Mapping and modifying objects within a HashMap

help
1

Hi there,

I need some help here. What I am trying to do is to mapp some objects and modify them if a new value is presented. In the test case below i have a mock-up of what i am trying to achieve.

Let say i iterate through a vec of keys for each key i want to check if it exists in my hashmap and if it does not i want to create a new object X with a key value set and another value (vals[i]) pushed to it. On the other hand if it already exists i would like to just push the value (vals[i]).

use std::collections::HashMap;
#[derive(Debug)]
struct X {
    r: String,
    v: Vec<u8>,
}

impl X {
    pub fn new() -> Self {
        X {
            r: "x".to_string(),
            v: Vec::new(),
        }
    }
    fn add(&mut self, x: u8) -> &mut Self {
        self.v.push(x);
        self
    }
    fn set(&mut self, x: &str) -> &mut Self {
        self.r = x.to_string();
        self
    }
}

fn main() {
    let vals  = vec![5u8; 4];
    let keys = vec!["X".to_string(), "X".to_string(), "Y".to_string(), "Y".to_string()];

    let mut res: HashMap<String, &mut X> = HashMap::default();
    let mut i = 0;

    while i < vals.len() {
        res.entry(keys[i].clone())
            .and_modify(|x| {
                x.add(vals[i] + 1u8);
            })
            .or_insert(X::new().set(&keys[i]).add(vals[i]));
        i += 1;
    }

    println!("{:#?}", res)
}

if i execute it i get

   Compiling playground v0.0.1 (/playground)
error[E0716]: temporary value dropped while borrowed
  --> src/main.rs:37:24
   |
33 |         res.entry(keys[i].clone())
   |         --- borrow later used here
...
37 |             .or_insert(X::new().set(&keys[i]).add(vals[i]));
   |                        ^^^^^^^^                            - temporary value is freed at the end of this statement
   |                        |
   |                        creates a temporary value which is freed while still in use
   |
   = note: consider using a `let` binding to create a longer lived value

For more information about this error, try `rustc --explain E0716`.
error: could not compile `playground` (bin "playground") due to 1 previous error

I understand what compiler is complaining about but i do not know how to fix it :frowning:

Can someone maybe provide some help here ?

Thank you

2

You've created a new X but it has no owner because you're only storing a reference &mut X in the HashMap. Are you sure you don't want to store X itself in the HashMap?

3

but how do i modify it then afterwords if it is not mutable? (sorry, now we are exposing my lack of understanding of rust in general)

4

Owned values are mutable, and your hashmap res is mutable, so you can modify its entries. The and_modify method gives your closure a mutable reference to the entry.

Also, you probably should use or_insert_with instead of or_insert, so that you don't create a new X in the case where an entry already exists. With or_insert_with, your closure will only be called to create a new X when one is needed.

1 Like
5

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