How can I update a value in a mutable HashMap?

by

Indexing immutably and indexing mutably are provided by two different traits: Index and IndexMut, respectively.

Currently, HashMap does not implement IndexMut, while Vec does.

The commit that removed HashMap‘s IndexMut implementation states:

This commit removes the IndexMut impls on HashMap and BTreeMap, in
order to future-proof the API against the eventual inclusion of an
IndexSet trait.

It’s my understanding that a hypothetical IndexSet trait would allow you to assign brand-new values to a HashMap, and not just read or mutate existing entries:

let mut my_map = HashMap::new();
my_map["key"] = "value";

For now, you can use get_mut:

*my_map.get_mut("a").unwrap() += 10;

Or the entry API:

*my_map.entry("a").or_insert(42) += 10;

More Related Contents:

  1. How to lookup from and insert into a HashMap efficiently?
  2. How to implement HashMap with two keys?
  3. How do I create a HashMap literal?
  4. How can I mutate other elements of a HashMap when using the entry pattern?
  5. How to iterate through a Hashmap, print the key/value and remove the value in Rust?
  6. Is it possible to use a HashSet as the key to a HashMap?
  7. How do I use the Entry API with an expensive key that is only constructed if the Entry is Vacant?
  8. Swagger complex response model with dynamic key value hash maps
  9. How can I use a HashMap with f64 as key in Rust?
  10. How do I create a HashMap with type erased keys?
  11. What’s the difference between placing “mut” before a variable name and after the “:”?
  12. Is a Java hashmap search really O(1)?
  13. How do I express mutually recursive data structures in safe Rust?
  14. Deriving a trait results in unexpected compiler error, but the manual implementation works
  15. Mutably borrow one struct field while borrowing another in a closure
  16. How to update-or-insert on a Vec?
  17. Find and replace words/lines in a file
  18. How do I include the end value in a range?
  19. How does Java order items in a HashMap or a HashTable?
  20. How do you set, clear and toggle a single bit in Rust?
  21. Limiting the max size of a HashMap in Java
  22. How can I distinguish between a deserialized field that is missing and one that is null?
  23. Why can’t `&(?Sized + Trait)` be cast to `&dyn Trait`?
  24. Cannot create a generic function that uses a literal zero
  25. How to avoid temporary allocations when using a complex key for a HashMap?
  26. Why does compilation not fail when a member of a moved value is assigned to?
  27. Why can Serde not derive Deserialize for a struct containing only a &Path?
  28. What is the Java equivalent of Objective-C’s NSDictionary?
  29. Implement fmt::Display for Vec
  30. Cannot pass self as callback parameter due to double borrowing

Leave a Comment