HashMap edit value and Iter Rust

Question

hi i have a function like that and a HashMap and the probleme is i want to iter and edit the HashMap but i have too much erro with a clone the code compil but the value of the HashMap

struct Piece(HashMap<(i32,i32), Couleur>);

fn press(&mut self, args: &Button) {
    let mut coco = self.0.clone();

    for (mut x, y) in coco {
        if let &Button::Keyboard(key) = args {
            match key {
                Key::Down => x.1 -= 1,
                Key::Left => x.0 += 1,
                Key::Right => x.0 -= 1,
                _ => {
                    println!("{:?}", x);
                }
            };
        }
    }
}

here the link of the full code if you need/want to try Link

and the dependecies of cargo

[dependencies]
piston_window = "0.93.0"
rand = "0.6.5"

Note that modifying a key in a `HashMap` in a way that its hash or eqaulity is affected is a logic error. — Sven Marnach, Dec 15 '20 at 16:58

vallentin · Accepted Answer · 2020-12-16T13:05:29.007

While you're cloning self.0 to coco, the following for loop you're consuming the HashMap. So while you're modifying x you're not actually affecting the key in coco, as you cannot mutate keys in a HashMap.

Instead wrap the body of you for loop in a map() and then collect() the result back into self.0.

Also your +=/-= for the keys are flipped.

fn press(&mut self, args: &Button) {
    let coco = self.0.clone();
    self.0 = coco
        .into_iter()
        .map(|(mut x, y)| {
            if let &Button::Keyboard(key) = args {
                match key {
                    // Key::Up => x.1 -= 1,
                    Key::Down => x.1 += 1,
                    Key::Left => x.0 -= 1,
                    Key::Right => x.0 += 1,
                    _ => {
                        println!("{:?}", x);
                    }
                };
            }
            (x, y)
        })
        .collect();
}

Alternatively, if you want to avoid cloning the whole HashMap up front, then you can use .iter() and clone() in map().

fn press(&mut self, args: &Button) {
    self.0 = self
        .0
        .iter()
        .map(|(x, &y)| {
            let mut x = x.clone();
            if let &Button::Keyboard(key) = args {
                match key {
                    // Key::Up => x.1 -= 1,
                    Key::Down => x.1 += 1,
                    Key::Left => x.0 -= 1,
                    Key::Right => x.0 += 1,
                    _ => {
                        println!("{:?}", x);
                    }
                };
            }
            (x, y)
        })
        .collect::<HashMap<_, _>>();
}

or you could mem::replace() and extend().

fn press(&mut self, args: &Button) {
    let coco = std::mem::replace(&mut self.0, HashMap::new());
    self.0.extend(coco.into_iter().map(|(mut x, y)| {
        if let &Button::Keyboard(key) = args {
            match key {
                // Key::Up => x.1 -= 1,
                Key::Down => x.1 += 1,
                Key::Left => x.0 -= 1,
                Key::Right => x.0 += 1,
                _ => {
                    println!("{:?}", x);
                }
            };
        }
        (x, y)
    }));
}

Also, I highly suggest using rustfmt to keep your code nicely formatted, not to mention that your mix of English and non-English names can create confusion.

thank you very much for your help i will try to use only English variable name and use rustfmt thank you very much — Zenmoa, Dec 15 '20 at 20:03
You don't need to clone the `HashMap`: you can do directly `self.0 = self.0.into_iter().map(...).collect();` — Jmb, Dec 16 '20 at 07:57
@Jmb No you can't use `self.0.into_iter()` because you cannot move `self.0`. Instead you could use `iter()` and `x.clone()` or `mem::replace()` and `extend()` (which I've updated the answer to include examples for) — vallentin, Dec 16 '20 at 13:05

HashMap edit value and Iter Rust

1 Answers1