# [R] intersection of 2 density curves

Monica Pisica pisicandru at hotmail.com
Thu Oct 8 16:26:59 CEST 2009

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Hi=2C
=20
Thanks so much for your answer. It works beautifully for my example and of =
course i will test it on the"real" data as well.
=20
Thanks again=2C
=20
Monica
________________________________
> Date: Thu=2C 8 Oct 2009 16:01:36 +0200
> Subject: Re: [R] intersection of 2 density curves
> From: r.m.krug at gmail.com
> To: pisicandru at hotmail.com
> CC: r-help at stat.math.ethz.ch
>
>
>
> On Thu=2C Oct 8=2C 2009 at 3:48 PM=2C Monica Pisica> wrote:
>
>
>
>
>
> Hi=2C
>
>
>
> 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=2C 1=2C 1)
>
> x2 <- rnorm(100=2C 0=2C 1)
>
> d1 <- density(x1)
>
> d2 <- density(x2)
>
>
>
> plot(d1=2C col =3D 2)
>
> lines(d2=2C col =3D 4)
>
>
>
> if you specify n=2C from ant to in density() and keep them the same=2C th=
en the density estimates are done for the same x-coordinates. Therefore you=
can compare d1\$y with d2\$y to deternine=2C where they intersect:
>
>
> d2 <- density(x2=2C from=3D-3=2C to=3D5=2C n=3D2048)
> d1 <- density(x1=2C from=3D-3=2C to=3D5=2C n=3D2048)
> plot(d1=2C col =3D 2)
> lines(d2=2C col =3D 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=3Dinters)
> abline(h=3DintersY)
>
> Cheers=2C
>
> 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 sev=
eral vertical and horizontal lines through the graph until visually i "hit"=
the intersection point)
>
>
>
>
> abline(v =3D 0.35=2C lty =3D 2)
>
> abline(h =3D 0.34=2C lty =3D 2)
>
>
>
> I really appreciate any hints you may have=2C thanks so much=2C
>
>
>
> Monica
>
> _________________________________________________________________
>

>
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tml
>
> and provide commented=2C minimal=2C self-contained=2C reproducible code.
>
>
>
>
> --
> Rainer M. Krug=2C PhD (Conservation Ecology=2C SUN)=2C MSc (Conservation =
Biology=2C UCT)=2C Dipl. Phys. (Germany)
>
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