[R] single row/column-indexing on matrices

[Murat Tasan]

awesome!  thanks much!

On Jul 24, 6:41 pm, Duncan Murdoch <murd... at stats.uwo.ca> wrote:
> 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
>
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