Loops in Rust

Loops in Rust are very intuitive if you have prior programming experience.

Infinite loop with breaks

The loop keyword creates an infinite loop. You can exit the loop using the break statement.

fn main() {
    let mut counter = 0;

    loop {
        counter += 1;
        println!("Counter: {}", counter); // Output: 5

        if counter == 5 {
            break;
        }
    }
}

Conditional loops

while Loop

The while loop continues running while a condition is true.

fn main() {
    let mut counter = 0;

    while counter < 5 {
        counter += 1;
        println!("Counter: {}", counter); // Output: 5
    }
}

If you are experienced with c, java or js you might've used the following variation of for loops extensively. for(i = 0; i < 5; i++), In Rust we have to write the above loop using while loops as shown in the example.

for loop

for loop in Rust works similar to for loop in python. That is it works only with iterators. What are iterators?

Iterator

An iterator is type which implements the following trait,

trait Iterator {
    type Item;

    fn next(&mut self) -> Option<Self::Item>;
}

For example consider Vec<T>, then the Item will be T. The next method will return None if we are at the end of the Vec or else Some(T).

Examples

fn main() {
    // Iterating over a range
    for i in 0..5 {
        println!("i: {}", i);
    }

    // Iterating over an array
    let numbers = [1, 2, 3, 4, 5];

    for number in numbers.iter() {
        println!("Number: {}", number);
    }
}

It is to be noted that both the range and the array implements the Iterator trait, If you try to for loop over a type which does not implement Iterator trait you will get a compile time error.