[Rd] bug (PR#13570)

[Prof Brian Ripley]

On Tue, 10 Mar 2009, Benjamin Tyner wrote:

> Many thanks Brian for tracking this down. Was it fixed by
>
>     c next line is not in current dloess
> 	        	  goto 7
>
> in ehg136? If this needs to be in the netlib version as well, we should 
> inform Eric Grosse.

The difference was in the argument list of one of the functions 
(ehg124?).  It was 'just' a question of looking at 354 diff sections, 
not all of which I understood, including that commented above.

> While we're at it, there are a few more inconsistencies (not nearly as 
> serious as PR#13570 so I hesitate to call them bugs) regarding the definition 
> of leaf cell membership (certain .lt. should be .le. ) in ehg128, ehg137, and 
> ehg138 (not currently used); it seems I neglected to mention these to Eric. 
> If you are interested in these I can submit a patch and will notify Eric as 
> well.

Please do let me know and I'll merge in.

> Finally, perhaps now is as good a time as any to point out that in the 
> documentation, the bit about cross-terms in
>
>   \item{drop.square}{for fits with more than one predictor and
>       \code{degree=2}, should the quadratic term (and cross-terms) be
>       dropped for particular predictors?
>
> is incorrect -- cross terms are not dropped in this implementation of loess.

Thanks, I will incorporate that.

> Thanks again,
> Ben
>
> Prof Brian Ripley wrote:
>> I've found the discrepancy, so the patched code from current dloess is now 
>> available in R-patched and R-devel.
>> 
>> On Fri, 6 Mar 2009, Prof Brian Ripley wrote:
>> 
>>> On Thu, 5 Mar 2009, Benjamin Tyner wrote:
>>> 
>>>> Hi
>>>> 
>>>> Nice to hear from you Ryan. I also do not have the capability to debug on 
>>>> windows; however, there is a chance that the behavior you are seeing is 
>>>> caused by the following bug noted in my thesis (available on ProQuest; 
>>>> email me if you don't have access):
>>>> 
>>>> "When lambda = 0 there are no local slopes to aid the blending algorithm, 
>>>> yet the
>>>> interpolator would still assume they were available, and thus use 
>>>> arbitrary values
>>>> from memory. This had implications for both fit and tr[L] computation. In 
>>>> the
>>>> updated code these are set equal to zero which seems the best automatic 
>>>> rule when
>>>> lambda = 0." [lambda refers to degree]
>>>> 
>>>> I submitted a bug fix to Eric Grosse, the maintainer of the netlib 
>>>> routines; the fixed lines of fortran are identified in the comments at 
>>>> (just search for my email address):
>>>> 
>>>> http://www.netlib.org/a/loess
>>>> 
>>>> These fixes would be relatively simple to incorporate into R's version of 
>>>> loessf.f
>>> 
>>> The fixes from dloess even more simply, since R's code is based on dloess. 
>>> Thank you for the suggestion.
>>> 
>>> Given how tricky this is to reproduce, I went back to my example under 
>>> valgrind.  If I use the latest dloess code, it crashes, but by selectively 
>>> importing some of the differences I can get it to work.
>>> 
>>> So it looks as if we are on the road to a solution, but something in the 
>>> current version (not necessarily in these changes) is incompatible with 
>>> the current R code and I need to dig further (not for a few days).
>>> 
>>>> Alternatively, a quick check would be for someone to compile the source 
>>>> package at https://centauri.stat.purdue.edu:98/loess/loess_0.4-1.tar.gz 
>>>> and test it on windows. Though this package incorporates this and a few 
>>>> other fixes, please be aware that it the routines are converted to C and 
>>>> thus there is a slight performance hit compared to the fortran.
>>>> 
>>>> Hope this helps,
>>>> Ben
>>> 
>>> [...]
>>> 
>>> -- 
>>> Brian D. Ripley,                  ripley at stats.ox.ac.uk
>>> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
>>> University of Oxford,             Tel:  +44 1865 272861 (self)
>>> 1 South Parks Road,                     +44 1865 272866 (PA)
>>> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>>> 
>> 
>
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595