Matrix*vector: coercing sparse to dense matrix for arithmetic
On 11/29/06, Tamas K Papp <tpapp at princeton.edu> wrote:
I have a sparse Matrix (kronecker product of spline design matrices), and I need to multiply each row by a number to get another matrix. If the matrix is A and the numbers are stored in a vector k, with plain vanilla matrices I would do
A*k
But when using the Matrix package (class of A is "dgCMatrix"), I get the warning "coercing sparse to dense matrix for arithmetic". The error message is perfectly reasonable, I am looking for a way to do it right (keeping operations in the realm of sparse matrices). Any help would be appreciated.
I would suggest premultiplying by a diagonal matrix. However, when I look at the method dispatch after an example it doesn't appear to be making a good choice. What I think should work well (and Martin or I will make the necessary changes to ensure that it does) is library(Matrix) data(KNex); mm <- KNex$mm system.time(Diagonal(x = rnorm(nrow(mm))) %*% mm)