R help - Dec 2008 - call lattice function in a function passing "groups" argument

I'm trying to use a lattice function within a function and have problems 
passing the "groups" argument properly. Let's say I have a data
frame

d <- data.frame(x = rnorm(100), y = c("a", "b"))

and want to plot variable x in a densityplot, grouped by the variable y, then 
I would do something like

densityplot(~ x, d, groups = y)

If however I wanted to call the function "densityplot" within a
function and
pass the "groups" argument as an argument of that function, how would
I have
to proceed? It is not as straightforward as

f <- function(data, groupvar) {
  densityplot(~ x, data, groups = groupvar)
}

probably because the lattice function "densityplot.formula"
preprocesses
the "groups" argument with

groups <- eval(substitute(groups), data, environment(formula))

It there a way how I could pass the "groups" argument in the function
"f"?

Thanks for any hints,
Thomas Zumbrunn

> 
> If however I wanted to call the function "densityplot" within a
function and
> pass the "groups" argument as an argument of that function, how
would I have
> to proceed? It is not as straightforward as
> 
> f <- function(data, groupvar) {
>   densityplot(~ x, data, groups = groupvar)
> }
> 
> probably because the lattice function "densityplot.formula"
preprocesses
> the "groups" argument with
> 
> groups <- eval(substitute(groups), data, environment(formula))
> 
> It there a way how I could pass the "groups" argument in the
function "f"?
Here's one approach.  Pass the 'groups' variable as a character,
then find
that variable in the data frame and rename it.

d <- data.frame(x = rnorm(100), y = c("a", "b"))

f <- function(data, groupvar) {
	names(data)[which(names(data) == groupvar)] <- "gp"
  	densityplot(~ x, data, groups = gp)
}

f(d, groupvar="y")

On Thu, Dec 11, 2008 at 2:55 AM, Thomas Zumbrunn <t.zumbrunn at unibas.ch>
wrote:
> I'm trying to use a lattice function within a function and have
problems
> passing the "groups" argument properly. Let's say I have a
data frame
>
> d <- data.frame(x = rnorm(100), y = c("a", "b"))
>
> and want to plot variable x in a densityplot, grouped by the variable y,
then
> I would do something like
>
> densityplot(~ x, d, groups = y)
>
> If however I wanted to call the function "densityplot" within a
function and
> pass the "groups" argument as an argument of that function, how
would I have
> to proceed? It is not as straightforward as
>
> f <- function(data, groupvar) {
>  densityplot(~ x, data, groups = groupvar)
> }
>
> probably because the lattice function "densityplot.formula"
preprocesses
> the "groups" argument with
Yes, that's the price of non-standard evaluation.
> groups <- eval(substitute(groups), data, environment(formula))
>
> It there a way how I could pass the "groups" argument in the
function "f"?
The obvious solution is to evaluate 'groupvar' yourself:

f <- function(data, groupvar) {
  groupvar <- eval(substitute(groupvar), data, parent.frame())
  densityplot(~ x, data, groups = groupvar)
}

A more general solution (where 'groupvar' may be missing) is to use
match.call() etc. (e.g., see lattice:::dotplot.formula)

-Deepayan