How to transform HashMap value in Rust

Question

I have a HashMap with key = id, value = Status; Now I want to update the status, but I have to remove and insert, because the new status is created from the old status. Is there any way to update without remove and insert?

enum Status {
    InCompleted(HashSet<WorkerId>),
    Working(WorkerId, HashSet<WorkerId>),
    Completed,
}
impl Status {
    fn to_working(self, worker_id: WorkerId) -> Self {
        match self {
            Self::InCompleted(w) => Self::Working(worker_id, w),
            _ => self,
        }
    }
// remove and insert new
let old_status = h.remove(&id).unwrap();
h.insert(id, old_status.to_working(cur_worker));

If you change the `Value` type of the map to `Option`, that would be easy. If not, think of what would happen if your `to_working()` panics, what would the state of the map be? — rodrigo, Feb 19 '22 at 13:58
Does this answer your question? [How can I update a value in a mutable HashMap?](https://stackoverflow.com/questions/30414424/how-can-i-update-a-value-in-a-mutable-hashmap) — Jonathan Feenstra, Feb 19 '22 at 15:13

score 4 · Answer 1 · edited Apr 06 '22 at 07:58

4

Check out the entry method for HashMap, and its and_modify method: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html#method.and_modify.

You can have something like:

h.entry(&id).and_modify(|status| 
    *status = status.to_working(cur_worker)
);

edited Apr 06 '22 at 07:58

spdrman

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answered Feb 19 '22 at 14:15

Jonathan Giddy

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The OP needs ownership of the value. – Chayim Friedman Feb 19 '22 at 23:38
@ChayimFriedman True. This only works if the `to_working` method can be modified to take `&mut self`. – Jonathan Giddy Feb 20 '22 at 17:55

score 1 · Answer 2 · answered Feb 19 '22 at 23:58

This is nothing that is exactly the same, unfortunately, but you can avoid looking up the value twice by replacing with the default, then inserting the new value. Note that it requires T: Default. If it is not, use a sensible default value (it'll stay there if the transformation will panic)

let status = h.get_mut(&id).unwrap();
let old_status = std::mem::take(status);
*status = old_status.to_working(cur_worker);

You can use a crate like replace_with:

replace_with::replace_with_or_default(
    h.get_mut(&id).unwrap(),
    |old_status| old_status.to_working(cur_worker),
);

How to transform HashMap value in Rust

2 Answers2