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

Martin Maechler maechler at stat.math.ethz.ch
Wed Jan 11 09:23:32 CET 2006

```>>>>> "Gabor" == Gabor Grothendieck <ggrothendieck at gmail.com>
>>>>>     on Tue, 10 Jan 2006 14:47:57 -0500 writes:

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

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

indeed, or, less succinct, but maybe more readable (and along the
"top-level function checks" I had proposed yesterday):

!is.null(d <- dim(x)) && all(d == c(3,5))

Gabor> 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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