R help - Jul 2009 - single row/column-indexing on matrices

hi all - quick question:

i have a matrix m, say nrow=5, ncol=4.
in a function i'd like to retrieve certain rows or columns from m, but
which rows/cols are not known ahead of time.
the function should return a sub-matrix (i.e. still of class
'matrix').

when selecting a single column (or row), the indexing operation
converts the object of class 'matrix' into a vector of it's mode.

e.g.:

myfun <- function(m, cols) m[,cols];
m <- matrix(1:20, nrow=5);
sm <- myfun(m, c(2));

the last line returns a vector, but i'd like to still get back a
matrix (of dimensions 5,1).  most of the time i'll be selecting groups
of rows or cols, so this isn't a problem (as the return value remains
a matrix), but on rare occasions a single selecting index will be
passed, causing a problem for code further down that expects the 'sm'
to be in matrix form (e.g. using the 'apply' function).

is there any way to preserve the class of the matrix object when
performing indexing?

On 24/07/2009 6:34 PM, Murat Tasan wrote:
> hi all - quick question:
> 
> i have a matrix m, say nrow=5, ncol=4.
> in a function i'd like to retrieve certain rows or columns from m, but
> which rows/cols are not known ahead of time.
> the function should return a sub-matrix (i.e. still of class
> 'matrix').
> 
> when selecting a single column (or row), the indexing operation
> converts the object of class 'matrix' into a vector of it's
mode.
> 
> e.g.:
> 
> myfun <- function(m, cols) m[,cols];
> m <- matrix(1:20, nrow=5);
> sm <- myfun(m, c(2));
> 
> the last line returns a vector, but i'd like to still get back a
> matrix (of dimensions 5,1).  most of the time i'll be selecting groups
> of rows or cols, so this isn't a problem (as the return value remains
> a matrix), but on rare occasions a single selecting index will be
> passed, causing a problem for code further down that expects the
'sm'
> to be in matrix form (e.g. using the 'apply' function).
> 
> is there any way to preserve the class of the matrix object when
> performing indexing?
Yes, use "drop=FALSE" when doing the indexing:

 > m <- matrix(1:10, 2,5)
 > m[1,]
[1] 1 3 5 7 9
 > m[1,,drop=FALSE]
      [,1] [,2] [,3] [,4] [,5]
[1,]    1    3    5    7    9

Duncan Murdoch