R help - Sep 2009 - Evaluating expresssions as parameter values

I need to play games with an expression similar to the following one:

print(xyplot(DepVar ~ Group|Covar, groups=Othergroup,
	data=mydf, pch = 18 ,main="Testcase",auto.key = TRUE))


The problem is that  the formula argument (the first argument)
an the groups argument are passed over from another program as strings.

The formula argument does not pose problems,
I can do as.formula("DepVar ~ Group|Covar")
bit I could not find an equivalent technique for the groups parameter.
as.expression(... does not work and neither does
parse(text=...

Is there a way of accomplishing what I need?

Try this:


f <- function(fo, data, groups) {
	g <- do.call("xyplot", list(as.formula(fo), groups =
as.name(groups), data))
	print(g)
}

f("yield ~ variety | site", data = barley, groups = "year")


On Tue, Sep 22, 2009 at 3:03 PM, Erich Neuwirth
<erich.neuwirth at univie.ac.at> wrote:
> I need to play games with an expression similar to the following one:
>
> print(xyplot(DepVar ~ Group|Covar, groups=Othergroup,
> ? ? ? ?data=mydf, pch = 18 ,main="Testcase",auto.key = TRUE))
>
>
> The problem is that ?the formula argument (the first argument)
> an the groups argument are passed over from another program as strings.
>
> The formula argument does not pose problems,
> I can do as.formula("DepVar ~ Group|Covar")
> bit I could not find an equivalent technique for the groups parameter.
> as.expression(... does not work and neither does
> parse(text=...
>
> Is there a way of accomplishing what I need?
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Thanks, that completely solves the problem.

On Sep 22, 2009, at 10:27 PM, Gabor Grothendieck wrote:
> Try parse(text=...):
>
> f <- function(fo, data, groups) {
> 	g <- do.call("xyplot", list(as.formula(fo),
> 		groups = parse(text = groups), data))
> 	print(g)
> }
>
> f("yield ~ variety | site", data = barley, groups =
"year")
>
>
>
> On Tue, Sep 22, 2009 at 4:20 PM, Erich Neuwirth
> <erich.neuwirth at univie.ac.at> wrote:
>> Thank you, this works for my example.
>> On Sep 22, 2009, at 9:17 PM, Gabor Grothendieck wrote:
>>
>> f <- function(fo, data, groups) {
>> g <- do.call("xyplot", list(as.formula(fo), groups =
as.name
>> (groups), data))
>> print(g)
>> }
>>
>> But xyplot allows expressions for the groups parameter also,
>> and using as.expression instead of as.name in your example does not  
>> work.
>> What is happening?
>>
>> f <- function(fo, data, groups) {
>> g <- do.call("xyplot", list(as.formula(fo), groups =
as.expression
>> (groups),
>> data))
>> print(g)
>> }
>>
>> f("yield ~ variety ~ site",barley,"year")
>> produces a very nicely labeled but empty plot.
>>
>

On Tue, Sep 22, 2009 at 12:03 PM, Erich Neuwirth
<erich.neuwirth at univie.ac.at> wrote:
> I need to play games with an expression similar to the following one:
>
> print(xyplot(DepVar ~ Group|Covar, groups=Othergroup,
> ? ? ? ?data=mydf, pch = 18 ,main="Testcase",auto.key = TRUE))
>
>
> The problem is that ?the formula argument (the first argument)
> an the groups argument are passed over from another program as strings.
>
> The formula argument does not pose problems,
> I can do as.formula("DepVar ~ Group|Covar")
> bit I could not find an equivalent technique for the groups parameter.

'groups' can be a formula too, e.g.,

xyplot(Sepal.Length ~ Petal.Length, iris, groups = ~Species)

-Deepayan

Let us walk through my problem step by step:

1.
print(xyplot(yield ~ variety | site,groups=year,data=barley))
2.
print(xyplot(as.formula("yield ~ variety |  
site"),groups=year,data=barley))

The above two statements work, the next one does not work

3.
print(xyplot(as.formula("yield ~ variety | site"),groups=parse 
(text="year"),data=barley))

But the following one works

4.
arglist<-list(as.formula("yield ~ variety | site"),groups=parse 
(text="year"),data=barley)
print(do.call("xyplot",arglist))


why does 3 not work and 4 work?
Is there a way of coercing the string "year" into something which can
directly be used in
3 as the argument used with groups= ???


The next one

print(xyplot(as.formula("yield ~ variety | site"),groups=eval(parse 
(text="year")),data=barley))

also works, but in this case the groups expression is evaluated before  
xyplot is applied,
and I think this is bad style (at least, if not worse).


On Sep 30, 2009, at 1:20 PM, Deepayan Sarkar wrote:
> xyplot(Sepal.Length ~ Petal.Length, iris, groups = ~Species)