[Rd] RFC: Proposal to make NROW() and NCOL() slightly more general

[Martin Maechler]

>>>>> Martin Maechler <maechler at stat.math.ethz.ch>
>>>>>     on Mon, 6 Feb 2012 15:35:36 +0100 writes:

    >> On Sat, Feb 4, 2012 at 10:38 AM, Martin Maechler
    >> <maechler at stat.math.ethz.ch> wrote:
    >> > The help has
    >> >
    >> >> Description:
    >> >
    >> >>   'nrow' and 'ncol' return the number of rows or columns present in 'x'.
    >> >>   'NCOL' and 'NROW' do the same treating a vector as 1-column matrix.
    >> >
    >> > and
    >> >
    >> >>   x: a vector, array or data frame
    >> >
    >> > I'm proposing to extend these two convenience functions
    >> > to also work ``correctly'' for generalized versions of matrices.
    >> >
    >> >
    >> > The current implementation :
    >> >
    >> > NROW <- function(x) if(is.array(x)||is.data.frame(x)) nrow(x) else length(x)
    >> > NCOL <- function(x) if(is.array(x) && length(dim(x)) > 1L || is.data.frame(x)) ncol(x) else 1L
    >> >
    >> > only treats something as matrix when  is.array(.) is true,
    >> > which is not the case, e.g., for multiprecision matrices from
    >> > package 'gmp' or for matrices from packages SparseM, Matrix or similar.
    >> >
    >> > Of course, all these packages could write methods for NROW, NCOL
    >> > for their specific matrix class, but given that the current
    >> > definition is so simple,
    >> > I'd find it an unnecessary complication.
    >> >
    >> > Rather I propose the following new version
    >> >
    >> > NROW <- function(x) if(length(dim(x)) || is.data.frame(x)) nrow(x) else length(x)
    >> > NCOL <- function(x) if(length(dim(x)) > 1L || is.data.frame(x)) ncol(x) else 1L

    >> That makes me wonder about:

    >> DIM <- function(x) if (length(dim(x)) > 1L) dim(x) else c(length(x), 1L)

    >> or maybe more efficiently:

    >> DIM <- function(x) {
    >> d <- dim(x)
    >> if (length(d) > 1L) dim(x) else c(length(x), 1L)
    >> }

    >> given that dim() is not always trivial to compute (e.g. for data
    >> frames it can be rather slow if you're doing it for hundreds of data
    >> frames)

    >> then NROW and NCOL could be exact equivalents to nrow and ncol.

    >> Hadley

    > Thank you, Hadley.
    > Indeed, your suggestion seems to make sense
    > {as far as it makes sense to have such simple functions to
    > exist in base at all, but as we already have NROW and NCOL ..}

    > So, I propose to adopt Hadley's  DIM() proposal, modified to

    > DIM <- function(x) if(length(d <- dim(x))) d else c(length(x), 1L)

    > and wait a day or so (or longer for reasons of vacation!) before
    > committing it, so the public can raise opinions.

Actually, the above --- building NROW() and NCOL() ond DIM() is
not quite correct:
 
  NCOL <- function(x) DIM(x)[2L]

will fail for   x <- array(1:3, 3)

so I think I'll stick for now with the generalizations to NROW()
and NCOL(), using

NROW <- function(x) if(length(d <- dim(x)))      d[1L] else length(x)
NCOL <- function(x) if(length(d <- dim(x)) > 1L) d[2L] else 1L

which incorporates Hadley's note that there are case where
dim(.) is ``relatively expensive''.
Note that the above are also (very slightly) more efficient than
basing them on DIM(.).

Martin