[Rd] Matrix subsetting {was "... vectorized nested loop..."}

From: Martin Maechler <maechler_at_stat.math.ethz.ch>
Date: Wed 24 Jan 2007 - 10:57:48 GMT

Hi Jose,
I'm answering your second batch of questions, since Chuck Berry has already well done so with the first one

>>>>> "Jose" == Jose Quesada <quesada@gmail.com> >>>>> on Tue, 23 Jan 2007 21:46:27 +0100 writes:

[........]

    Jose> # example
    Jose> library(Matrix)
    Jose> x = as(x,"CsparseMatrix")

[..........]

    Jose> Also, I have noticed that getting a row from a Matrix
    Jose> object produces a normal array (i.e., it does not
    Jose> inherit Matrix class). 

This is very much on purpose, following the principle of "least surprise" so I'm surprised you're suprised.. :

The 'Matrix' behavior has been modelled to follow the more than 20 years old 'matrix' behavior :

> matrix(1:9, 3) [,2]

 [1] 4 5 6
> matrix(1:9, 3) [,2 , drop=FALSE]

      [,1]

 [1,]    4
 [2,]    5
 [3,]    6

> library(Matrix)

 Loading required package: lattice
> Matrix(1:9, 3) [,2]

 [1] 4 5 6
> Matrix(1:9, 3) [,2, drop = FALSE]

 3 x 1 Matrix of class "dgeMatrix"

      [,1]

 [1,]    4
 [2,]    5
 [3,]    6

>

But then I should not be surprised, because there has been the R FAQ

>> 7.5 Why do my matrices lose dimensions?

for quite a while.

*And* I think that there is only one "thing" in the S language about which every "knowledgable one" agrees that it's a "design bug", and that's the fact that 'drop = TRUE' is the default, and not 'drop = FALSE' {but it's not possible to change now, please don't start that discussion!}     

Given what I say above, I wonder if our ("new-style") 'Matrix' objects should not behave differently than ("old-style") 'matrix' and indeed do use a default 'drop = FALSE'.
This might break some Matrix-based code though, but then 'Matrix' is young enough, and working Matrix indexing is much younger, and there are only about 4 CRAN/Bioconductor packages depending on 'Matrix'.
--> This discussion (about changing this behavior in the "Matrix" package) should definitely be lead on the R-devel mailing list --> CC'ing to R-devel
{hence one (but please *only* one !) cross-post}

    Jose> However, selecting >1 rows,
    Jose> does produce a same-class matrix. If I convert with
    Jose> as() the output of selecting one row, am I losing
    Jose> performance? Is there any way to make the resulting
    Jose> vector be a 1-D Matrix object?

yes, ", drop = FALSE", see above

Martin



R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel Received on Wed Jan 24 22:01:27 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 Wed 24 Jan 2007 - 11:31:19 GMT.

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