On Saturday, 24 June 2023 at 07:36:26 UTC, Cecil Ward wrote:
Jonathan, is it possible that I wanted one thing and got another? My description in the earlier post was of the _aim_ of the program. What I ended up with might be something else? I wanted an array of uints whose values are the results/outputs of the mapping function. Since it is keyed by strings I assumed that the runtime generates some kind of hash for fast lookup when I ask it to retrieve an entry by the string (key) associated with it. I assumed that in some sense the hashing was sort of separate with some degree of independence from the underlying array, if that makes sense. The lookup is just assumed to be fast but how it is done we don’t really care. I just wanted to expand the array as I did successfully elsewhere with reserve, as I built this structure by successive additions of data. I have a number of strings and the map is meant to output the ordinal number in which I first saw them, zero-based. Then I want to come back and randomly look up one ordinal given a string preferably with a very fast lookup. The number of entries can not practically be more than 30, and even that would be highly unusual, maybe ten is the practical limit in my particular case, so it’s hardly MySQL.
I just realised something, your point about altering the table and having to rehash, is well taken. I hadn’t considered that. The reason for my foolishness in failing to realise that I’m asking the impractical is my pattern of usage. I add all the entries into the mapping table and have no interest in any lookups until it is fully built. Then a second function starts to do lookups while the data remains unchanging and that usage pattern can be guaranteed. I could even idup it if that would help, as copying < 32 uints wouldn’t take forever. A typical value would be a mere 5 or less. I only picked 32 to be completely safely ott.
