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

[Martin Maechler]

>>>>> peter dalgaard 
>>>>>     on Sun, 8 Dec 2019 12:11:50 +0100 writes:

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

agreed.

    > 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...)

I think you are spot on, Peter.
All of this code has a longish history, with incremental border
case improvements.
Let's hope (somewhat unrealistically) this is the last one for
dnorm().

NB: dlnorm() and some of the gamma/chisq/.. may need a
    similar adjustment

Lastly, regression tests for this
(either in  tests/d-p-q-r-tests.{R,Rout.save}
 or easier in reg-tests-1d.R)  should be added too.

    > -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
    > Solbjerg Plads 3, 2000 Frederiksberg, Denmark
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