[R] logical masking of a matrix converts it to a vector

[David Winsemius]

On Dec 4, 2009, at 4:03 PM, Jorge Ivan Velez wrote:

> 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

Doesn't work on a Mac (admittedly not the latest version, but it's  
only a month old.
 > b <- a[rmask, ]
 > colSums(b)
Error in colSums(b) : 'x' must be an array of at least two dimensions

 > sessionInfo()
R version 2.10.0 Patched (2009-10-29 r50258)
x86_64-apple-darwin9.8.0

locale:
[1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

loaded via a namespace (and not attached):
[1] tools_2.10.0


>
> 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 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.
>>
>
> 	[[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.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT