shifting a huge matrix left or right efficiently ?

A little algebra up front can often help.

Note that

 	X <- shiftMatrixL(X, 1)*3 + shiftMatrixL(X,2)*5

(and similarly with more terms on the r.h.s)

is just

 	X <- X %*% mat

where mat is is a matrix of zeroes except that

 	mat[ i+1, i ] == 3
 	mat[ i+2, i ] == 5

and dim(mat) == dim(X).

So forget about explicitly shifting the matrix if you do this in native R 
- just construct a suitable version of mat and use '%*%'. If you must do

this in C shift coefficient vector implicitly using a pointer before 
finding the inner product with each row, and if the matrix is truly large 
follow Tony Plate's advice to transform X first (and look at Chapter 1 of 
Matrix Computations by Golub and Van Loan, 1996, if you need to know why).

I'm wondering what's the best way to shift a huge matrix left or right.
My current implementation is the following:

shiftMatrixL <- function(X, shift, padding=0) {
 cbind(X[, -1:-shift], matrix(padding, dim(X)[1], shift))
}

X <- shiftMatrixL(X, 1)*3 + shiftMatrixL(X,2)*5...

However, it's still slow due to heavy use of this function.
The resulting matrix will only be read once and then discarded,
so I believe the best implementation of this function is in C,
manipulating the internal data structure of this matrix.
Anyone know similar package for doing this job ?

Huang-Wen

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Charles C. Berry                        (858) 534-2098
                                          Dept of Family/Preventive Medicine
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