[Rd] RFC: Proposal to make NROW() and NCOL() slightly more general
Martin Maechler
maechler at stat.math.ethz.ch
Tue Feb 7 17:32:47 CET 2012
>>>>> 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
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