Iterator Intersperse with Another Iterator

I'm looking for an Iterator method that will intersperse an existing iterator, but with another iterator. For example, if I have 2 iterators that produce:

it1 => {1,2,3,4}
it2 => {6,7,8}

... then I'd like to produce: {1,6,7,8,2,6,7,8,3,6,7,8}.

In my actual code, I want to separate values in an iterator by n 0s, but I can do this easily enough with repeat_n from Itertools. Sometime with lazy eval and no memory allocations would be great :slight_smile:

I guess what I'm really asking is if there is anything out-of-the-box so I don't have to go with my poorly named iterator adaptor:

struct InterspersePaddingIterator<T, I> {
    base_iter: I,
    padding: T,
    pad_amt: usize,
    cur: usize

impl <T, I: Iterator<Item=T>> InterspersePaddingIterator<T, I> {
    pub fn new(base_iter: I, padding: T, pad_amt: usize) -> Self {
        InterspersePaddingIterator {
            cur: pad_amt

impl <T: Clone, I: Iterator<Item=T>> Iterator for InterspersePaddingIterator<T, I> {
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        if self.cur >= self.pad_amt {
            self.cur = 0;
        } else {
            self.cur += 1;
            return Some(self.padding.clone())

Something like this?

fn main() {
    let vals = vec![1, 2, 3, 4].into_iter();
    let padding = [7, 8, 9];

    let interleaved = vals.flat_map(
        |x| std::iter::once(x).chain(padding.iter().copied())


If you want the first and last element to both come from the value iterator, it gets a little more complicated:

fn main() {
    let mut vals = vec![1, 2, 3, 4].into_iter();
    let padding = vec![7, 8, 9].into_iter();

    let interleaved =
            |x| padding.clone().chain(std::iter::once(x))


1 Like

The first one is what I'm looking for... thanks @2e71828 !

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