[R] order of points in spline

[Charles C. Berry]

On Fri, 6 Nov 2009, _nico_ wrote:

>
> Hello everyone,
>
> I was trying to fit a spline to some points and I was quite surprised to
> find out that the function spline does not take into account the order of
> the points themselves, but orders them by x.
>
> For instance, I have:
> x <- c(262, 275, 264, 250, 247, 242, 238, 233)
> y <- c(422, 389, 359, 308, 269, 229, 191, 176)
> plot(x, y, xlim=c(0, 500), ylim=c(0,500))
> s <- spline(x,y)
> points(s, type="l", lwd=2)
>
> As you can see from the above examplethe spline is fitted not using the
> points in the order that was passed, but ordering them by x.
>
> Is there a way to prevent this?

Huh?

Prevent spline from using (x[n],y[n]) as the coordinates of the n^{th} 
data point?

Why on earth would you want to do that?

Do you think that this:

> all.equal( spline(x,y) , spline( rev(x) , rev(y)))
[1] TRUE
>

should have returned FALSE??

Chuck

p.s. Hint: Respond by citing a reference in the peer reviewed math/stats 
literature that describes a function that does what you want and folks may 
be able to help you.


> I also tried loess and other packages that fit curves to points but didn't
> have any luck either
>
> thanks
> nico
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>
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Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901