On Thu, 5 Mar 2009, Peter Dalgaard wrote:
Prof Brian Ripley wrote:
Undortunately the example is random, so not really reproducible (and I
see nothing wrong on my Mac). However, Linux valgrind on R-devel is
showing a problem:
==3973== Conditional jump or move depends on uninitialised value(s)
==3973== at 0xD76017B: ehg141_ (loessf.f:532)
==3973== by 0xD761600: lowesa_ (loessf.f:769)
==3973== by 0xD736E47: loess_raw (loessc.c:117)
(The uninitiialized value is in someone else's code and I suspect it was
either never intended to work or never tested.) No essential change has
been made to the loess code for many years.
I would not have read the documentation to say that degree = 0 was a
reasonable value. It is not to my mind 'a polynomial surface', and
loess() is described as a 'local regression' for degree 1 or 2 in the
reference. So unless anyone wants to bury their heads in that code I
think a perfectly adequate fix would be to disallow degree = 0.
(I vaguely recall debating allowing in the code ca 10 years ago.)
The code itself has
if (!match(degree, 0:2, 0))
stop("'degree' must be 0, 1 or 2")
though. "Local fitting of a constant" essentially becomes kernel
smoothing, right?
I do know the R code allows it: the question is whether it is worth
the effort of finding the problem(s) in the underlying c/dloess code,
whose manual (and our reference) is entirely about 1 or 2. I am
concerned that there may be other things lurking in the degree=0 case
if it was never tested (in the netlib version: I am sure it was only
minmally tested through my R interface).
I checked the original documentation on netlib and that says
29 DIM dimension of local regression
1 constant
d+1 linear (default)
(d+2)(d+1)/2 quadratic
Modified by ehg127 if cdeg<tdeg.
which seems to confirm that degree = 0 was intended to be allowed, and
what I dimly recall from ca 1998 is debating whether the R code should
allow that or not.
If left to me I would say I did not wish to continue to support degree
= 0.
On Thu, 5 Mar 2009, Uwe Ligges wrote:
Berwin A Turlach wrote:
G'day Peter,
On Thu, 05 Mar 2009 09:09:27 +0100
Peter Dalgaard <p.dalga...@biostat.ku.dk> wrote:
rha...@stat.purdue.edu wrote:
<<insert bug report here>>
This is a CRITICAL bug!!! I have verified it in R 2.8.1 for mac
and for windows. The problem is with loess degree=0 smoothing.
For example, try the following:
x <- 1:100
y <- rnorm(100)
plot(x, y)
lines(predict(loess(y ~ x, degree=0, span=0.5)))
This is obviously wrong.
Obvious? How? I don't see anything particularly odd (on Linux).
Neither did I on linux; but the OP mentioned mac and windows. On
windows, on running that code, the lines() command added a lot of
vertical lines; most spanning the complete window but some only part.
Executing the code a second time (or in steps) gave sensible
results. My guess would be that some memory is not correctly
allocated or
initialised. Or is it something like an object with storage mode
"integer" being passed to a double? But then, why doesn't it show on
linux?
Happy bug hunting. If my guess is correct, then I have no idea how to
track down such things under windows.....
Cheers,
Berwin
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Please can you folks try under R-devel (to be R-2.9.0 in a couple of
weeks) and report if you still see it. I do not under R-devel (but do
under R-release), so my guess is that something called by loess() has
been fixed in the meantime.
Moreover it is not the plot stuff that was wrong under R-2.8.1
(release) but the loess computations.
Uwe Ligges
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O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
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~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907
--
Brian D. Ripley, rip...@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
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