Cycle a Rust iterator a given number of times

Question

How do I cycle through an iterator a finite number of times?

I would expect the output of something like this to be 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3 and then stop:

vec![1, 2, 3].iter().cycle(4)
//                         ^ but .cycle() doesn't take an argument...

I don't know the length of the iterator to begin with.

Answer 1

One simple way is to repeat the iterator itself, take the first 4 and flatten:

fn main() {
    let v = vec![1, 2, 3];
    let res = std::iter::repeat(v.iter())
        .take(4)
        .flatten()
        .collect::<Vec<_>>();
    dbg!(res);
}

---

Some micro-benchmark result using code in this gist comparing 3 different approaches:

• repeat-take-flatten in this answer
• hand-rolled loop
• a cycle_n implementation mimicking Iterator::cycle.

Kudos to rustc, cycle_n consistently outperforms the other two when the input is reasonably large whereas repeat-take-flatten performs the best for small input.