[R] integration problem
Sundar Dorai-Raj
sundar.dorai-raj at PDF.COM
Tue Oct 26 18:19:36 CEST 2004
Christoph Scherber wrote:
> Dear R users,
>
> I have spectral data (say, wavelength vs. extinction coefficient) for
> which IÂ´d like to calculate an integral (i.e. the area underneath the
> curve).
>
> Suppose the (artificial) dataset is
>
> lambda E
> 1 2
> 2 4
> 3 5
> 4 8
> 5 1
> 6 5
> 7 4
> 8 9
> 9 8
> 10 2
>
>
>
> How can I calculate an integral for these values using R?
>
> Many thanks for any help!
> Regards
>
> Christoph
>
Hi Christoph,
You can write some code to do trapezoidal integration or use ?approx
in the following manner:
f <- function(xnew, x, y) approx(x, y, xnew)$y
integrate(f, min(x$lambda), max(x$lambda), x = x$lambda, y = x$E)
where `x' is your data above. Using approx is perhaps overkill but it
works. A better solution would be to use trapezoids or perhaps piecewise
linear integration. I don't know of a package that has the latter
approaches off the top of my head but that doesn't mean they doesn't
exist somewhere.
--sundar
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