The entry
API is designed for this. In manual form, it might look like
let values = match map.entry(key) {
Entry::Occupied(o) => o.into_mut(),
Entry::Vacant(v) => v.insert(default),
};
One can use the briefer form via Entry::or_insert_with
:
let values = map.entry(key).or_insert_with(|| default);
If default
is already computed, or if it’s OK/cheap to compute even when it isn’t inserted, you can use Entry::or_insert
:
let values = map.entry(key).or_insert(default);
If the HashMap
‘s value implements Default
, you can use Entry::or_default
, although you may need to provide some type hints:
let values = map.entry(key).or_default();