[Rd] NaN in R distribution functions

[Gregor Gorjanc]

Prof Brian Ripley wrote:
> On Wed, 28 Dec 2005, Gregor Gorjanc wrote:
> 
>> Dear R developers,
>>
>> I noticed that core R distribution functions return NaN, when parameter
>> values are out of parameter space. I have looked in source code and
>> found that warnings and return of NaN are done internally in C code. For
>> dgamma.c the line 49 is:
>>
>>     if (shape <= 0 || scale <= 0) ML_ERR_return_NAN;
>>
>> OK. How should this be implemented if distribution functions are written
>> directly in R? I came up with this
>>
>>     if (any(shape <= 0)) {
>>         warning("shape must be positive")
>>         return(NaN)
>>     }
> 
> 
> As the R-level code is vectorized, NaN is unlikely to be the appropriate 
> return value. You will find that only the relevant entries are NaN, for 
> example
> 
>> dgamma(1, shape=-1:2)
> 
> [1]       NaN       NaN 0.3678794 0.3678794
> Warning message:
> NaNs produced in: dgamma(x, shape, scale, log)
>

Yes, you are right.

>> I think that it would be nice that returning NaN for undefined parameter
>> values in distribution functions would also be documented in Writing R
>> Extension.
> 
> It is purely a convention for use in the standalone libRmath with which 
> the code is shared.  Otherwise returning NA or giving an error would 
> seem more appropriate.
> 
> `Writing R Extensions' does not cover writing distribution functions, 
> and is certainly not intended to mandate how such extensions are written.

I understand, however it would be really nice to have some guideline so 
that developers would write more or less similar code. I had a problem 
with one such functions, since it stopped if parameters values were out 
of parameter space. I used that function in optim() and it was 
annoyingly stopping the optimization. It was not much to change the 
function to return NaN (this could be as well NA as you have pointed 
out), but it would be nice that this would be the default.

-- 
Lep pozdrav / With regards,
     Gregor Gorjanc

----------------------------------------------------------------------
University of Ljubljana     PhD student
Biotechnical Faculty
Zootechnical Department     URI: http://www.bfro.uni-lj.si/MR/ggorjan
Groblje 3                   mail: gregor.gorjanc <at> bfro.uni-lj.si 

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