How to skip n items from inside of a iterator loop

Question

This code:

play

fn main() {
    let text = "abcd";

    for char in text.chars() {
        if char == 'b' {
            // skip 2 chars
        }
        print!("{}", char);
    }
    // prints `abcd`, but I want `ad`
}

prints abcd, but I want to skip 2 chars if b was found, so that it prints ad. How do I do that?

I tried to put the iterator into a variable outside the loop and manipulate that iterator within the loop, but the Borrow Checker doesn't allow that.

Answer 1

AFAIK you can't do that with a for loop. You will need to desugar it by hand:

let mut it = text.chars();
while let Some(char) = it.next() {
    if char == 'b' {
        it.nth(1); // nth(1) skips/consumes exactly 2 items
        continue;
    }
    print!("{}", char);
}

Playground

Answer 2

If you want to keep an iterator style, you can use std::iter::successors (I've replaced the special char with '!' for being more readable:

fn my_iter<'a>(s: &'a str) -> impl Iterator<Item = char> + 'a {
    let mut it = s.chars();

    std::iter::successors(it.next(), move |c| {
        if *c == '!' {
            it.next().and_then(|_| it.next())
        } else {
            it.next()
        }
    })
    .filter(|c| *c != '!')
}

fn main() {
    assert!(my_iter("a!bc").eq("ac".chars()));
    assert!(my_iter("!abcd").eq("bcd".chars()));
    assert!(my_iter("abc!d").eq("abc".chars()));
    assert!(my_iter("abcd!").eq("abcd".chars()));
}