[R] evaluating variables in the context of a data frame

[Zack Weinberg]

On 6/7/07, Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:
> >> f <- function(x, dat) evalq(x, dat)
> >> f(o, D)
> > Error in eval(expr, envir, enclos) : object "o" not found
> >> g <- function(x, dat) eval(x, dat)
> >> g(o, D)
> > Error in eval(x, dat) : object "o" not found
> >
> > What am I doing wrong?  This seems to be what the helpfiles say you do
> > to evaluate arguments in the context of a passed-in data frame...
>
> When you call f(o, D), the argument 'o' is evaluated in the current
> environment ('context' in R means something different).  Because of lazy
> evaluation, it is not evaluated until evalq is called, but it evaluated as
> if it was evaluated greedily.
>
> g(quote(o), D) will work.

Thanks.

After a bit more experimentation I figured out that this does what I want:

> h <- function(x, d) eval(substitute(x), d, parent.frame())

but I don't understand why the substitute() helps, or indeed why it
has any effect at all...

zw