Replacing context:
Hello everyone.
I have created a 100*100 matrix in R.
Let's now say that I have a line that starts from (2,3) point and
ends to the
(62,34) point. In other words this line starts at cell (2,3) and
ends at cell
(62,34).
Is it possible to get by some R function all the matrix's cells
that this line
transverses?
I would like to thank you for your feedback.
Best Regards
Alex
On Sep 15, 2010, at 6:52 AM, Michael Bedward wrote:
Hello Alex,
Here is one way to do it. It works but it's not pretty :)
If you want an alternative, consider that produces the Y cell indices
(since the x cell indices are already 2:62):
> linefn <- function(x) 3+((34-3)/(62-2)) *(x-2)
> findInterval(linefn(2:62), 3:34)
[1] 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11
11 12 12 13 13 14
[28] 14 15 15 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25
26 26 27 27 28
[55] 28 29 29 30 30 31 32
# that seems "off" by two
> linefn(62)
[1] 34
> linefn(2)
[1] 3 # but that checks out and I realized those were just indices for
the 3:34 findInterval vector
> (3:34)[findInterval(linefn(2:62), 3:34)]
[1] 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13
13 14 14 15 15 16
[28] 16 17 17 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27
28 28 29 29 30
[55] 30 31 31 32 32 33 34
( no rounding and I think the logic is clearer.)
--
David.
interp <- approx(c(2, 62), c(3, 34), method="linear", xout=2:62)
m <- matrix(c(interp$x, round(interp$y)), ncol=2)
tie <- m[,2] == c(-Inf, m[-nrow(m),2])
m <- m[ !tie, ]
You might want to examine the result like this...
plot(m) # plots points
lines(c(2,26), c(3, 34)) # overlay line for comparison
you can add a grid with
abline(v=2:62, h=3:34)
Michael
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