Rodolfo Federico Gamarra wrote:
Hi all,
The documentation for the normal distribution
(http://www.boost.org/doc/libs/1_38_0/libs/random/random-distributions.html#n...)
states:
"Members explicit normal_distribution(const result_type& mean = 0, const result_type& sigma = 1); Requires: sigma > 0"
In the actual code of this constructor
(http://www.boost.org/doc/libs/1_38_0/boost/random/normal_distribution.hpp)
it can be seen
"assert(_sigma >= result_type(0));".
That is, acepts sigma == 0.
Indeed, there's no "division-by-zero" or so, because the method that is used ("Box-Muller") doesn't actually divide by sigma:
return _cached_rho * (_valid ? cos(result_type(2)*pi*_r1) : sin(result_type(2)*pi*_r1)) * _sigma + _mean;
So, in this case, the numbers will be all equal to _mean. From the
As a user, that seems like a reasonable behavior. It might sometimes be more convenient to set sigma = 0 than to use a specific class for a degenerate distribution. The sample mean has asymptotic sd = 0, after all.
mathematical point of view there's no such thing as a normal distribution with sigma equal to zero; but, computationally, may be handy to allow that.
What's the expected behavior? Is there a typo in the docs? Or is it that the case sigma equal to zero is indeed undefined (so that anything could happen)?
Thanks in advance, cheers. -- Rodolfo Federico Gamarra