Create an iterator from a single element

Question

I would like to prepend an element to an iterator. Specifically, I would like to create an iterator that steps through the sequence [2, 3, 5, 7, 9, ...] up to some maximum. The best I've been able to come up with is

range_step_inclusive(2,2,1).chain(range_step_inclusive(3, max, 2))

But the first iterator is kind of a hack to get the single element 2 as an iterator. Is there a more idiomatic way of creating a single-element iterator (or of prepending an element to an iterator)?

Answer 1

This is the exact use case of std::iter::once.

Creates an iterator that yields an element exactly once.

This is commonly used to adapt a single value into a chain of other kinds of iteration. Maybe you have an iterator that covers almost everything, but you need an extra special case. Maybe you have a function which works on iterators, but you only need to process one value.

range_step_inclusive is long gone, so let's also use the inclusive range syntax (..=):

iter::once(2).chain((3..=max).step_by(2))

Answer 2

You can use the Option by-value iterator, into_iter:

Some(2).into_iter().chain((3..).step_by(2))