is.matrix

-----Original Message-----
From: r-devel-bounces at r-project.org 
[mailto:r-devel-bounces at r-project.org] On Behalf Of Peter Dalgaard
Sent: Tuesday, November 11, 2008 1:31 PM
To: Daniel H?yer Iversen
Cc: r-devel at r-project.org
Subject: Re: [Rd] is.matrix

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

)


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