R help - Feb 2011 - Accessing sub diagonals / spdiag in R ?

Hello, I'm attempting to access a specific number of sub diagonals in a
MATRIX and have been accustomed to using spdiags in MATLAB or Octave. I've
got a solution pieced together using for loops and it works though isn't
vectorized and liable to run very slow
for large matrices.

As an example:

A 1 2 3 4 5
9 8 7 6 5
4 5 6 7 8
5 4 3 2 1
8 7 6 0 1

The subdiagonals are: 9,5,3,0   4,4,6   5,7  and 8,
I know about lower.tri and can fetch the data in a resulting vector
which ,in this case, would be:

9,4,5,8,5,4,7,3,6,0

though I would have to manipulate this some more to extract the other
diagonals (imagine this being done for say a 1000 x 1000 matrix). I looked
at CRAN and didn't see anything corresponding to
spdiags. The closest package appeared to be the one relating to sparse
matrices and band symmetry. Would you have any suggestions about 1) how to
emulate spdiags or 2) working with the lower.tri returned-data and
extracting the remaining diags efficiently. I can live with what I have but
imagine that there is something more direct. Thanks

	[[alternative HTML version deleted]]

On Fri, Feb 25, 2011 at 11:26 AM, Mingo <catojones at gmail.com>
wrote:
> Hello, I'm attempting to access a specific number of sub diagonals in a
> MATRIX and have been accustomed to using spdiags in MATLAB or Octave.
I've
> got a solution pieced together using for loops and it works though
isn't
> vectorized and liable to run very slow
> for large matrices.
>
> As an example:
>
> A > 1 2 3 4 5
> 9 8 7 6 5
> 4 5 6 7 8
> 5 4 3 2 1
> 8 7 6 0 1
>
> The subdiagonals are: 9,5,3,0 ? 4,4,6 ? 5,7 ?and 8,
> I know about lower.tri and can fetch the data in a resulting vector
> which ,in this case, would be:
>
> 9,4,5,8,5,4,7,3,6,0
>
> though I would have to manipulate this some more to extract the other
> diagonals (imagine this being done for say a 1000 x 1000 matrix). I looked
> at CRAN and didn't see anything corresponding to
> spdiags. The closest package appeared to be the one relating to sparse
> matrices and band symmetry. Would you have any suggestions about 1) how to
> emulate spdiags or 2) working with the lower.tri returned-data and
> extracting the remaining diags efficiently. I can live with what I have but
> imagine that there is something more direct. Thanks
A[ col(A) == row(A) - i ] is the ith subdiagonal

-- 
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com