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

[Erik Iverson]

> 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]