[R] Matrix package: band matrix

[Martin Maechler]

>>>>> "TL" == Thomas Lumley <tlumley at u.washington.edu>
>>>>>     on Fri, 20 Feb 2009 05:49:10 -0800 (PST) writes:

    TL> On Fri, 20 Feb 2009, David Winsemius wrote:
    >> If you can tolerate the subdiagonal and superdiagonal zero elements being 
    >> populated, then perhaps this is useful. If there is a subset function as you 
    >> suggest, then perhaps further reduction would be feasible from this as a 
    >> starting point. At least it would reduce the size from 10^5 x 10^5 to 10^5 x 3:
    >> 
    >> library(Matrix)
    >> xxx <- data.frame(x1 =1:5, y1=c(0,6,0,7,0) )
    >> M= Matrix(0,5,5)
    >> M[row(M) == col(M)] <- xxx$x1
    >> M[row(M)-1 == col(M)] <- xxx$y1[1:(length(xxx$y1)-1)]
    >> M[row(M) == col(M)-1] <- xxx$y1[1:(length(xxx$y1)-1)]

    TL> That provides the right structure, but row(M) and col(M) are dense matrices with 10^10 elements, so it still uses too much memory.

Yes, definitely, thank you, David, anyway.

I'll work out a nice solution, (but am currently busy with
something else, so will not be within the next hour).

For a future version of Matrix, I'd really like a nice user
interface, i.e., function, for constructing  band matrices; this
has been on my TODO list, at least mentally, for quite a while,
and as I had never gotten close to use case, never gained enough
priority.

Maybe I can get some suggestions now on desired functionality of
such a function, bandMatrix(.) say ?
It probably should have an option for specifying symmetric
vs. "general" band matrices, something that the "corresponding"
Matlab function [ spdiags() ] does not.

Martin Maechler, ETH Zurich


    TL> -thomas

    TL> Thomas Lumley			Assoc. Professor, Biostatistics
    TL> tlumley at u.washington.edu	University of Washington, Seattle

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