John Maddock wrote:
I seriously hope I'm doing something wrong here, but the last 20 bits in a double returned from uniform_real appear to always be zero. Is this by design? I really hope not!
This seems to be dependant on the type of generator you use. Try using lagged_fibonacci. The casting and dividing involved in uniform_real appears to loose a lot of accuracy, but lagged_fibonacci generates doubles between 0 and 1, so there's no cast and it's dividing by 1. Interestingly, minstd_rand also seems to work better. Purely speculatively, I think this might be because of the ranges of values it generates - with mt19937 the random number is divided by 0xffffffff, while for minstd_rand it is divided by 0x7ffffffd. Although, I wouldn't be surprised if it performs badly by some other measure. Come to think of it, uniform_real is meant to be generating a half-open range - so maybe it should be diving by (max() - min() + 1) for integers. Looking at the concept documentation: 'For integer generators (i.e. integer T), the generated values x fulfill min() <= x <= max(), for non-integer generators (i.e. non-integer T), the generated values x fulfill min() <= x < max().' Or are you only meant to use floaing point generators when generating a floating point distribution? Daniel