Sorry if this is a stupid question, but I’m struggling with what I think is ownership in rust.

I’m completely new to rust, but have done low level languages before so I know the concept of pointers pretty well. However, this rust ownership I can’t quite wrap my head around.

I’m trying to refactor my solution to AoC Day 4 Part 2 using a struct reference instead of a stand-alone vector.

The error I’m getting, and can’t figure out is in the process function at line

cards.iter_mut().for_each(|card | {

The error is

cannot borrow cards as mutable more than once at a time second mutable borrow occurs here

There is a lot of parsing in my code, so I stuck that in a spoiler below.

The relevant code is:

#[derive(Debug)]
struct Card {
    id: u32,
    score: u32,
    copies: u32,
}

fn process(input: &str) -> u32 {
    let mut cards: Vec = parse_cards(input);

    cards.iter_mut().for_each(|card| {
        let copy_from = card.id as usize + 1;
        let copy_to: usize = copy_from + card.score as usize - 1;

        if card.score == 0 || copy_from > cards.len() {
            return;
        }

        for card_copy in cards[copy_from - 1..copy_to].iter() {
            let id = card_copy.id as usize - 1;
            let add = cards[card.id as usize - 1].copies;
            cards[id].copies += add;
        }
    });

    return cards.iter().map(|c| c.copies).sum();
}

Other code:

spoiler 
fn main() {
    let input = include_str!("./input1.txt");
    let output = process(input);
    dbg!(output);
}

fn parse_cards(input: &str) -> Vec {
    return input.lines().map(|line| parse_line(line)).collect();
}

fn parse_line(line: &str) -> Card {
    let mut card_split = line.split(':');
    let id = card_split
        .next()
        .unwrap()
        .replace("Card", "")
        .trim()
        .parse::()
        .unwrap();

    let mut number_split = card_split.next().unwrap().trim().split('|');

    let winning: Vec = number_split
        .next()
        .unwrap()
        .trim()
        .split_whitespace()
        .map(|nbr| nbr.trim().parse::().unwrap())
        .collect();
    let drawn: Vec = number_split
        .next()
        .unwrap()
        .trim()
        .split_whitespace()
        .map(|nbr| nbr.trim().parse::().unwrap())
        .collect();

    let mut score = 0;

    for nbr in &drawn {
        if winning.contains(&nbr) {
            score = score + 1;
        }
    }

    return Card {
        id,
        score,
        copies: 1,
    };
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn full_test() {
        let result = process(
            "Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
        Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
        Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
        Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
        Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
        Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11",
        );
        assert!(result != 0);
        assert_eq!(result, 30);
    }
}
  • rutrum@lm.paradisus.dayEnglish
    5·
    1 year ago

    Yeah when you iterate over cards mutably, you’re borrowing it again at the line cards[id].copies +=… I think. If you’re going to access the card by index there then you loop over indicies at the top lop instead of iter_mut

    • bia@programming.devOP
      2·
      1 year ago

      Thanks, I tried that. But now I get the error.

      cannot borrow cards as mutable because it is also borrowed as immutable