R devel - Aug 2000 - Re: [R] too large alpha or beta in dbeta ? (PR#643)

>>>>> "TL" == Thomas Lumley
<thomas@biostat.washington.edu> writes:
    TL> On Thu, 24 Aug 2000, Troels Ring wrote:
    >> Dear friends.
    >> 
    >> Is this as expected ? Is alpha and beta too large simply ?
    >> 
    >> > dbeta(.1,534,646)
    >> [1] NaN
    >> Warning message:
    >> NaNs produced in: dbeta(x, shape1, shape2, log)

    TL> well, it should work, but the correct answer is effectively zero.
    TL> pbeta(.1,534,646) gives 3.6e-213

    TL> Perhaps more worrying is 
    >> dbeta(.25,534,646)
    TL> [1] Inf
yes.
and I see that it is one case where the log-density seems to be alright:
> dbeta(.25,534,646,log=TRUE)
[1] -109.939612

and also for the NaN case :
> dbeta(.1,534,646, log=TRUE)
[1] -480.725168

A workaround is  using   exp( log-density  ), i.e.

  exp(dbeta(x,a,b, log = TRUE)) :

Look at
> plot(function(x)    dbeta(x, 534,646, log = TRUE), n = 1001)
or
> plot(function(x)exp(dbeta(x, 534,646, log = TRUE)), n = 1001)
--
I'll have a look.

Martin


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On Fri, 25 Aug 2000 15:20:10 +0200 (MET DST), Martin Maechler wrote in
message <200008251320.PAA05152@pubhealth.ku.dk>:
>Look at
>
>> plot(function(x)    dbeta(x, 534,646, log = TRUE), n = 1001)
I never knew that plot could take a function!  

Could I make some suggestions?

1.  It should be mentioned more prominently in the documentation.
Since functions aren't a class, I scanned over the list of plot
methods without ever noticing that functions were mentioned there.
I'd suggest modifying the description of parameter x to read:

 x: the coordinates of points in the plot. Alternatively, a
single plotting structure, function or R object with a `plot' method
can be provided.


2.  If xlim is specified and from and to aren't, the from and to
values should default to xlim[1] and xlim[2], not 0 and 1.  

3.  The help for plot.function says:

"This used to be a quick hack which seems to serve a useful
purpose, but can give bad results for functions which are not
smooth."

That doesn't read right:  it makes me ask, "If it used to be a quick
hack, what is it now?"  I'd suggest:

"This function was a quick hack which seems to serve a useful
purpose, but it can give bad results for functions which are not
smooth."


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