Find two densities' point of intersection in R - r

Question

I have two densities that overlap as seen in the attached picture. I want to find out where the two lines meet. How would I go about doing that?
This is the code that produced the image:
... #reading in files etc.
pdf("test-plot.pdf")
d1 <- density(somedata)
d2 <- density(someotherdata)
plot(d1)
par(col="red")
lines(d2)
dev.off()
The original data is just two monodimensional vectors, so what I'm interested in is the intersection point of their densities.
I tried to use the solution shown in here, but unfortunately, it neither gives me a number nor even draws the lines correctly:
edit: I have found what I was looking for

Answer 1

# create and plot example data
set.seed(1)
plotrange <- c(-1,8)
d1 <- density(rchisq(1000, df=2), from=plotrange[1], to=plotrange[2])
d2 <- density(rchisq(1000, df=3)-1, from=plotrange[1], to=plotrange[2])
plot(d1)
lines(d2)
# look for points of intersection
poi <- which(diff(d1$y > d2$y) != 0)
# Mark those points with a circle:
points(x=d1$x[poi], y=d1$y[poi], col="red")
# or with lines:
abline(v=d1$x[poi], col="orange", lty=2)
abline(h=d1$y[poi], col="orange", lty=2)

Answer 2

intersect(x,y)
see this help file
For example: If your data are in the same data.frame df
intersect(df$col1, df$col2)

Answer 3

Here is a small example extending John's answer with an example.
require(ggplot2)
require(reshape2)
set.seed(12)
df <- data.frame(x = round(rnorm(100, 20, 10),1), y = round((100/log(100:199)),1))
str(df)
# 'data.frame': 200 obs. of 2 variables:
# $ variable: Factor w/ 2 levels "x","y": 1 1 1 1 1 1 1 1 1 1 ...
# $ value : num 16.8 25.7 20.5 22 19 ...
# Melt and plot
mdf <- melt(df)
ggplot(mdf) +
geom_density(aes(x = value, color = variable))
# Find points that intersect
intersect(df$x, df$y)
# [1] 18.9 20.1 21.3 21.5 21.0 19.6 19.0 20.0 19.8
# To make the answer more complete, here is the source code of intersect.
function (x, y)
{
y <- as.vector(y)
unique(y[match(as.vector(x), y, 0L)])
}
<bytecode: 0x10285d400>
<environment: namespace:base>
>
# It's actually posible to use unique and match to produce the same output
unique(as.vector(df$y)[match(as.vector(df$x), df$y, 0L)])
# [1] 18.9 20.1 21.3 21.5 21.0 19.6 19.0 20.0 19.8!

Answer 4

I'm sure your answers are correct, but here's what finally worked for me:
d1$x[abs(d1$y-d2$y) < 0.00001 && d1$x < 1000 && d1$x > 500]
(because I really only needed to find out one value and am a total R newbie, which made it difficult to understand your answers, since I don't even understand most basic R concepts yet. Thank you for your help and sorry.