# [R] integration problem

Petr Pikal petr.pikal at precheza.cz
Wed Oct 27 09:47:47 CEST 2004

```Hi Christoph

I use this function to calculate an area under curve. It integrates
along the whole wavelength set or you can restrict the calculation
by setting dm and hm to some appropriate values.

It also gives you two values. The first is total area to the x axis
(y=0) and the second is only area below the curve and above line
from min(x) to max(x).

integ1<-function (x,y,dm=-Inf,hm=+Inf)
{
if(dm==-Inf)dm<-min(x)
if(hm==+Inf)hm<-max(x)
vyber<-x<=hm&x>=dm
l<-length(x[vyber])
v<-diff(x[vyber])
z<-y[vyber][1:l-1]+y[vyber][2:l]
o<-z*v/2
osum<-sum(o)
o1<-
(y[x==min(x[vyber])]+y[x==max(x[vyber])])*(max(x[vyber])-
min(x[vyber]))/2
cista<-osum-o1
return(c(osum,cista))
}

I have also a function for specifying region for integration from
plotted picture, if you are interested.

Cheers
Petr

On 26 Oct 2004 at 17:54, 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
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help