[Rd] Problem using model.frame with argument subset in own function

[Greg B. Hill]

Gavin,

I ran into the same cryptic "invalid subscript type 'closure'" message in
a slightly less complicated scenario, and wanted to post the cause in 
my case (the root cause is probably the same either way).

Similarly to your case, I was subsetting a data frame. I had a list
of variable names corresponding to columns in the frame. 
Unfortunately the variable name I had assigned to this list, var, 
coincided with the name of a base package function in R for variance.

When I attempted to subset df[, var], I got the 'closure' error message,
but if I renamed the list of variable names so the collision didn't occur,
e.g. df[, vars] instead of df[, var], it worked as expected.

Sincerely, 
Greg B. Hill


Gavin Simpson wrote:
> 
> Dear List,
> 
> I am writing a formula method for a function in a package I maintain. I
> want the method to return a data.frame that potentially only contains
> some of the variables in 'data', as specified by the formula.
> 
> The problem I am having is in writing the function and wrapping it
> around model.frame. Consider the following data frame:
> 
> dat <- data.frame(A = runif(10), B = runif(10), C = runif(10))
> 
> And the wrapper function:
> 
> foo <- function(formula, data = NULL, ..., subset = NULL,
>                 na.action = na.pass) {
>     mt <- terms(formula, data = data, simplify = TRUE)
>     mf <- model.frame(formula(mt), data = data, subset = subset,
>                       na.action = na.action)
>     ## real function would do more stuff here and pass mf on to
>     ## other functions
>     mf
> }
> 
> This is how I envisage the function being called. The real world use
> would have a data.frame with tens or hundreds of components where only a
> few need to be excluded. Hence wanting formulas of the form below to
> work.
> 
> foo(~ . - B, data = dat)
> 
> The aim is to return only columns A and C in an object returned by
> model.frame. However, when I run the above, I get the following error:
> 
>> foo(~ A + B, data = dat)
> Error in xj[i] : invalid subscript type 'closure'
> 
> I've tracked this down to the line in model.frame.default
> 
>     subset <- eval(substitute(subset), data, env)
> 
> After evaluating this line, subset contains:
> 
> Browse[1]> subset
> function (x, ...) 
> UseMethod("subset")
> <environment: namespace:base>
> 
> Not NULL, and hence the error later on when calling the internal
> model.frame code.
> 
> So the question is, what am I doing wrong?
> 
> If I leave the subset argument out of the definition of foo and rely
> upon the default in model.frame.default, the function works as
> expected. 
> 
> Perhaps the question should be, how do I modify foo() to allow it to
> have a formal subset argument, passed to model.frame?
> 
> Any other suggestions gratefully accepted.
> 
> Thanks in advance,
> 
> G
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