[Rd] (PR#13132) splinefun gives incorrect derivs when extrapolating to the left

[Prof Brian Ripley]

Berwin,

Thanks, I've merged this (second version) patch into 2.8.0 beta.

Brian

On Tue, 7 Oct 2008, Berwin A Turlach wrote:

> On Tue, 7 Oct 2008 19:31:03 +0800
> Berwin A Turlach <berwin at maths.uwa.edu.au> wrote:
>
>> The attached patch (against the current SVN version of R) implements
>> the latter strategy.  With this patch applied, "make check
>> FORCE=FORCE" passes on my machine.  The version of R that is build
>> seems to give the correct answer in your example:
>
> Perhaps I should have thought a bit more about this.  For a natural
> spline c[1] is zero, and d[1] is typically not but for evaluations
> left of the first knot it should be taken as zero.  So the attached
> patch solves the problem in what some might consider a more elegant
> manner. :)
>
> With the patch "make check FORCE=FORCE" works on my machine and it
> also solves your example:
>
> R> x <- 1:10
> R> y <- sin(x)
> R>
> R> splfun <- splinefun(x,y, method='natural')
> R>
> R> # these should be linear (and are)
> R> splfun( seq(0,1, 0.1) )
> [1] 0.5682923 0.5956102 0.6229280 0.6502459 0.6775638 0.7048816
> [7] 0.7321995 0.7595174 0.7868352 0.8141531 0.8414710
> R>
> R> # these should all be the same
> R> splfun( seq(0,1, 0.1), deriv=1 )
> [1] 0.2731787 0.2731787 0.2731787 0.2731787 0.2731787 0.2731787
> [7] 0.2731787 0.2731787 0.2731787 0.2731787 0.2731787
> R>
> R> # these should all be 0
> R> splfun( seq(0,1, 0.1), deriv=2 )
> [1] 0 0 0 0 0 0 0 0 0 0 0
> R> splfun( seq(0,1, 0.1), deriv=3 )
> [1] 0 0 0 0 0 0 0 0 0 0 0
>
> HTH.
>
> Cheers,
>
> 	Berwin
>
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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)
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