R help - May 2007 - Passing variable names as function arguments for lme model specification

Hello,

I have what I hope is a relatively straightforward question.

I'm trying to write a function that runs lme using variables specified  
in the function argument, along with some other tasks.

However, I'm having some problems with lme accessing the variables I'm  
interested in.

Here's a simplified version of my test code:

testfunction<-function(var1, var2)
{
lme(var1 ~ var2,random=~1,data=testdata)
#other stuff...
}


If I run testfunction(var_a, var_b), I get the following error:
Error in model.frame(formula, rownames, variables, varnames, extras,  
extranames,  :
         invalid variable type

I tested the same procedure with lm rather than lme, and it seems to  
be an acceptable way of passing arguments with this procedure.

This works:
testfunction<-function(var1, var2)
{
lm(var1 ~ var2,data=testdata)
#other stuff...
}

Any help on this matter would be much appreciated,

Jane Lange
Department of Biostatistics, UCLA