Re: [Boost-users] Boost random not looking so random

On Sat, Oct 27, 2012 at 09:30:45PM -0700, Robin Rowe wrote:

I'm using boost to shuffle an array, but it's not looking as random as I would expect. Am I not using it right?

Randomness doesn't give a hoot about discernable patterns. A generator that avoids "non-random patterns" would be properly defective. If you want to measure the randomness of something, consider using the proper statistical tests designed to measure the uncorrellation of sources.

The code below outputs sequences that look random mostly, but I see patterns such as 7 7 12 12 11.

boost::random::mt19937 rng; boost::minstd_rand lcggen; lcggen.seed(clockTime); rng.seed(lcggen); unsigned min=1; unsigned max=15; for(unsigned i=0;i<5;i++) { boost::random::uniform_int_distribution<> dist(min,max); const int j = dist(rng); ... code that shuffles stuff using j ... min++; cout<

You may of course be using the results of an uniform distribution in a way that causes the resulting shuffle have some configurations that are more likely than others, but ocular observation is still not a reliable metric for that. -- Lars Viklund | zao@acc.umu.se