R help - Jan 2006 - Correct way to test for exact dimensions of matrix or array

Dear R Users,

I want to test the dimensions of an incoming vector, matrix or array safely
and succinctly.  Specifically I want to check if the unknown object has
exactly 2 dimensions with a specified number of rows and columns.

I thought that the following would work:
> obj=matrix(1,nrow=3,ncol=5)
> identical( dim( obj) , c(3,5) )
[1] FALSE  

But it doesn't because c(3,5) is numeric and the dims are integer.  I
therefore ended up doing something like:
> identical( dim( obj) , as.integer(c(3,5)))
OR
> isTRUE(all( dim( obj) == c(3,5) ))
Neither of which feel quite right.  Is there a 'correct' way to do this?

Many thanks,

Greg Jefferis.

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) ))
[1] TRUE

OR 
> obj=numeric(10)
> isTRUE(all( dim( obj) == c(3,5) ))
[1] TRUE

(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

you could use: isTRUE(all.equal(dim(obj), c(3, 5)))


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://www.med.kuleuven.be/biostat/
     http://www.student.kuleuven.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Gregory Jefferis" <gsxej2 at cam.ac.uk>
To: <r-help at stat.math.ethz.ch>
Sent: Tuesday, January 10, 2006 3:47 PM
Subject: [R] Correct way to test for exact dimensions of matrix or 
array

> Dear R Users,
>
> I want to test the dimensions of an incoming vector, matrix or array 
> safely
> and succinctly.  Specifically I want to check if the unknown object 
> has
> exactly 2 dimensions with a specified number of rows and columns.
>
> I thought that the following would work:
>
>> obj=matrix(1,nrow=3,ncol=5)
>> identical( dim( obj) , c(3,5) )
> [1] FALSE
>
> But it doesn't because c(3,5) is numeric and the dims are integer. 
> I
> therefore ended up doing something like:
>
>> identical( dim( obj) , as.integer(c(3,5)))
>
> OR
>
>> isTRUE(all( dim( obj) == c(3,5) ))
>
> Neither of which feel quite right.  Is there a 'correct' way to do 
> this?
>
> Many thanks,
>
> Greg Jefferis.
>
> 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) ))
> [1] TRUE
>
> OR
>> obj=numeric(10)
>> isTRUE(all( dim( obj) == c(3,5) ))
> [1] TRUE
>
> (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
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 

Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm

>>>>> "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!)