On 2010-04-23 11:46, David Winsemius wrote:
On Apr 23, 2010, at 1:06 PM, Muhammad Rahiz wrote:
Does anyone know of a method that I can get the intersection where the
red and blue curves meet i.e. the value on the x-axis?
x <- 1:10
y <- 10:1
plot(x,y)
abline(lm(y~x),col="blue")
abline(h=2.5,col="red")
Two ways :
> xy <- lm(y~x)
> xyf <- function(x) coef(xy)[1] +x*coef(xy)[2]
# absolute difference
> optimise(f=function(x) abs(xyf(x)-2.5), c(1,10) )
$minimum
[1] 8.49998
$objective
(Intercept)
1.932015e-05
#N minimize squared difference
> optimise(f=function(x) (xyf(x)-2.5)^2, c(1,10) )
$minimum
[1] 8.5
$objective
(Intercept)
3.155444e-30
Another (crude) way is to use locator(). I usually maximize
the plot window for this.
--
Peter Ehlers
University of Calgary
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