Stanislas: > I've read that, since we're programming in Objective-C, it's more elegant > and reliable to use an NSArray stuffed with NSNumber instead of a classic C > array. But what I want to do, it's performing classic calculation, like mean > value, variance... etc, on floating point numbers. Typically around 1000 of > them. > > I know that our computers are fast now, but this doesn't mean that we have > to not take care of optimization.
I faced this very same situation with a scientific data mining and visualization application. In my case, it was a permutation test against a whole lot of doubles (for reasons of precision). When dealing with any level of precision, you may have to "roll the dice" 10,000 times per comparison or more. Because the application takes advantage of Core Data and (to an extent Cocoa Bindings) I first implemented this using NS[Mutable]Arrays of NSNumbers. It did not perform well. :-) The standard test set I used took literally around a minute to run. Unacceptable for a "explore as you go" visualization UI. I then asked myself why I felt everything had to be shoehorned into Cocoa / Objective-C ... of course I was embarrassed and had no explanation to offer myself. After I'd thoroughly chastised myself for being so ridiculous, I re-implemented the test by extracting the necessary values into a C array of doubles (and another of randomized vector indices) then doing the calculations. First, I'll say that this felt wasteful because "they're already neatly-packaged objects - won't performance suffer copying values back and forth like that?" Once I got over that and actually reasoned it out, I informed myself that the answer was, of course, "no, because comparatively speaking, copying the values around is nothing next to 100,000 iterations and many more sufficiently-random-number generations to shuffle the vectors". To which I replied, "Oh." :-) All told, the minute-long permutation test was reduced to about two seconds. Wow. I hope this is helpful. -- I.S. _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
