site_archiver@lists.apple.com Delivered-To: darwin-dev@lists.apple.com User-agent: Mutt/1.5.18 (2008-05-17) On Wed, Nov 25, 2009 at 10:26:51AM -0700, Derek Gaston scratched on the wall:

Hello everyone... I've been struggling with rand and srand on OSX 10.6 and 10.5. It seems to me that something is terribly broken. Here is a short program I wrote to convince myself of that:

#include <stdlib.h> #include <iostream>

int main() { for(unsigned int i=0; i<100; i++) { srand(i+1); double value = rand()/(RAND_MAX + 1.0); std::cout<<value<<std::endl; } }

This piece of code should generate 100 random numbers between 0 and 1... I am expecting that they be fairly spaced out as well...

I wouldn't trust it to do that. A good PRNG generates *sequences* that are well spaced and seemingly random. You don't have sequences.

This is actually indicative of what we actually do in one of our applications (I know it doesn't usually make much sense to reseed all the time... but you're just going to have to trust me that it's necessary in our case ;-)

Not only does it not make sense, as this code shows, it is a very bad idea. You're setting the seed over and over with very similar numbers (in the scope of a full unsigned integer) and getting similar first values. Not exactly a surprise. Since this isn't a sequence, there is little reason to assume they'd be fairly spaced out. You need a sequence off a single seed for that.

Am I doing something wrong??

Yeah, stop re-seeding the PRNG. Do it once on startup and never again. Or, find a much better source of semi-random seeds. Clearly the Linux implementation does a better job at producing first-value numbers that cover the number domain a bit better. But that doesn't discount the fact you're using the PRNG incorrectly. -j -- Jay A. Kreibich < J A Y @ K R E I B I.C H > "Our opponent is an alien starship packed with atomic bombs. We have a protractor." "I'll go home and see if I can scrounge up a ruler and a piece of string." --from Anathem by Neal Stephenson _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-dev mailing list (Darwin-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-dev/site_archiver%40lists.appl... This email sent to site_archiver@lists.apple.com