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

[Phil Spector]

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
>
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