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

From: Martin Maechler <maechler_at_stat.math.ethz.ch>
Date: Wed 11 Jan 2006 - 02:13:30 EST

>>>>> "Gregory" == Gregory Jefferis <gsxej2@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!)



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