# Re: [R] question on graphs and finding area under a curve

Date: Tue 02 Aug 2005 - 23:24:05 EST

Hi,

To find the area lying between the curve y = y(x) and 45 degree line (which, assuming it goes through the origin, is y = x), you can use the following function based on trapezoidal rule:

trap.rule <- function(x,f) {sum(diff(x)*(f[-1]+f[-length(f)]))/2}

trap.rule(x,f=y-x)

This area will be negative if y(x) is below the 45 degree line.

However, your question is not complete, I think. You need to specify the interval of integration. For this you may need to determine the points of intersection of the two curves, which involves the solution of a fixed point problem.

Hope this helps,
Ravi.

> -----Original Message-----
> From: r-help-bounces@stat.math.ethz.ch [mailto:r-help-
> bounces@stat.math.ethz.ch] On Behalf Of Renuka Sane
> Sent: Tuesday, August 02, 2005 8:51 AM
> To: r-help@stat.math.ethz.ch
> Subject: [R] question on graphs and finding area under a curve
>
> Question on graphs:
>
> The default case for drawing a graph in R, is where a little space is left
> on the x and y axis before the first tick i.e. even if I say xlim=c(0,1) -
> -
> there will be some space between the edge of the x-axis and where 0 is
> placed. If I want 0 on the edge, how do I do it in R?
>
> Area under the curve:
>
> I have a 45 degree line and a curve above or below it. Is there a way in R
> to find the area between the two?
>
> Thanks,
> Renuka
>
> --
> Renuka Sane
> http://www.nyx.net/~rsane
>
> [[alternative HTML version deleted]]
>
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