Parameter range not checked in Incomplete Beta Function

[Sylvain Jasson <jasson@…>]

The incomplete beta function is defined for a > 0, b > 0 and 0 <= x <= 1.

A call to boost::math::ibeta(0,1,0.5) should raise an std::domain_error under default policy. The domain error on a (first parameter) is not detected.

Here is some very simple code : #include <iterator> #include <algorithm> #include "boost/math/special_functions/beta.hpp" #include "boost/math/special_functions/gamma.hpp" #include "boost/math/special_functions/log1p.hpp"

int main() {

double df1,df2; double x; double ret_val;

df1=0.0; df2=1; x=0.5;

ret_val=boost::math::ibeta(df1,df2,x);

std::cout<<" df1 = "<<df1<<" df2= "<<df2<<" x= "<< x <<" I_x(df1,df2)= "<<ret_val<<std::endl;

}

and the output :

df1 = 0 df2= 1 x= 0.5 I_x(df1,df2)= 1

(using g++ 4.3.2)

[John Maddock]

Whether it's a good idea or not I don't know, but the behaviour is deliberate as Ix(0,b) for b > 0 is well defined, finite, and real valued. Likewise Ix(a,0) for a > 0.

HTH, John.

[Sylvain Jasson <jasson@…>]

Many thanks for this quick response.

I would then suggest : 1) to change the doc. ( which says "All of these functions require a > 0, b > 0 and 0 <= x <= 1. ") 2) to raise an exception for a=b=0 as I_x(0,0) is indeterminate.

Any way, the workaround is more than easy and the library very usefull !

[John Maddock]

(In [57045]) Improve ibeta error handling, update and regenerate docs. Update tests. Fixes #3533.