Accessor functions in lattice graphics

Hi all:

Thanks, David. This is a good example of knowledgeable "R forensic
investigation." I leave it to Frank whether it meets his criteria.

However, I would argue that that this is bad practice and quite
unwise. In general, these details are implementation dependent and
could change. Yes, they are exposed, but only because everything is in
R. The more desirable and safer way to do things would be to use
accessor functions. I believe Frank's question was to ask whether such
accessor functions exist. If they do not, then of course one is stuck
with writing one based on details like thoseyou have elucidated. But
this is really not good programming practice, imo.

Contrary and more informed views welcome.

Cheers,
Bert

On Mon, Jun 20, 2011 at 7:17 AM, David Winsemius
<dwinsemius at comcast.net> wrote:

Frank Harrell wrote:

I know about the current.row, current.column, and panel.number functions
that are useful within panel functions written for lattice. ?Are there
easy ways to obtain the names of the conditioning variables (those
appearing after |) and their values for the current panel?
Thanks
Frank

Using one of the examples in help(xyplot):

states <- data.frame(state.x77,
? ? ? ? ? ? ? ? ? ? state.name = dimnames(state.x77)[[1]],
? ? ? ? ? ? ? ? ? ? state.region = state.region)
testp <- xyplot(Murder ~ Population | state.region, data = states,
? ? ? groups = state.name,
? ? ? panel = function(x, y, subscripts, groups) {
? ? ? ? ? ltext(x = x, y = y, labels = groups[subscripts], cex=1,
? ? ? ? ? ? ? ? fontfamily = "HersheySans")
? ? ? })
str(testp)

I see that the names of the state.regions are in an attribute of
testp$packet.sizes, so:

attr(testp$packet.sizes, "dimnames")[["state.region"]][4]

#[1] "West"

#Looking for a further example in help(xyplot with 2 "panel dimensions" I
tried:

testp2 <- dotplot(variety ~ yield | year * site, data=barley)

#I'm not quite sure how the "rows" and "columns" are defined , but I get:

attr(testp2$packet.sizes, "dimnames")[[1]][2]

[1] "1931"

attr(testp2$packet.sizes, "dimnames")[[2]][1]

[1] "Grand Rapids"

I'm not sure this is the "right answer" because after looking further, I
now
see a list node entitled $condlevels which returns the same information
is a
much more straightforward fashion:

testp2 $ condlevels

$year
[1] "1932" "1931"

$site
[1] "Grand Rapids" ? ?"Duluth" ? ? ? ? ?"University Farm" "Morris"
[5] "Crookston" ? ? ? "Waseca"

(And other nodes that contain information about conditioning levels:
$ index.cond ? ? ? :List of 2
?..$ : int [1:2] 1 2
?..$ : int [1:6] 1 2 3 4 5 6
?$ perm.cond ? ? ? ?: int [1:2] 1 2
?$ condlevels ? ? ? :List of 2
?..$ year: chr [1:2] "1932" "1931"
?..$ site: chr [1:6] "Grand Rapids" "Duluth" "University Farm" "Morris"
...

HTH;

David.

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-- 
"Men by nature long to get on to the ultimate truths, and will often
be impatient with elementary studies or fight shy of them. If it were
possible to reach the ultimate truths without the elementary studies
usually prefixed to them, these would not be preparatory studies but
superfluous diversions."

-- Maimonides (1135-1204)

Bert Gunter
Genentech Nonclinical Biostatistics