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

[Duncan Murdoch]

On 6/8/2007 11:33 AM, Zack Weinberg wrote:
> 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...

Within the evaluation frame of h, x is a promise to evaluate an 
expression.  substitute(x) extracts the expression.  If you just use x, 
it gets evaluated in the frame from which h was called, rather than in a 
frame created from d.

Duncan Murdoch