Get a random element from BTreeMap


Suppose we have a very large BTreeMap<K, V> and in addition to the ‘proper’ usage, we want to choose entries randomly from time to time.

Is this possible? A naive solution would be to store another “structure” with say <i64, K> pairs. Then generate integers at random, get the key and use it to find V in the BTreeMap. However if the BTreeMap is very large, say in the GBs, then this additional structure is also large…

Maybe there is a way to do it, without much additional overhead?


Not the most performant solution, but if you need to do it for whatever reason (without having a side structure) you can just grab the length and then run iterate you hit it. It’s not ideal, but it’s probably the cheapest way to do it without knowing what K is. Logically I’d write a small function which takes the tree (mutably or not, depending on needs), then iterating until it hits, then returning. You can even do the random roll there, or return key + value by reference (again mutable or not).

I’m sure there’s a better solution I’m not thinking of, but since it’s a BTree, it limits your options.


Yes you can just iterate a random number of times through. But we really need something fast, i.e. O(1) for each access.


Maybe use the IndexMap crate which is like the hashmap but keeps the insertion order. It has a method called get_index

After looking at the crate again I realized it doesn’t support btreemaps, only hashmaps, so I guess that doesn’t help much.


Oh wow, thanks! IndexMap is indeed much better suited for our purposes. I didn’t knew that it exists as a resizeable hash-table. Great!