How to lookup from and insert into a HashMap efficiently?

Question

I\'d like to do the following:

• Lookup a Vec for a certain key, and store it for later use.
• If it doesn\'t exist, create an empty Ve

Answer 1

The entry API is designed for this. In manual form, it might look like

use std::collections::hash_map::Entry;

let values: &Vec = match map.entry(key) {
    Entry::Occupied(o) => o.into_mut(),
    Entry::Vacant(v) => v.insert(default)
};

Or one can use the briefer form:

map.entry(key).or_insert_with(|| default)

If default is OK/cheap to compute even when it isn't inserted, it can also just be:

map.entry(key).or_insert(default)