[R] use of expand.model.frame

[John Fox]

Dear R-help list members,

I'm encountering problems using expand.model.frame. To illustrate, consider 
the function

     > test <- function(model){
     +     expand.model.frame(model, "income")
     +     }
     >

The data frame Prestige (from the car library) has several variables, 
including prestige, income and education. I've attached this data frame

     > search()
     [1] ".GlobalEnv"      "Prestige"        "package:lattice" 
"package:grid"    "package:car"     "package:ctest"
     [7] "Autoloads"       "package:base"
     >

and fit the following model

     > mod.1 <- lm(prestige ~ (log(income) + education)*type)
     >

 From the documentation for expand.model.frame, the default environment in 
which "things" are evaluated is environment(formula(model)), where model is 
the first argument to expand.model frame; in this case

     > environment(formula(mod.1))
     <environment: R_GlobalEnv>
     >

When I call the test function with mod.1, however, I get the following error:

     > test(mod.1)
     Error in eval(expr, envir, enclos) : Object "prestige" not found
     >

But if I fit the same model specifying the data argument to lm as Prestige, 
then test works,

     > mod.2 <- lm(prestige ~ (log(income) + education)*type, data=Prestige)
     > test(mod.2)
                             prestige log(income) education type income
     GOV.ADMINISTRATORS            68.8    9.421492     13.11 prof  12351
     GENERAL.MANAGERS              69.1   10.161187     12.26 prof  25879
     ACCOUNTANTS                   63.4    9.134647     12.77 prof   9271
     . . .
     TYPESETTERS                   42.2    8.773694     10.00   bc   6462
     BOOKBINDERS                   35.2    8.193400      8.55   bc   3617
     >

even though

     > environment(formula(mod.2))
     <environment: R_GlobalEnv>
     >

that is, still the global environment.

Obviously, I'm not thinking correctly about the scoping issue.

What I need is a version of test that works in both cases -- whether or not 
the data argument is specified to lm. (Of course, I need to use the 
expanded model frame as part of a more complicated function.)

I'm running R version 1.4.1 under Windows 2000.

Any help would be greatly appreciated.

John
-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

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