Hi Nathan,
On Jul 8, 2009, at 10:20 PM, Nathan S. Watson-Haigh wrote:
> I have two matrices:
>
>> m1 <- matrix(1,4,4)
>> m1
> [,1] [,2] [,3] [,4]
> [1,] 1 1 1 1
> [2,] 1 1 1 1
> [3,] 1 1 1 1
> [4,] 1 1 1 1
>
>> m2 <- matrix(0,3,3)
>> diag(m2) <- 1
>> m2
> [,1] [,2] [,3]
> [1,] 1 0 0
> [2,] 0 1 0
> [3,] 0 0 1
>
> I want to get indicies from m2 such that they match indicies as
> though they came
> from the lower right of m1. Here's how things work:
>
>> ind1 <- which(m1 == 1)
>> ind1
> [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
>> ind2 <- which(m2 == 1)
>> ind2
> [1] 1 5 9
>
>
> I would like ind2 to be offset so they look like indicies from the
> lower right
> of m1:
>> ind2
> [1] 6 11 16
I have written some utility functions I use often[1], and have
"sub2ind" function that works in a similar fashion to the MATLAB
function of the same name. It's somehow long and convoluted because
you can send in your arguments 12 different ways from sunday, so:
sub2ind <- function(x, y, nrow, ncol=NULL) {
## Returns a linear index for the (x,y) coordinates passed in.
if (is.matrix(x) || is.data.frame(x)) {
stopifnot(ncol(x) == 2)
if (!missing(y)) {
if (missing(nrow)) {
nrow <- y
} else {
ncol <- nrow
nrow <- y
}
}
y <- x[,2]
x <- x[,1]
}
if (is.matrix(nrow)) {
d <- dim(nrow)
nrow <- d[1]
ncol <- d[2]
} else if (is.null(ncol)) {
stop("Dimensions of matrix under-specified")
}
# Sanity check to ensure we got each var doing what it should be
doing
if (length(x) != length(y) || length(nrow) != 1 || length(ncol) !=
1) {
stop("I'm confused")
}
((x - 1) + ((y - 1) * nrow)) + 1
}
R> m1 <- matrix(1,4,4)
R> m2 <- matrix(0,3,3)
R> ind2 <- which(m2 == 1, arr.ind=T) + 1
R> ind2
row col
[1,] 2 2
[2,] 3 3
[3,] 4 4
R> my.ind2 <- sub2ind(ind2, nrow=4, ncol=4)
# my.ind2 <- sub2ind(ind2[,1], ind2[,2], 4, 4)
# my.ind2 <- sub2ind(ind2[,1], ind2[,2], m1)
# my.ind2 <- sub2ind(ind2, m1)
R> my.ind2
[1] 6 11 16
I think that gets you to where you want to be :-)
-steve
[1] They can be found here if your intersted, there isn't much
documentation there, but I'll fix that some time later :-)
http://github.com/lianos/ARE.utils/tree/master--
Steve Lianoglou
Graduate Student: Physiology, Biophysics and Systems Biology
Weill Medical College of Cornell University
Contact Info:
http://cbio.mskcc.org/~lianos______________________________________________
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