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

From: Martin Maechler <maechler_at_stat.math.ethz.ch>
Date: Tue, 07 Feb 2012 17:32:47 +0100

>>>>> 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



R-devel_at_r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel Received on Tue 07 Feb 2012 - 16:42:49 GMT

This quarter's messages: by month, or sorted: [ by date ] [ by thread ] [ by subject ] [ by author ]

All messages

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.2.0, at Thu 09 Feb 2012 - 01:20:15 GMT.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-devel. Please read the posting guide before posting to the list.

list of date sections of archive