = 2;

    while let Some(next) = get_next(f1, f2) {
        sum = sum + next;
        f1 = f2;
        f2 = next;    
    }
    println!("{}", sum);
}

fn get_next(f1: i32, f2: i32) -> Option<i32> {
    if f1 + f2 < 1000 {
        return Some(f1 + f2);
    } else {
        return None;
    };
}