From: Martin Maechler <maechler_at_stat.math.ethz.ch>

Date: Mon 29 Jan 2007 - 08:06:34 GMT

Warning message:

data length [4] is not a sub-multiple or multiple of the number of rows [3] in matrix > (2 * x) + t(x)

3 x 3 sparse Matrix of class "dgCMatrix"

> ((2 * x) + t(x)) * t(x)

3 x 3 sparse Matrix of class "dgCMatrix"

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon Jan 29 19:12:41 2007

Date: Mon 29 Jan 2007 - 08:06:34 GMT

>>>>> "Jose" == Jose Quesada <quesada@gmail.com> >>>>> on Sat, 27 Jan 2007 23:42:34 +0100 writes:

Jose> Hi Martin, Thanks for your detailed answer.

Jose> x <- Matrix(1:12, 3,4, sparse = TRUE)

>> I hope that you are aware of the fact that it's not

* >> efficient at all to store a dense matrix (it has *no* 0
** >> entry) as a sparse one..
** >>
** >> and your posting is indeed an incentive for the Matrix
** >> developers to improve that part ... ;-)
** >>
*

Jose> Yes, the toy example is not sparse but the actual data Jose> is, and very large; I'm aware that coercing a dense Jose> matrix into the Sparse format is not leading to any Jose> saving (on the contrary). I'm talking about a real Jose> application with large sparse matrices; from now on, Jose> I'll post small examples using sparse matrices as well Jose> to avoid confusion.

ok.

Jose> so I tried

Jose> x = matrix(1:12,3,4) Jose> x = as(x, "CsparseMatrix") Jose> xnorms = sqrt(colSums(x^2)) Jose> xnorms = as(xnorms, "CsparseMatrix") Jose> (xnormed = t(x) * (1/xnorms)) Jose> But now, instead of a warning I get Jose> "Error: Matrices must have same dimensions in t(x) * (1/xnorms)"

>> yes. And the same happens with traditional matrices -- and well so:

* >> For arithmetic with matrices (traditional or "Matrices"),
** >>
** >> A o B (o in {"+", "*", "^", ....})
** >> -----
** >>
** >> does require that matrices A and B are ``conformable'', i.e.,
** >> have exact same dimensions.
** >>
** >> Only when one of A or B is *not* a matrix,
** >> then the usual S-language recycling rules are applied,
** >> and that's what you were using in your first example
** >> (<Matrix> * <numeric>) above.
** >>
*

Jose> Right. So this means that the * operator is not Jose> overloaded in Matrix (that is, if I use it, I'll get Jose> my Matrix coherced to matrix. Is that correct?

no. The "*" is overloaded (read on)

Jose> Does this mean that there is no easy way to do element-by-element Jose> multiplication without leaving the sparse Matrix format?

No. There is an easy way:

If you multiply (or add or ..) two sparse matrices of matching dim(), the
result will be sparse. Also if use a "scalar" (length-1 vector)
with a Matrix, the result remains sparse (where appropriate) :

> (x <- Matrix(c(0,1,0,0), 3,3))

3 x 3 sparse Matrix of class "dtCMatrix"

[1,] . . . [2,] 1 . . [3,] . 1 .

Warning message:

data length [4] is not a sub-multiple or multiple of the number of rows [3] in matrix > (2 * x) + t(x)

3 x 3 sparse Matrix of class "dgCMatrix"

[1,] . 1 . [2,] 2 . 1 [3,] . 2 .

> ((2 * x) + t(x)) * t(x)

3 x 3 sparse Matrix of class "dgCMatrix"

[1,] . 1 . [2,] . . 1 [3,] . . .

What you tried to do, <sparse> * <vector-of-length-gt-1>, will only result in a sparse matrix in the next version of the Matrix package.

Jose> I suspect I'm facing the drop=T as before...

>> why??

Jose> Because when I got a row out of a Matrix object, the Jose> resulting vector is not of class Matrix but numeric, Jose> and then (<Matrix> * <numeric>) is applied. Jose> Last, I shouldn't consider myself the most standard Jose> user of the matrix package, since my lineal algebra is Jose> really basic. But in any case, you should know that Jose> your package is being enormously useful for me. KeepJose> up the good work. And if I can help by posting my very Jose> basic questions, I'm glad to help.

Ok, thanks for the flowers :-)

Martin

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon Jan 29 19:12:41 2007

Archive maintained by Robert King, hosted by
the discipline of
statistics at the
University of Newcastle,
Australia.

Archive generated by hypermail 2.1.8, at Mon 29 Jan 2007 - 09:30:25 GMT.

*
Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help.
Please read the posting
guide before posting to the list.
*