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);
}
}
Why are you doing
cards.iter_mut().for_each(|card| {
I don’t see that you mutate card anywhere, am I missing something?
No, that’s probably wrong.
I’ve just been trying different combinations of borrowing, but I can’t get it to works.
I’m pretty sure it’s the
cards[id].copies += add
that is the cause of my issues.
Might be best to share a rust playground link https://play.rust-lang.org/
Others can then execute and see the error.
Edit: here’s the link
Thank you. I made the change suggested below to the linked one, but it still fails with the error.
cannot borrow
cards
as mutable because it is also borrowed as immutable
OK, so I’m thinking…
The error says I can’t combine immutable and mutable in the same function. I guess this makes sense, and that it’s because I’m not allowed to mutate an object while accessing it.
Is this a correct understanding of the error, or is there anything I can do to use this approach?