id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
7177	Students t returns rubbihs values for undefined input instead of throw	Paul A. Bristow	Paul A. Bristow	"Reported by Thomas Mang

According to the documentation, in boost 1-50 non-member functions of moments (e.g. mean, variance) which are not defined for a given distribution shall issue compile-time errors, or throw a domain_error at runtime otherwise. For the Cauchy Distribution, that's what it does. For the t-distribution, that's not what it does - the non-member functions happily return some complete rubbish values for mean and variance (and hence I'd also suppose standard deviation). For kurtosis and skewness it seems to work correctly but are subject to mathematical definitions. I have not tested other non-member functions.

typedef  boost::math::students_t_distribution<double>  t_distr;

// this is ok
std::cout << ""mean(t(2)): "" << boost::math::mean( t_distr(2) ) << std::endl;

// this is NOT ok - first moment not defined for given distribution std::cout << ""mean(t(1)): "" << boost::math::mean( t_distr(1) ) << std::endl;


// this is ok
std::cout << ""var(t(3)): "" << boost::math::variance( t_distr(3) ) << std::endl;

// this might be ok or not
// depending on point of view, either Inf or undefined std::cout << ""var(t(2)): "" << boost::math::variance( t_distr(2) ) << std::endl;

// this is NOT ok - second moment not defined for the given distribution std::cout << ""var(t(1)): "" << boost::math::variance( t_distr(1) ) << std::endl;
"	Bugs	closed	Boost 1.52.0	math	Boost Development Trunk	Problem	fixed	math students t