Sparse matrix no longer sparse (Matrix Package)

I have zero'd in on what appears to be the issue. This seems to be a bug
in Matrix, but I am not sure yet. I am attaching files that would allow
others to replicate this with my toy data.

Notice the elements of D1 in the attached data are all integers. It is a
sparse, diagonal matrix.

library(Matrix)
class(D1)

[1] "ddiMatrix"
attr(,"package")
[1] "Matrix"

Now, I find the inverse of the matrix A as follows:

A <- Ir + ZtZ %*% D1
A.Inv <- solve(A, Ir)

Notice now the inverse of A remains a dgCMatrix and it is relatively
small in size, only 33424 bytes.

class(A.Inv)

[1] "dgCMatrix"
attr(,"package")
[1] "Matrix"

object.size(A.Inv)

33424 bytes

Now, if I change an element of the matrix D1 to be non-integer, D1 still
has the same class as it did before

D1[1] <- 1.2

class(D1)

[1] "ddiMatrix"
attr(,"package")
[1] "Matrix"

Now, if I use this new version of D1 in the same calculations as above,
notice that A.Inv is no longer a dgCMatrix but instead becomes a
dgeMatrix. It then increases from an object of size 33424 bytes to an
object of size 2001112 bytes!

A <- Ir + ZtZ %*% D1
A.Inv <- solve(A, Ir)
class(A.Inv)

[1] "dgeMatrix"
attr(,"package")
[1] "Matrix"

object.size(A.Inv)

2001112 bytes

What I desire is that the object A.Inv remain sparse at all times and not
become dense. But, perhaps there is a reason this change occurs that I
don't fully understand.

I can of course coerce it back to a sparse matrix and it reduces back in
size.

 object.size(as(A.Inv, 'sparseMatrix'))

33424 bytes

I of course recognize it requires more memory to store floating points
than integers, but is this large increase on the order of magnitude that
seems about right?

Is there a reason the floating point in D1 causes for A.Inv to no longer
remain sparse?

Thank you for your help,
Harold





-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Doran, Harold
Sent: Wednesday, July 10, 2013 11:42 AM
To: r-help at r-project.org
Cc: dmbates at gmail.com; 'maechler at stat.math.ethz.ch'
Subject: [R] Sparse matrix no longer sparse (Matrix Package)

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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