HashMap borrow issue when trying to implement find or insert

Question

I tried to implement own analogue of find_or_insert method that looks like this:

use std::collections::HashMap;

pub struct SomeManager {
    ne         


        

Answer 1

Note that in your first case you're doing one lookup when the key exists in the map and three when it does not exist. Your last attempt does two lookups in either case. This is somewhat prettified version of the latter:

pub fn get_type<'a>(&'a mut self, k: i32) -> &'a i32 {
    let contains = self.types.contains_key(&k);
    if !contains {
        self.types.insert(k, self.next);
        self.next += 1;
    }
    self.types.get(&k).unwrap()
}

I don't think it is possible to avoid the second lookup without some support from the map's implementation because of borrowing restrictions.

In any case, using the solution by Chris Morgan is superior to the above one (for example, it may be more efficient and in fact require less lookups), so I suggest sticking with it.