I have a large function computing an iterative algorithm for fitting mixed linear models. Almost all code relies on functions from the Matrix package. I've come across an issue that I do not believe previously occurred in earlier versions of R or Matrix.

I have a large, sparse matrix, A as

> class(A);dim(A)
[1] "dgCMatrix"
attr(,"package")
[1] "Matrix"
[1] 12312 12312

I am in a position where I must find its inverse.  I realize this is less than ideal, and I have two ways of doing this

A.Inv <- solve(A, Ir) or just solve(A)

Where Ir is an identity matrix with the same dimensions as A and it is also sparse

> class(Ir)
[1] "ddiMatrix"
attr(,"package")
[1] "Matrix"

The issue, however, is that the inverse of A is converted into a dense matrix and this becomes a huge memory hog, causing the rest of the algorithm to fail. In prior versions this remained as a sparse matrix.

> A.Inv[1:5, 1:5]
5 x 5 Matrix of class "dgeMatrix"
          [,1]      [,2]      [,3]      [,4]      [,5]
[1,] 0.6878713 0.0000000 0.0000000 0.0000000 0.0000000
[2,] 0.0000000 0.6718767 0.0000000 0.0000000 0.0000000
[3,] 0.0000000 0.0000000 0.5076945 0.0000000 0.0000000
[4,] 0.0000000 0.0000000 0.0000000 0.2324122 0.0000000
[5,] 0.0000000 0.0000000 0.0000000 0.0000000 0.2139975

I could coerce this matrix to become sparse such as

> AA <- as(A.Inv, 'sparseMatrix')
> class(AA)
[1] "dgCMatrix"
attr(,"package")
[1] "Matrix"

> AA[1:5, 1:5]
5 x 5 sparse Matrix of class "dgCMatrix"

[1,] 0.6878713 .         .         .         .
[2,] .         0.6718767 .         .         .
[3,] .         .         0.5076945 .         .
[4,] .         .         .         0.2324122 .
[5,] .         .         .         .         0.2139975

But I don't think this is best.

So, my question is why is a matrix that is sparse turning into a dense matrix? Can I avoid that and keep it sparse without having to coerce it to be sparse after it is created?

Thank you very much
Harold


> sessionInfo()
R version 3.0.1 (2013-05-16)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C
[5] LC_TIME=English_United States.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] lme4_0.999999-2 Matrix_1.0-12   lattice_0.20-15

loaded via a namespace (and not attached):
[1] grid_3.0.1   nlme_3.1-109 stats4_3.0.1 tools_3.0.1

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