R help - Nov 2006 - Matrix dimnames

R version : 2.4.0
O/S: Windows XP SP2 :-(

How does one assign names to the columns of a matrix so that the columns 
can be accesses using the '$' operator? It seem that x$c1 below should 
return column 1.

 > x = matrix(1:4,2)
 > dimnames(x) = list(c("r1", "r2"), c("c1",
"c2"))
 > x
    c1 c2
r1  1  3
r2  2  4
 > x$c1
NULL
 > x$c2
NULL

Daniel Gatti
UNC-CH

Daniel Gatti wrote:
> How does one assign names to the columns of a matrix so that the columns 
> can be accesses using the '$' operator? It seem that x$c1 below
should
> return column 1.
> 
>  > x = matrix(1:4,2)
>  > dimnames(x) = list(c("r1", "r2"),
c("c1", "c2"))
>  > x
>     c1 c2
> r1  1  3
> r2  2  4
>  > x$c1
> NULL
>  > x$c2
> NULL
> 
Is this useful?--
> x[,"c1"]
r1 r2 
 1  2 

I have only seen "$" used with lists.  A data frame is a kind
of list, but a matrix isn't.  
>From the R Reference Manual: "The form [of indexing] using $
applies to recursive objects such as lists and pairlists."

HTH

Mike Prager
Southeast Fisheries Science Center, NOAA
Beaufort, North Carolina  USA