[R] x, y for point of intersection

[R. Michael Weylandt <michael.weylandt@gmail.com>]

If it's a one off, the identify() function might be of help -- if you need something algorithmic it's harder due to floating point stuff and sampling frequencies. Let me know if that's the case. 

Michael

On Nov 22, 2011, at 3:40 PM, Monica Pisica <pisicandru at hotmail.com> wrote:

> 
> 
> 
> Hi everyone,
> 
>  
> 
> I am trying to get a point of intersection between a
> polyline and a straight line ….. and get the x and y coordinates of this point.
> For exemplification consider this:
> 
>  
> 
>  
> 
> set.seed(123)
> 
>  
> 
> k1 <-rnorm(100, mean=1.77, sd=3.33)
> 
>  k1 <- sort(k1)
> 
> q1 <- rnorm(100, mean=2.37, sd=0.74)
> 
> q1 <- sort(q1, decreasing = TRUE)
> 
> plot(k1, q1, xlim <- c((min(k1)-5), (max(k1)+5)),
> type="l")
> 
>  
> 
> ya <- 2
> 
> xa = -5
> 
> yb=4
> 
> xb=12
> 
>  
> 
> lines(c(xa, xb), c(ya, yb), col = 2)
> 
>  
> 
> # I want to get the x and y coordinates of the
> intersection of the 2 lines ….
> 
>  
> 
> m <- (ya-yb)/(xa-xb)
> 
> b <- ya-m*xa
> 
> ln <- loess(q1~k1)
> 
> lines(ln)
> 
>  
> 
> It is clear that the x, y will satisfy both linear
> equations, y = m*x + b and the ln polyline ….. but while I can visualize the
> equation of the straight line – I have problems with the polyline. I will appreciate
> any ideas to solve this problem. I thought it a trivial solution but it seems I
> cannot see it.
> Thanks,
> Monica
> 
> 
>                         
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