R devel - Jun 2012 - An extreme quantile of the geometric distribution

Hi,

 With R version 2.10.0 (2009-10-26) on Windows, I computed the p=1.e-20 
 quantile of the geometric distribution with parameter prob=0.1.
> qgeom(1.e-20,0.1)
 [1] -1

 But this is not possible, since X=0,1,2,...

 I guess that this might be a bug in the quantile function, which should 
 use the log1p function, instead of the naive formula.

 Am I correct ?

 Best regards,

 Micha?l

On Jun 28, 2012, at 22:49 , <michael.baudin at contrib.scilab.org>
<michael.baudin at contrib.scilab.org> wrote:
> Hi,
> 
> With R version 2.10.0 (2009-10-26) on Windows, I computed the p=1.e-20
quantile of the geometric distribution with parameter prob=0.1.
> 
>> qgeom(1.e-20,0.1)
> [1] -1
> 
> But this is not possible, since X=0,1,2,...
> 
> I guess that this might be a bug in the quantile function, which should use
the log1p function, instead of the naive formula.
> 
> Am I correct ?
Nope. (The source is availably, you know....).

The problem is that a slight fuzz is subtracted inside ceil(....), but
there's no check that the result is positive.

qnbinom(...., size=1) is equivalent and does get right, by the way.  

-pd
> 
> Best regards,
> 
> Micha?l
> 
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