On Thu, Oct 8, 2009 at 3:48 PM, Monica Pisica <pisicandru@hotmail.com>wrote:

>
>
> Hi,
>
> I would like to find out the coordinates of the intersection points of 2
> density curves. I did a search but i didn't get any significant results. I
> really hope some of you have some ideas. here it is an example:
>
> set.seed(123)
> x1 <- rnorm(100, 1, 1)
> x2 <- rnorm(100, 0, 1)
> d1 <- density(x1)
> d2 <- density(x2)
>
> plot(d1, col = 2)
>  lines(d2, col = 4)
>
>
if you specify n, from ant to in density() and keep them the same, then the
density estimates are done for the same x-coordinates. Therefore you can
compare d1$y with d2$y to deternine, where they intersect:

d2 <- density(x2, from=-3, to=5, n=2048)
d1 <- density(x1, from=-3, to=5, n=2048)
plot(d1, col = 2)
lines(d2, col = 4)
d1$y < d2$y
intersX <- d1$x[as.logical(abs(diff(d1$y < d2$y)))]
intersY <- d1$y[as.logical(abs(diff(d1$y < d2$y)))]
abline(v=inters)
abline(h=intersY)

Cheers,

Rainer


> Now i would like to know the coordinates of the intersection point. I did
> some "trial and error" and i come up with these .... (that means i did
> several vertical and horizontal lines through the graph until visually i
> "hit" the intersection point)
>
> abline(v = 0.35, lty = 2)
> abline(h = 0.34, lty = 2)
>
> I really appreciate any hints you may have, thanks so much,
>
> Monica
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-- 
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation Biology,
UCT), Dipl. Phys. (Germany)

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