> In a nutshell, formulas carry the environment in which they are defined along
> with the variable names, and your dfr was defined in the test.FN environment,
> but the formulas were defined in the global environment. I got this to work by
> defining the formula character strings in the global environment, and then
> converting those strings to formulas in the function. I don't think you can
> trick
> lm into referring to the global environment from within test.FN so that
> summaries refer to the X.des data frame instead of dfr (but someone could
> prove me wrong).
If you want a function to refer to something in the global environment, just
refer to the global object in the function. If the object name isn't used in
the function's scope, it is sought in the parent environment. So the original
code works if
test.FN <- function(scope, k=2){
temp.lm=lm(scope$lower, data=X.des) ## X.des is sought in parent
environment
step(temp.lm, scope=scope$upper, k=k)
}
Admittedly, I'd not regard that kind of thing as a good idea; fragile and
inflexible. But if you are clear about scope it does work.
Another way to proceed, somewhat more safely, is to wrap the whole thing in a
function, passing X.des as dfr, then defining test.FN inside the outer function
so that you know where it's going to get dfr from. Something along the lines of
test.step <- function(dfr, Y) {
test.FN <- function(scope, k=2){
temp.lm=lm(scope$lower, data=dfr) ## X.des
print(temp.lm)
step(temp.lm, scope=as.formula(scope$upper), k=k)
}
scope <- list( lower= as.formula('Y~1'),
upper=as.formula(paste('Y~', paste(names(dfr[1:20]),
collapse="+")))
)
test.FN(scope=scope)
}
test.step(X.des, Y)
S Ellison
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