Dear Piddlers,
I'm in the process of binding the GSL random distribution functions
(since currently only the samplers are implemented, in PDL::GSL::RNG,
not the pdf/cdf functions).
I'm trying to bind a function whose prototype has a mix of types, like
so:
double gsl_ran_poisson_pdf (unsigned int k, double mu)
I want to bind it so that it is threadable on both parameters, like:
PDL::gsl_ran_poisson_pdf(long(20), 10.5);
PDL::gsl_ran_poisson_pdf(long(10), pdl(8,9,10,11,12));
PDL::gsl_ran_poisson_pdf(long(8,9,10,11,12), pdl(8,9,10,11,12));
My first attempt is the obvious:
pp_def('gsl_ran_poisson_pdf',
Pars => 'int numdraws(); double lambda(); double [o] out()',
Code => q{
$out() = gsl_ran_poisson_pdf( $numdraws(), $lambda() );
},
);
This works, but then I started wondering how cognizant people will be
about passing an integer type as the first parameter, since in Perl
most people don't worry about types too much. I'd prefer automatically
rounding floating point values correctly, so the following three lines
produce the same value. The above will truncate 9.9 to 9. (I'm not
sure if 10.0 is guaranteed to cast down to the 'properly' to 10,
or could it possibly to be cast to 9?)
PDL::gsl_ran_poisson_pdf(double(9.9), pdl(8,9,10,11,12));
PDL::gsl_ran_poisson_pdf(double(10.0), pdl(8,9,10,11,12));
PDL::gsl_ran_poisson_pdf(double(10.1), pdl(8,9,10,11,12));
Maybe I should drop the type spec from numdraws() in Pars and use the
type of the loop to decide whether or not to round?:
pp_def('gsl_ran_poisson_pdf2',
Pars => 'numdraws(); double lambda(); double [o] out()',
Code => q{
/* the generic loop type is the type of numdraws, right? */
types(BSUL) %{
$out() = gsl_ran_poisson_pdf( $numdraws(), $lambda() );
%}
types(FD) %{
$out() = gsl_ran_poisson_pdf(
/* need to round numdraws to nearest int */
($numdraws() > 0.0) ?
floor($numdraws() + 0.5) :
ceil($numdraws() - 0.5),
$lambda()
);
%}
},
);
This achieves automatic rounding but seems hackish. Any advice? Should
I just go with the first version and leave it to the user to be aware of
the types? Or is there a smarter way of properly rounding floating
points down to integers? Maybe keep the first one, but write a wrapper
in Perl?
Also, side question: how come there is no 'unsigned int' type in PDL?
ushort is the only unsigned int type.
thanks,
Tom
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