Iterator that skips every nth element - rust

Question

Rather than taking every Nth element from an iterator which I can do with Iterator::step_by, I would like to skip every Nth element. How can I achieve this idiomatically? Is there maybe even a standard library or itertools function?
This is what I came up with to skip every 7th say. It requires enumerate, filter, and map, though one could use a filter_map instead of the latter two.
(0..100).enumerate()
.filter(|&(i, x)| (i + 1) % 7 != 0)
.map(|(i, x)| x);
How could I cast this into a function so that I could simply write:
(0..100).skip_every(7)

Answer 1

If you want to get the exact interface you asked for, your best option at this time is to implement a custom iterator adapter type. Here's a basic version of such a type:
pub struct SkipEvery<I> {
inner: I,
every: usize,
index: usize,
}
impl<I> SkipEvery<I> {
fn new(inner: I, every: usize) -> Self {
assert!(every > 1);
let index = 0;
Self {
inner,
every,
index,
}
}
}
impl<I: Iterator> Iterator for SkipEvery<I> {
type Item = I::Item;
fn next(&mut self) -> Option<Self::Item> {
if self.index == self.every - 1 {
self.index = 1;
self.inner.nth(1)
} else {
self.index += 1;
self.inner.next()
}
}
}
pub trait IteratorSkipEveryExt: Iterator + Sized {
fn skip_every(self, every: usize) -> SkipEvery<Self> {
SkipEvery::new(self, every)
}
}
impl<I: Iterator + Sized> IteratorSkipEveryExt for I {}
(Playground)
A more complete implementation could also add optimized versions of further Iterator methods, as well as implementations of DoubleEndedIterator and ExactSizeIterator -- see the implementation of StepBy as an example.

Answer 2

Your code is pretty easy to turn into a function:
fn skip_every<I: Iterator> (iter: I, n: usize) -> impl Iterator<Item = <I as Iterator>::Item> {
iter.enumerate()
.filter_map(move |(i, v)| if (i + 1) % n != 0 { Some (v) } else { None })
}
fn main() {
println!("{:?}", skip_every (0..20, 7).collect::<Vec<_>>());
}
Playground
Or avoiding the expensive modulo:
fn skip_every2<I: Iterator> (iter: I, n: usize) -> impl Iterator<Item = <I as Iterator>::Item> {
iter.zip ((0..n).rev().cycle()).filter_map (|(v, i)| if i != 0 { Some (v) } else { None })
}
Playground