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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Dr. Gavin Simpson [t] +44 (0)20 7679 0522
ECRC, UCL Geography, [f] +44 (0)20 7679 0565
Pearson Building, [e] gavin.simpsonATNOSPAMucl.ac.uk
Gower Street, London [w] http://www.ucl.ac.uk/~ucfagls/
UK. WC1E 6BT. [w] http://www.freshwaters.org.uk
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