[Rd] interp.surface() error (PR#7745)

[Prof Brian Ripley]

R does not contain a function interp.surface!

This is from package 'fields', and according to the FAQ only the 
maintainer is allowed to send bug reports on a contributed package to 
R-bugs.

I am closing this report: please contact the maintainer (and do read the 
FAQ).

On Wed, 23 Mar 2005 murdoch at stats.uwo.ca wrote:

> On Wed, 23 Mar 2005 22:16:30 +0100 (CET), takahama at andrew.cmu.edu
> wrote :
>
>> R version: 2.001

(There is no such version of R.)

>> OS: Windows XP SP2
>>
>> When I use the interp.surface() function, on occasion I have encountered
>> the following error message:
>>
>> Error in interp.surface(a, loc) : subscript out of bounds
>>
>> Since it is somewhat dependent on the data set, I cannot say exactly in
>> which cases these occur, but I believe I have found the problem and a
>> solution for it:
>>
>> The scaling calculations in the code,
>>    lx <- ((nx - 1) * (loc[, 1] - xa))/xr + 1
>>    ly <- ((ny - 1) * (loc[, 2] - ya))/yr + 1
>> can lead to numerical error, so in the following lines
>>    lx1 <- ifelse(lx == nx, nx - 1, trunc(lx))
>>    ly1 <- ifelse(ly == ny, ny - 1, trunc(ly))
>> the equality statements which are the first arguments to the IF-ELSE
>> statements return false when they are supposed to be true. I believe the
>> remedy is to simply avoid absolute equalities, as in the following example
>> (which has worked for my problem):
>>    lx1 <- ifelse(abs(lx - nx) < 1e-10, nx - 1, trunc(lx))
>>    ly1 <- ifelse(abs(ly - ny) < 1e-10, ny - 1, trunc(ly))
>>
>> I hope this is an appropriate posting for an error report. Thank you very
>> much for your hard work,
>
> I think
>
> ifelse(lx >= nx, nx - 1, trunc(lx))
> ifelse(ly >= ny, ny - 1, trunc(ly))
>
> would probably be a better fix, but I'd really like to see an example
> of this in action.  Could you email me a dataset where you see it?
>
> Duncan Murdoch
>
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>
>

-- 
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