Hi everybody,
Thank you so much for your answers. The easiest and most straight forward
solution is using the function segm_dist from package "pracma" as suggested by
Hans Borchers.
Thanks again and Happy Thanksgiving for those who celebrate!
Monica
----------------------------
Message: 99
Date: Wed, 23 Nov 2011 08:11:22 +0000
From: Hans W Borchers <hwborch...@googlemail.com>
To: <r-h...@stat.math.ethz.ch>
Subject: Re: [R] x, y for point of intersection
Message-ID: <loom.20111123t085346-...@post.gmane.org>
Content-Type: text/plain; charset="utf-8"
Monica Pisica <pisicandru <at> hotmail.com> writes:
> 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")
xa <- -5; ya <- 2
xb <- 12; yb <- 4
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.
You could apply the function segm_distance in package 'pracma'. If the
distance between two segments is 0, it returns the intersection point:
p1 <- c(xa, ya); p2 <- c(xb, yb)
for (i in 2:100) {
p3 <- c(k1[i-1], q1[i-1]); p4 <- c(k1[i], q1[i])
s <- segm_distance(p1, p2, p3, p4)
if (s$d == 0) break
}
s$p # 0.2740154 2.6204724
points(s$p[1], s$p[2], pch="+", col="red")
> Thanks,
> Monica
>
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