[Rd] What should dnorm(0, 0, -Inf) return?

[peter dalgaard]

Yes, that looks like a bug and an easily fixable one too.

However, I spy another issue: Why do we check the !R_FINITE(x) && mu == x before checking for sd < 0 ? The difference is whether we 

return ML_NAN;
or
ML_ERR_return_NAN;

but surely negative sd should always be an error?

I'd be inclined to do

    if (sigma < 0) ML_ERR_return_NAN;
    if(!R_FINITE(sigma)) return R_D__0;
    if(!R_FINITE(x) && mu == x) return ML_NAN;/* x-mu is NaN */
    if (sigma == 0) 
        return (x == mu) ? ML_POSINF : R_D__0;
    x = (x - mu) / sigma;


(Ping Martin...)

-pd

> On 7 Dec 2019, at 23:40 , Wang Jiefei <szwjf08 using gmail.com> wrote:
> 
> Good question, I cannot speak for R's developers but I would like to
> provide some information on the problem. Here are the first few lines of
> the dnorm function located at src\nmath\dnorm.c:
> 
> ```
> double dnorm4(double x, double mu, double sigma, int give_log)
> {
> #ifdef IEEE_754
>    if (ISNAN(x) || ISNAN(mu) || ISNAN(sigma))
> return x + mu + sigma;
> #endif
>    if(!R_FINITE(sigma)) return R_D__0;
>    if(!R_FINITE(x) && mu == x) return ML_NAN;/* x-mu is NaN */
>    if (sigma <= 0) {
>    if (sigma < 0) ML_ERR_return_NAN;
>        /* sigma == 0 */
>        return (x == mu) ? ML_POSINF : R_D__0;
>    }
>    ....
> }
> ```
> 
> You can clearly see where the problem is. I think either the document or
> the code needs a modification.
> 
> Best,
> Jiefei
> 
> On Sat, Dec 7, 2019 at 5:05 PM Weigand, Stephen D. via R-devel <
> r-devel using r-project.org> wrote:
> 
>> Hi,
>> Apropos of a recent Inf question, I've previously wondered if dnorm "does
>> the right thing" with
>> 
>>  dnorm(0, 0, -Inf)
>> 
>> which gives zero. Should that be zero or NaN (or NA)?
>> 
>> The help says "'sd < 0' is an error and returns 'NaN'" and since -Inf < 0
>> is TRUE, then... is this a bug?
>> 
>> Thank you,
>> Stephen
>> Rochester, MN USA
>> 
>> ______________________________________________
>> R-devel using r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>> 
> 
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> 
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
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