R help - Dec 2009 - logical masking of a matrix converts it to a vector

One problem I've been having is the special case in which only one
row/column remains and the variable gets converted into a vector when
entries are removed by logical masking. This is a problem because subsequent
code may rely on matrix operations (apply, colsums, dim, etc) For example:
> a <- matrix(c(1, 2, 3, 4), nrow = 2)
> a
     [,1] [,2]
[1,]    1    3
[2,]    2    4
> rmask <- c(TRUE, FALSE)
> b <- a[rmask, ]
> colSums(b)
Error in colSums(b) : 'x' must be an array of at least two dimensions

To ensure the code works regardless of how the matrix gets modified, I need
to explicitly recast results to a matrix, for example:

b <- matrix(a[rmask, ], ncol = 2)

This can get messy, requiring extra work to maintain column/row names or
additional commands to determine the correct dimensions of the matrix (for
example, if I'm masking both rows and columns simultaneously). Is there a
more elegant way to deal with this in R?

~A

	[[alternative HTML version deleted]]

On Dec 4, 2009, at 3:52 PM, Austin Huang wrote:
> One problem I've been having is the special case in which only one
> row/column remains and the variable gets converted into a vector when
> entries are removed by logical masking. This is a problem because  
> subsequent
> code may rely on matrix operations (apply, colsums, dim, etc) For  
> example:
>
>> a <- matrix(c(1, 2, 3, 4), nrow = 2)
>> a
>     [,1] [,2]
> [1,]    1    3
> [2,]    2    4
>> rmask <- c(TRUE, FALSE)
>> b <- a[rmask, ]
>> colSums(b)

 > b <- a[rmask, , drop=FALSE]
 > colSums(b)
[1] 1 3
> Error in colSums(b) : 'x' must be an array of at least two
dimensions
>
> To ensure the code works regardless of how the matrix gets modified,  
> I need
> to explicitly recast results to a matrix, for example:
>
> b <- matrix(a[rmask, ], ncol = 2)
>
> This can get messy, requiring extra work to maintain column/row  
> names or
> additional commands to determine the correct dimensions of the  
> matrix (for
> example, if I'm masking both rows and columns simultaneously). Is  
> there a
> more elegant way to deal with this in R?
-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT

Check out `drop'

?drop

a <- matrix(c(1, 2, 3, 4), nrow = 2)

rmask <- c(TRUE, FALSE)

b <- a[rmask, , drop=FALSE]

colSums(b)

This will maintain the matrix structure.

Ravi.

----------------------------------------------------------------------------
-------

Ravi Varadhan, Ph.D.

Assistant Professor, The Center on Aging and Health

Division of Geriatric Medicine and Gerontology 

Johns Hopkins University

Ph: (410) 502-2619

Fax: (410) 614-9625

Email: rvaradhan at jhmi.edu

Webpage:
http://www.jhsph.edu/agingandhealth/People/Faculty_personal_pages/Varadhan.h
tml

 

----------------------------------------------------------------------------
--------

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On
Behalf Of Austin Huang
Sent: Friday, December 04, 2009 3:52 PM
To: R-help at r-project.org
Subject: [R] logical masking of a matrix converts it to a vector

One problem I've been having is the special case in which only one
row/column remains and the variable gets converted into a vector when
entries are removed by logical masking. This is a problem because subsequent
code may rely on matrix operations (apply, colsums, dim, etc) For example:
> a <- matrix(c(1, 2, 3, 4), nrow = 2)
> a
     [,1] [,2]
[1,]    1    3
[2,]    2    4
> rmask <- c(TRUE, FALSE)
> b <- a[rmask, ]
> colSums(b)
Error in colSums(b) : 'x' must be an array of at least two dimensions

To ensure the code works regardless of how the matrix gets modified, I need
to explicitly recast results to a matrix, for example:

b <- matrix(a[rmask, ], ncol = 2)

This can get messy, requiring extra work to maintain column/row names or
additional commands to determine the correct dimensions of the matrix (for
example, if I'm masking both rows and columns simultaneously). Is there a
more elegant way to deal with this in R?

~A

	[[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org 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.

> One problem I've been having is the special case in which only one
> row/column remains and the variable gets converted into a vector when
> entries are removed by logical masking. This is a problem because
> subsequent
> code may rely on matrix operations (apply, colsums, dim, etc) For example:
> 
> > a <- matrix(c(1, 2, 3, 4), nrow = 2)
> > a
>      [,1] [,2]
> [1,]    1    3
> [2,]    2    4
> > rmask <- c(TRUE, FALSE)
> > b <- a[rmask, ]
> > colSums(b)
> Error in colSums(b) : 'x' must be an array of at least two
dimensions
> 
> To ensure the code works regardless of how the matrix gets modified, I
> need
> to explicitly recast results to a matrix, for example:
> 
> b <- matrix(a[rmask, ], ncol = 2)
> 
> This can get messy, requiring extra work to maintain column/row names or
> additional commands to determine the correct dimensions of the matrix (for
> example, if I'm masking both rows and columns simultaneously). Is there
a
> more elegant way to deal with this in R?
See ?[ and ?drop

b <- a[rmask, , drop = FALSE]

Hi Austin,

What version of R are you using? It works for me for R 2.10.0 Patched on Win
XP Pro:

R> a <- matrix(c(1, 2, 3, 4), nrow = 2)
R> a
# [1] 1 3
# [2] 2 4
R> rmask <- c(TRUE, FALSE)
R> a[rmask,]
# [1] 1 3

HTH,
Jorge


On Fri, Dec 4, 2009 at 3:52 PM, Austin Huang <> wrote:
> One problem I've been having is the special case in which only one
> row/column remains and the variable gets converted into a vector when
> entries are removed by logical masking. This is a problem because
> subsequent
> code may rely on matrix operations (apply, colsums, dim, etc) For example:
>
> > a <- matrix(c(1, 2, 3, 4), nrow = 2)
> > a
>     [,1] [,2]
> [1,]    1    3
> [2,]    2    4
> > rmask <- c(TRUE, FALSE)
> > b <- a[rmask, ]
> > colSums(b)
> Error in colSums(b) : 'x' must be an array of at least two
dimensions
>
> To ensure the code works regardless of how the matrix gets modified, I need
> to explicitly recast results to a matrix, for example:
>
> b <- matrix(a[rmask, ], ncol = 2)
>
> This can get messy, requiring extra work to maintain column/row names or
> additional commands to determine the correct dimensions of the matrix (for
> example, if I'm masking both rows and columns simultaneously). Is there
a
> more elegant way to deal with this in R?
>
> ~A
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org 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.
>
	[[alternative HTML version deleted]]

b <- a[rmask,,drop=FALSE]

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu



On Fri, 4 Dec 2009, Austin Huang wrote:
> One problem I've been having is the special case in which only one
> row/column remains and the variable gets converted into a vector when
> entries are removed by logical masking. This is a problem because
subsequent
> code may rely on matrix operations (apply, colsums, dim, etc) For example:
>
>> a <- matrix(c(1, 2, 3, 4), nrow = 2)
>> a
>     [,1] [,2]
> [1,]    1    3
> [2,]    2    4
>> rmask <- c(TRUE, FALSE)
>> b <- a[rmask, ]
>> colSums(b)
> Error in colSums(b) : 'x' must be an array of at least two
dimensions
>
> To ensure the code works regardless of how the matrix gets modified, I need
> to explicitly recast results to a matrix, for example:
>
> b <- matrix(a[rmask, ], ncol = 2)
>
> This can get messy, requiring extra work to maintain column/row names or
> additional commands to determine the correct dimensions of the matrix (for
> example, if I'm masking both rows and columns simultaneously). Is there
a
> more elegant way to deal with this in R?
>
> ~A
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org 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.
>