R help - May 2005 - Using "|"

Dear all,

I have a rodent dataset that I am reanalyzing.  The set consists of 
several variables (#Microtus captured, Grass stems/m^2, etc), among 
which is a Grid factor variable c(North,South).  I have evidence that 
there are significant differences in vegetation and rodent populations 
between the two locations (from chi-square tests) and would like to run 
separate analyses along North/South lines on the rest of my data.  For 
example, I type:

 > plot(Microtus.T~Grass | Grid)

However, I get the following:

Error in plot.window(xlim, ylim, log, asp, ...) :
	need finite xlim values
In addition: Warning messages:
1: "|" not meaningful for factors in: Ops.factor(Grass, Grid)
2: no finite arguments to min; returning Inf
3: no finite arguments to max; returning -Inf

Other than adding xlim arguments, what else can I do, especially in 
light of warning message 1?

Thank you in advance.

Sincerely,
Jim Milks

Graduate Student
Environmental Sciences Ph.D. Program
Wright State University
3640 Colonel Glenn Hwy
Dayton, OH 45435

Jim Milks wrote on 5/3/2005 8:34 AM:
> Dear all,
> 
> I have a rodent dataset that I am reanalyzing.  The set consists of 
> several variables (#Microtus captured, Grass stems/m^2, etc), among 
> which is a Grid factor variable c(North,South).  I have evidence that 
> there are significant differences in vegetation and rodent populations 
> between the two locations (from chi-square tests) and would like to run 
> separate analyses along North/South lines on the rest of my data.  For 
> example, I type:
> 
>  > plot(Microtus.T~Grass | Grid)
> 
> However, I get the following:
> 
> Error in plot.window(xlim, ylim, log, asp, ...) :
>     need finite xlim values
> In addition: Warning messages:
> 1: "|" not meaningful for factors in: Ops.factor(Grass, Grid)
> 2: no finite arguments to min; returning Inf
> 3: no finite arguments to max; returning -Inf
> 
> Other than adding xlim arguments, what else can I do, especially in 
> light of warning message 1?
> 
> Thank you in advance.
> 
> Sincerely,
> Jim Milks

Are you confusing "plot.formula" with "lattice:xyplot"? Your
example
should work if you use:

library(lattice)
xyplot(Microtus.T ~ Grass | Grid)

HTH,

--sundar