[Rd] is.matrix

[Peter Dalgaard]

Daniel Høyer 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

)


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