Issue #20300 has been updated by matheusrich (Matheus Richard). Rust [calls this method `insert`](https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.insert):
Inserts a key-value pair into the map.
If the map did not have this key present, None is returned.
If the map did have this key present, the value is updated, and the old value is returned.
```rs let mut map = HashMap::new(); assert_eq!(map.insert(37, "a"), None); assert_eq!(map.is_empty(), false);
map.insert(37, "b"); assert_eq!(map.insert(37, "c"), Some("b")); assert_eq!(map[&37], "c"); ```
---------------------------------------- Feature #20300: Hash: set value and get pre-existing value in one call https://bugs.ruby-lang.org/issues/20300#change-107020 * Author: AMomchilov (Alexander Momchilov) * Status: Open ---------------------------------------- When using a Hash, sometimes you want to set a new value, **and** see what was already there. Today, you **have** to do this in two steps: ```ruby h = { k: "old value" } # 1. Do a look-up for `:k`. old_value = h[:k] # 2. Do another look-up for `:k`, even though we just did that! h[:k] = "new value" use(old_value) ``` This requires two separate `Hash` look-ups for `:k`. This is fine for symbols, but is expensive if computing `#hash` or `#eql?` is expensive for the key. It's impossible to work around this today from pure Ruby code. One example use case is `Set#add?`. See https://bugs.ruby-lang.org/issues/20301 for more details. I propose adding `Hash#update_value`, which has semantics are similar to this Ruby snippet: ```ruby class Hash # Exact method name TBD. def update_value(key, new_value) old_value = self[key] self[key] = new_value old_value end end ``` ... except it'll be implemented in C, with modifications to `tbl_update` that achieves this with a hash-lookup. I'm opening to alternative name suggestions. @nobu came up with `exchange_value`, which I think is great. Here's a PR with a PoC implementation: https://github.com/ruby/ruby/pull/10092 ```ruby h = { k: "old value" } # Does only a single hash look-up old_value = h.update_value(:k, "new value") use(old_value) ``` -- https://bugs.ruby-lang.org/