[R] Correct way to test for exact dimensions of matrix or array

Gabor Grothendieck ggrothendieck at gmail.com
Tue Jan 10 20:47:57 CET 2006

```If its just succint you are after then this is slightly
shorter:

identical(dim(x)+0, c(3,5))

On 1/10/06, Gregory Jefferis <gsxej2 at cam.ac.uk> wrote:
> Thanks for suggestions.  This is a simple question in principle, but there
> seem to be some wrinkles - I am always having to think quite carefully about
> how to test for equality in R.  I should also have said that I would like
> the check to be efficient as well safe and succinct.
>
> One suggestion was:
>
>    isTRUE(all.equal(dim(obj), c(3, 5)))
>
> But that is not so efficient because all.equal does lots of work esp if it
> the objects are not equal.
>
> Another suggestion was:
>
>    all( dim( obj) == c(3,5) )
>
> But that is not safe eg because dim(vector(10)) is NULL and
> all(NULL==c(3,5)) is actually TRUE (to my initial surprise) so vectors would
> pass through the net.
>
> So, so far the only way that is efficient, safe and succinct is:
>
>    identical( dim( obj) , as.integer(c(3,5)))
>
> Martin Maechler pointed out that at the beginning of a function you might
> want to break down the test into something less succinct, that printed more
> specific error messages - a good suggestion for a top level function that is
> supposed to be user friendly.
>
> Any other suggestions?  Many thanks,
>
> Greg Jefferis.
>
> On 10/1/06 15:13, "Martin Maechler" <maechler at stat.math.ethz.ch> wrote:
>
> >>>>>> "Gregory" == Gregory Jefferis <gsxej2 at cam.ac.uk>
> >>>>>>     on Tue, 10 Jan 2006 14:47:43 +0000 writes:
> >
> >     Gregory> Dear R Users,
> >
> >      Gregory> I want to test the dimensions of an incoming
> >      Gregory> vector, matrix or array safely
> >
> >
> >     Gregory> and succinctly.  Specifically I want to check if
> >     Gregory> the unknown object has exactly 2 dimensions with a
> >     Gregory> specified number of rows and columns.
> >
> >     Gregory> I thought that the following would work:
> >
> >>> obj=matrix(1,nrow=3,ncol=5)
> >>> identical( dim( obj) , c(3,5) )
> >     Gregory> [1] FALSE
> >
> >     Gregory> But it doesn't because c(3,5) is numeric and the dims are
> > integer.  I
> >     Gregory> therefore ended up doing something like:
> >
> >>> identical( dim( obj) , as.integer(c(3,5)))
> >
> >     Gregory> OR
> >
> >>> isTRUE(all( dim( obj) == c(3,5) ))
> >
> > the last one is almost perfect if you leave a way the superfluous
> > isTRUE(..).
> >
> > But, you say that it's part of your function checking it's
> > arguments.
> > In that case, I'd recommend
> >
> >      if(length(d <- dim(obj)) != 2)
> >   stop("'d' must be matrix-like")
> >      if(!all(d == c(3,5)))
> >   stop("the matrix must be  3 x 5")
> >
> > which also provides for nice error messages in case of error.
> > A more concise form with less nice error messages is
> >
> >   stopifnot(length(d <- dim(obj)) == 2,
> >             d == c(3,50))
> >
> >   ## you can leave away  all(.)  for things in stopifnot(.)
> >
> >
> >
> >
> >     Gregory> Neither of which feel quite right.  Is there a 'correct' way to
> > do this?
> >
> >     Gregory> Many thanks,
> >
> > You're welcome,
> > Martin Maechler, ETH Zurich
> >
> >     Gregory> Greg Jefferis.
> >
> >     Gregory> PS Thinking about it, the second form is (doubly) wrong because:
> >
> >>> obj=array(1,dim=c(3,5,3,5))
> >>> isTRUE(all( dim( obj) == c(3,5) ))
> >     Gregory> [1] TRUE
> >
> >     Gregory> OR
> >>> obj=numeric(10)
> >>> isTRUE(all( dim( obj) == c(3,5) ))
> >     Gregory> [1] TRUE
> >
> >     Gregory> (neither of which are equalities that I am happy with!)
> >
>
> --
> Gregory Jefferis, PhD                               and:
> Research Fellow
> Department of Zoology                               St John's College
> University of Cambridge                             Cambridge
> Downing Street                                      CB2 1TP
> Cambridge, CB2 3EJ
> United Kingdom
>
> Tel: +44 (0)1223 336683                             +44 (0)1223 339899
> Fax: +44 (0)1223 336676                             +44 (0)1223 337720
>
> gsxej2 at cam.ac.uk
>
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