On 2012-06-30 09:04, David Winsemius wrote:
On Jun 30, 2012, at 9:35 AM, mlell08 wrote:
Dear List,
I've created a two-dimensional array which shall contain a value and
its
error, respectively.
These two values are concatenated in al list and bear the names "sl"
and
"sl_err"
But I can't adress them using the $-notation.
a<- array(list(NULL),dim=c(2,2))
a[[1,1]]<- c(a=2,b=3)
a[[1,1]]$a
## Fehler in a[[1, 1]]$a : $ operator is invalid for atomic vectors
a[[1,1]]["a"] # This works however.
## a
## 2
I always thought these two methods of indexing are equal?
You thought wrong (on two accounts as it happens). The "$" methods
translate to "[[" with a quoted argument and there is no matrix/array
equivalent since vectors loose their names (if they had any to begin
with) when they are put into a matrix or array. The equivalent method
to x$a is x[["a"]], not x["a"].
Actually, to be picky (and David knows this), quoting the
help page (and the OP should have read this):
" x$name is equivalent to x[["name", exact = FALSE]] ".
In my view, there is far too much eagerness to use "$" rather
than getting accustomed to the other, more powerful, extraction
methods.
Peter Ehlers
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