Ben Bolker <bbolker <at> gmail.com> writes:
>
> I'm not quite sure how to do it, but I think you should look
> at the ?band function in Matrix. In combination with diag() of a
> suitably truncated matrix, you should be able to extract bands
> of sparse matrices efficiently ...
>
getband <- function(A,k) {
n <- nrow(A)
if (abs(k)>(n-1)) stop("bad band requested")
if (k>0) {
v <- seq(n-k) ## -seq((n-k+1),n)
w <- seq(k+1,n) ## -seq(n-k-1)
} else if (k<0) {
v <- seq(-k+1,n)
w <- seq(n+k)
} else return(diag(A))
diag(band(A,k,k)[v,w,drop=FALSE])
}
PS: I think this should extract the k^th off-diagonal
band in a way that should (?) work reasonably efficiently
with sparse matrices. I have not tested it carefully,
nor benchmarked it.
Ben Bolker
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