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

[Gregory Jefferis]

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
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United Kingdom

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