R: keeping a 1-d subset of a matrix a matrix

By default, if a subsetting operations yields a one-dimensional matrix (row or column vector), the dimensions are dropped.

> (m <- matrix(1:4, nrow=2))
     [,1] [,2]
[1,]    1    3
[2,]    2    4
> (v <- m[1, 1:2])
[1] 1 3

This can cause problems when we want to go ahead and continue using the result in matrix operations:
> v %*% m %*% t(v)
Error in v %*% m %*% t(v) : non-conformable arguments

The solution is to explicitly tell R not to drop the dimensions:

> (v <- m[1, 1:2, drop=FALSE])
     [,1] [,2]
[1,]    1    3
> v %*% m %*% t(v)
     [,1]
[1,]   52

Something else to look at: removing a column, given its name in a variable: Chris Handorf, subsetting a dataframe