如何索引R矩阵而不将其恢复为向量

Question

I declared a 1 by 6 matrix A by saying:

A <- matrix(1:6, nrow=1)

I then do dim(A) and as expected I get 1 by 6...but then I do A[,2:5] and I would expect that to be a matrix of dimension 1 by 4 with entries 2,3,4,5...but instead dim( A[,2:5] ) gives me NULL! it degraded into a vector or something. How can I avoid this?

I am ultimately trying to do something like:

A[,a:b] %*% X[a:b,a:b] %*% t(A[,a:b])

varying a and b so I can multiply only parts of the above matrices together..but this breaks when A decays into a vector...

Thanks

Answer 1

Use ,drop=FALSE as an additional (trailing) argument to involving ] .

Example:

R> M <- matrix(1:4,2,2)
R> M[,2]                  ## looses matrix class
[1] 3 4
R> M[,2,drop=FALSE]       ## forced to a n x 1 matrix
     [,1]
[1,]    3
[2,]    4
R> 

This may well be the main FAQ, but for compatibility reasons the behaviour is unlikely to change.