On Mon, Jan 19, 2015 at 3:08 PM, Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote: > Zachary Gilmartin wrote: > >> Why aren't there trees in the python standard library? > > Possibly because they aren't needed? Under what circumstances would you use > a tree instead of a list or a dict or combination of both? > > That's not a rhetorical question. I am genuinely curious, what task do you > have that you think must be solved by a tree?
In general, any time you want to maintain a sorted list or mapping, balanced search tree structures come in handy. Here's an example task: suppose you want to represent a calendar, where timeslots can be reserved for something. Calendar events are not allowed to intersect. The most important query is: What events are there that intersect with the timespan between datetimes d1 and d2? (To draw a daily agenda, figure out if you should display an alert to the user that an event is ongoing or imminent, etc.) You also want to be able to add a new event to the calendar, that takes place between d1 and d2, and to remove a event. I leave it to the reader to implement this using a sorted map. (hint: sort by start.) This maybe seems contrived, but I've used this exact datatype, or a remarkably similar one, in a few different circumstances: sequenced actions of characters in a strategy game, animation, motion planning... There are a few possible implementations using Python data structures. You can do it using a linear scan, which gets a little slow pretty quickly. You can make insertion slow (usually OK) by sorting on insertion, but if you ever forget to resort your list you will get a subtle bug you might not notice for a while. And so on. It's better in every way to use the third-party blist module, so why bother? -- Devin -- https://mail.python.org/mailman/listinfo/python-list