How do I pass an iterator I am iterating on to a function?

Question

I'm iterating through an array, and depending on the current value, I'd like to pass the iterator to a sub function and have it deal with a number of values, and upon exiting the sub function, carry on iterating through the array. Below is the closest I've managed to get so far, but I keep getting error: use of moved value: 'iter'.

I've tried looking into lifetimes, but that hasn't worked for me either. I've spent most of a day on this now, and can't seem to get anywhere with it. Any help would be greatly appreciated. Thanks.

enum Thing {
    Three(char, char, char),
    Four(char, char, char, char),
}

fn take_three <'a>(iter: &mut std::slice::Iter<'a, char>) -> Thing {
    let a = iter.next().unwrap();
    let b = iter.next().unwrap();
    let c = iter.next().unwrap();
    Thing::Three(*a,*b,*c)
}

fn take_four <'a>(iter: &mut std::slice::Iter<'a, char>) -> Thing {
    let a = iter.next().unwrap();
    let b = iter.next().unwrap();
    let c = iter.next().unwrap();
    let d = iter.next().unwrap();
    Thing::Four(*a,*b,*c,*d)
}

fn parse_tokens (tokens: &Vec<char>) {
    let mut iter = tokens.iter();
    let mut things: Vec<Thing> = vec![];
    for token in iter {
        match token {
            &'a' => things.push(take_three(&mut iter)),
            &'b' => things.push(take_four(&mut iter)),
            _ => {},
        }
    }
}

fn main() {
    let tokens = vec!['a', '1', '2', '3', 'b', '1', '2', '3', '4', 'a', '4', '5', '6'];
    parse_tokens(&tokens);
}

Answer 1

The for construct consumes the iterator, and doing what you want using it will be quite tricky (if not impossible, I'm really not sure about that).

However, you can have it working pretty easily by switching to a while let construct, like this:

fn parse_tokens (tokens: &Vec<char>) {
    let mut iter = tokens.iter();
    let mut things: Vec<Thing> = vec![];
    while let Some(token) = iter.next() {
        match token {
            &'a' => things.push(take_three(&mut iter)),
            &'b' => things.push(take_four(&mut iter)),
            _ => {},
        }
    }
}