with vs. attach

On May 5, 2016, at 5:12 PM, Spencer Graves <spencer.graves at effectivedefense.org> wrote:

I want a function to evaluate one argument
in the environment of a data.frame supplied
as another argument.  "attach" works for
this, but "with" does not.  Is there a way
to make "with" work?  I'd rather not attach
the data.frame.


With the following two functions "eval.w.attach"
works but "eval.w.with" fails:


dat <- data.frame(a=1:2)
eval.w.attach <- function(x, dat){
  attach(dat)
  X <- x
  detach()
  X
}

eval.w.with <- function(x, dat){
  with(dat, x)
}

eval.w.attach(a/2, dat) # returns c(.5, 1)