R devel - Aug 2014 - Subscripting Matrices

There seems to be a result type difference when subscripting a 6 x 1 
matrix as compared to a 3 x 2 matrix that is caused by the ncol = 1 
compared to ncol > 1.

 > ThinMatrix <- matrix(1:6,ncol=1)
 > ThinMatrix
      [,1]
[1,]    1
[2,]    2
[3,]    3
[4,]    4
[5,]    5
[6,]    6
 > FatMatrix <- matrix(1:6,ncol=2)
 > FatMatrix
      [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6
 > dim(ThinMatrix[TRUE,])
NULL                                      #Though this value should be 6 1
 > dim(FatMatrix[TRUE,])
[1] 3 2

Thanks for your help.

Terry Ireland

You want `drop=FALSE`:
> dim(ThinMatrix[TRUE, , drop=FALSE])
[1] 6 1
>From ?"[":
    drop: For matrices and arrays.  If ?TRUE? the result is coerced to
          the lowest possible dimension (see the examples).  This only
          works for extracting elements, not for the replacement.  See
          ?drop? for further details.

And R inferno 8.1.44:
http://www.burns-stat.com/pages/Tutor/R_inferno.pdf

Gabor

On Wed, Aug 6, 2014 at 11:07 AM, Terrence Ireland <kyzyl at his.com>
wrote:
> There seems to be a result type difference when subscripting a 6 x 1 matrix
> as compared to a 3 x 2 matrix that is caused by the ncol = 1 compared to
> ncol > 1.
>
>> ThinMatrix <- matrix(1:6,ncol=1)
>> ThinMatrix
>      [,1]
> [1,]    1
> [2,]    2
> [3,]    3
> [4,]    4
> [5,]    5
> [6,]    6
>> FatMatrix <- matrix(1:6,ncol=2)
>> FatMatrix
>      [,1] [,2]
> [1,]    1    4
> [2,]    2    5
> [3,]    3    6
>> dim(ThinMatrix[TRUE,])
> NULL                                      #Though this value should be 6 1
>> dim(FatMatrix[TRUE,])
> [1] 3 2
>
> Thanks for your help.
>
> Terry Ireland
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

On Aug 6, 2014, at 10:07 AM, Terrence Ireland <kyzyl at his.com> wrote:
> There seems to be a result type difference when subscripting a 6 x 1 matrix
as compared to a 3 x 2 matrix that is caused by the ncol = 1 compared to ncol
> 1.
> 
> > ThinMatrix <- matrix(1:6,ncol=1)
> > ThinMatrix
>     [,1]
> [1,]    1
> [2,]    2
> [3,]    3
> [4,]    4
> [5,]    5
> [6,]    6
> > FatMatrix <- matrix(1:6,ncol=2)
> > FatMatrix
>     [,1] [,2]
> [1,]    1    4
> [2,]    2    5
> [3,]    3    6
> > dim(ThinMatrix[TRUE,])
> NULL                                      #Though this value should be 6 1
> > dim(FatMatrix[TRUE,])
> [1] 3 2
> 
> Thanks for your help.
> 
> Terry Ireland

Hi,

This question really should have gone to R-Help, not R-Devel and it is a FAQ:

 
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-do-my-matrices-lose-dimensions_003f
> str(ThinMatrix[TRUE,])
 int [1:6] 1 2 3 4 5 6


Regards,

Marc Schwartz