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

[David Winsemius]

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