Does order matter? Do you need to access items by key or just at random? Map has been mentioned, but there is also MapSet if you don’t need keys. Both implement the enumerable protocol, so to get a random value you can pass it to Enum.random. What I’ve done in the past when I want constant time lookups by key and also maintain order is to create two structures: a map for the constant lookups, and a list of keys for the order.
You may also consider looking into using a Registry if you want a container for processes, though for your use case it might be easier to just keep simple collection of pids.
