2009/10/19 Danny Woods <dannywo...@gmail.com> > > harto wrote: > > Hello, > > > > I've just started learning Clojure, so I'm trying to figure out the > > correct way of doing things. I've been trying to create and 'modify' a > > large vector for an online programming exercise, but I'm running into > > some performance issues. > > > > Any general tips would be appreciated! > > > > Well, I'm a beginner too, but I'll take a stab at answering your questions. > > > Firstly, I'm creating a vector of booleans like this: > > > > (defn vector-of [n value] > > (vec (for [_ (range n)] value))) > > > > It takes quite a long time for large values of n, though: > > > > user=> (time (dorun (vector-of 1e7 true))) > > "Elapsed time: 6734.509528 msecs" > > > I suspect you're being bitten by the repeated creation of the underlying > data structures. 'range' is going to create a lazy sequence, from which > 'for' is going to create its own, lazy structure, which 'vec' then > iterates over to create its own. Just running '(time (dorun (range > 1e7)))' shows that it's taking up a non-trivial amount of time just on > its own. >
Ah, OK. I didn't understand the implications of using those lazy sequences. I think I need to do some more reading on lazy data structures. Thanks for your input! > > This modified version does the same thing a little more quickly: > > (defn vector-of [n value] > (loop [v [] > c 0] > (if (< c n) (recur (assoc v c value) (inc c))))) > > user> (time (dorun (vector-of 1e7 true))) > "Elapsed time: 1000.409039 msecs" > > > Secondly, I'm iterating across one of these large vectors using > > something like the following (contrived) function: > > > > (defn set-flags [v] > > (loop [i 0 > > v v] > > (if (< i (count v)) > > (recur (inc i) (assoc v i false)) > > v))) > > > > user=> (let [v (vector-of 1e7 true)] (time (dorun (set-flags v)))) > > "Elapsed time: 15563.916114 msecs" > > > > Am I missing anything obvious here? That seems like a really long time > > to me. > > > With the modified vector-of, above, this problem seems to go away for me: > > user> (time (dorun (set-flags (vector-of 1e7 true)))) > "Elapsed time: 1009.362723 msecs" > > Cheers, > Danny. > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
