R devel - Nov 2008 - is.matrix

a=c(1,1,2);
is.matrix(a) gives FALSE
is.matrix(t(a)) gives TRUE
is.matrix(t(t(a))) gives TRUE

Is this correct? Shouldn't all give FALSE?
I think is.matrix should give FALSE when dimension is 1*n or n*1.

Daniel H?yer Iversen wrote:
> a=c(1,1,2);
> is.matrix(a) gives FALSE
> is.matrix(t(a)) gives TRUE
> is.matrix(t(t(a))) gives TRUE
> 
> Is this correct? Shouldn't all give FALSE?
> I think is.matrix should give FALSE when dimension is 1*n or n*1.
No this is correct. is.matrix() returns TRUE if and only if the argument
has a two-dimensional 'dim' attribute, and

 > dim(a)
NULL
> dim(t(a))
[1] 1 3
> dim(t(t(a)))
[1] 3 1

(And is.array() depends on having a 'dim' attribute of positive length,
so
> dim(a) <- 3
> is.matrix(a)
[1] FALSE
> is.array(a)
[1] TRUE

)


-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907

Daniel H?yer Iversen wrote:
> a=c(1,1,2);
> is.matrix(a) gives FALSE
> is.matrix(t(a)) gives TRUE
> is.matrix(t(t(a))) gives TRUE
>
> Is this correct? Shouldn't all give FALSE?
> I think is.matrix should give FALSE when dimension is 1*n or n*1.
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>   
All of the above is consistent with the documentation for is.matrix():

|  is.matrix| returns |TRUE| if |x| is a matrix and has a |dim 
<dim.html>| attribute of length 2) and |FALSE| otherwise

[There seems to be a typo in this sentence from ?is.matrix : an 
unmatched ")"]

This is also useful behavior -- when programming it is often useful to 
know whether something is a matrix or not because that can affect 
computations performed with the object.

For the more informal definition of "matrix" that it looks like want, 
you could use
 is.matrix(x) && all(dim(x)>1)
(or maybe all(dim(x) != 1) depending on how you want to treat matrices 
that have a dimension with zero extent)

-- Tony Plate