[R] Subsetting a matrix

[Adelchi Azzalini]

On Monday 14 July 2003 11:59, you wrote:
> I'd welcome some comments or advice regarding the situation described
> below.
>
> The following illustrates what seems to me to be an inconsistency
> in the behaviour of matrix subsetting:
>
>   > Z<-matrix(c(1.1,2.1,3.1,1.2,2.2,3.2,1.3,2.3,3.3),nrow=3)
>   > Z
>        [,1] [,2] [,3]
>   [1,]  1.1  1.2  1.3
>   [2,]  2.1  2.2  2.3
>   [3,]  3.1  3.2  3.3
>   > dim(Z)
>   [1] 3 3
>
>   > Z0<-Z[c(T,F,F),c(F,T,T)]
>   > Z0
>   [1] 1.2 1.3
>   > dim(Z0)
>   NULL
>
> whereas, of course, with
>
>   > Z1<-Z[c(T,T,F),c(F,T,T)]
>   > Z1
>        [,1] [,2]
>   [1,]  1.2  1.3
>   [2,]  2.2  2.3
>   > dim(Z1)
>   [1] 2 2
>
> i.e. a fully-paid-up matrix.
>
> What I would have expected is that Z0 should come out as a 1x2 matrix:
>
>          [,1] [,2]
>   [1,]  1.2  1.3
>
> with dim(Z0) --> [1] 1 2

This seems to me yet another example of the side-effects caused by
the automatic conversion of matrix to "vector" (in Splus/R sense)
when the one of the dimension is 1.  There are many examples of this 
sort.

Of course the remedy is to do
    Z0 <- Z[c(T,F,F),c(F,T,T), drop=FALSE]

Personally, I find this automatic conversion to "vector" a somewhat confusing 
feature (although I can see its reasons),  and I know of many people that would 
have preferred  that drop=FALSE was the default  behaviour, but surely now 
is difficult to change it.

regards
Adelchi Azzalini 


-- 
Adelchi Azzalini  <azzalini at stat.unipd.it>
Dipart.Scienze Statistiche, Università di Padova, Italia
http://azzalini.stat.unipd.it/