[R] Converting indices of a matrix subset

[Nathan S. Watson-Haigh]

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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 can be done with a simple offset when using arr.ind=TRUE:
> ind2 <- which(m2 == 1, arr.ind=T) + 1
> ind2
     row col
[1,]   2   2
[2,]   3   3
[3,]   4   4

But I don't want to use this, as I eventually want to be able to do things like:

> m1[-c(ind2)] <- 0
> m1
     [,1] [,2] [,3] [,4]
[1,]    0    0    0    0
[2,]    0    1    0    0
[3,]    0    0    1    0
[4,]    0    0    0    1


Any thoughts?
Cheers,
Nathan

- --
- --------------------------------------------------------
Dr. Nathan S. Watson-Haigh
OCE Post Doctoral Fellow
CSIRO Livestock Industries
Queensland Bioscience Precinct
St Lucia, QLD 4067
Australia

Tel: +61 (0)7 3214 2922
Fax: +61 (0)7 3214 2900
Web: http://www.csiro.au/people/Nathan.Watson-Haigh.html
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