Can I find out if a filtered iterator is empty? [duplicate]

Question

I have a filtered iterator like so:

let filt_it = a_vector.iter().filter(|x| condition_on_x);

Is there a way I can find out whether it's empty?

I need the iterator to stay an iterator for later use and it doesn't seem possible to clone filtered iterators.

Answer 1

You can make your iterator peekable and peek the first item; if it's None, then the iterator is empty. peek doesn't consume the item¹, so the next call to next will return it. However, peekable consumes the iterator, so you'll have to pass the Peekable around — but Peekable is also an Iterator, so if your code expects an Iterator or an IntoIterator, it'll work fine.

let filt_it = a_vector.iter().filter(|x| condition_on_x).peekable();
if filt_it.peek().is_some() {
    // The iterator is not empty, process it.
    process(filt_it);
}

---

Another option is to clone the iterator, call next once on the cloned iterator; again, if the result is None, then the iterator is empty. Then, you can process the original iterator, which will yield the first item again.

This option might not always work; not all iterators can be cloned. However, Filter implements Clone when both the iterator and the predicate function are Clone. Starting with Rust 1.26.0, closures implement Clone when possible (before 1.26.0, closures would never implement Clone).

Also, this option is probably not viable if the iterator has a side effect, since the side effect may occur twice, or if the iterator is not deterministic, as both copies of the iterator may yield different results.

---

¹ If you call peekable with a mutable reference to your iterator, you can actually keep the original iterator around after dropping the Peekable. However, if you had called peek before dropping the Peekable, you'll see that it actually consumed the first item in the original iterator. That's completely normal; Iterator itself doesn't provide an interface for peeking. Peekable will store the peeked-at item so it can return it in the following call to next, but if you drop the Peekable, then that stored item will also be dropped.