Iterators

Iterators are pervasive in Rust, and delving into them in full detail would necessitate renaming this course to Slow Track to Rust. We'll cover the basics and direct you to the documentation for all the intricate details about iterators.

The Control Flow section explained that the call to lines() in the for loop returns an iterator. We say that the for loop consumes the iterator. However, in Rust, we can do much more than just consume iterators!

Iterator Adaptors

New iterators can be created using iterator adaptors. These adaptors generate new iterators that modify some aspect of the original. Let's use one to print line numbers.

The enumerate function creates an iterator that yields the current iteration count along with the value from the previous iterator, as tuples¹ in the form (index, value). In the loop, the type for i is usize, and the type for line is &str.

fn main() {
    let pattern = "him";
    let poem = "I have a little shadow that goes in and out with me,
                And what can be the use of him is more than I can see.
                He is very, very like me from the heels up to the head;
                And I see him jump before me, when I jump into my bed.

                The funniest thing about him is the way he likes to grow -
                Not at all like proper children, which is always very slow;
                For he sometimes shoots up taller like an india-rubber ball,
                And he sometimes gets so little that there’s none of him at all.";

    for (i, line) in poem.lines().enumerate() {
        if line.contains(pattern) {
            println!("{}: {line}", i + 1);
        }
    }
}

Advanced Iterators

We can take it a step further by using the filter_map iterator adapter to return an iterator that includes only the items we want!

fn main() {
    let pattern = "him";
    let poem = "I have a little shadow that goes in and out with me,
                And what can be the use of him is more than I can see.
                He is very, very like me from the heels up to the head;
                And I see him jump before me, when I jump into my bed.

                The funniest thing about him is the way he likes to grow -
                Not at all like proper children, which is always very slow;
                For he sometimes shoots up taller like an india-rubber ball,
                And he sometimes gets so little that there’s none of him at all.";

    for (line_no, line) in
        poem.lines()
            .enumerate()
            .filter_map(|(i, line)| match line.contains(pattern) {
                true => Some((i + 1, line)),
                false => None,
            })
    {
        println!("{line_no}: {line}");
    }
}

Whoa! What just happened? We're on the fast track to learning Rust, so we're picking up the pace! Let's break this down because this code snippet needs some unpacking.