On 5/6/2016 6:46 AM, peter dalgaard wrote:
On 06 May 2016, at 02:43 , David Winsemius <dwinsem...@comcast.net> wrote:
On May 5, 2016, at 5:12 PM, Spencer Graves
<spencer.gra...@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)
How about using eval( substitute( ...))?
eval.w.sub <- function(expr, datt){
eval( substitute(expr), env=datt)
}
eval.w.sub(a/2, dat)
#[1] 0.5 1.0
Actually, I think a better overall strategy is to say that if you want to pass
an expression to a function, then pass an expression object (or a call object
or maybe a formula object).
Once you figure out _how_ your eval.w.attach works (sort of), you'll get the
creeps:
Lazy evaluation causes the argument x to be evaluated after the attach(), hence
the evaluation environment of an actual argument is being temporarily modified
from inside a function.
Apart from upsetting computer science purists, there could be hidden problems: One major
issue is that values in "dat" could be masked by values in the global
environment, another issue is that an error in evaluating the expression will leave dat
attached. So at a minimum, you need to recode using on.exit() magic.
So my preferences go along these lines:
dat <- data.frame(a=1:2)
eval.expression <- function(e, dat) eval(e, dat)
eval.expression(quote(a/2), dat)
[1] 0.5 1.0
eval.expression(expression(a/2), dat)
[1] 0.5 1.0
eval.formula <- function(f, dat) eval(f[[2]], dat)
eval.formula(~a/2, dat)
[1] 0.5 1.0
Hi, Peter:
I don't like eval.expression or eval.formula, because they don't
automatically accept what I naively thought should work and require more
knowledge of the user. What about David's eval.w.sub:
a <- pi
dat <- data.frame(a=1:2)
eval.w.sub <- function(a, Dat){
eval( substitute(a), env=Dat)
}
> eval.w.sub(a/2, dat)
[1] 0.5 1.0
This produces what's desired in a way that seems simpler to me.
By the way, I really appreciate Peter's insightful comments:
eval.w.attachOops <- function(x, Dat){
attach(Dat)
X <- x
detach()
X
}
> eval.w.attachOops(a/2, dat)
The following object is masked _by_ .GlobalEnv:
a
[1] 1.570796
> eval.w.attachOops(b/2, dat)
The following object is masked _by_ .GlobalEnv:
a
Error in eval.w.attachOops(b/2, dat) : object 'b' not found
> search()
[1] ".GlobalEnv" "Dat" "package:graphics"
[4] "package:grDevices" "package:utils" "package:datasets"
[7] "package:methods" "Autoloads" "package:base"
> objects(2)
[1] "a"
*** NOTES:
1. This gives a likely wrong answer with a warning if "a" exists
in .GlobalEnv, and leaves "Dat" (NOT "dat") attached upon exit.
2. A stray "detach()" [not shown here] detached
"package:stats". oops.
*** Using "on.exit" fixes the problem with failure to detach but not the
likely wrong answer:
detach()
search()
eval.w.attachStillWrong <- function(x, dat){
attach(dat)
on.exit(detach(dat))
X <- x
X
}
The following object is masked _by_ .GlobalEnv:
a
[1] 1.570796
> eval.w.attachStillWrong(b/2, dat)
The following object is masked _by_ .GlobalEnv:
a
Error in eval.w.attachStillWrong(b/2, dat) : object 'b' not found
> search()
[1] ".GlobalEnv" "package:grDevices" "package:utils"
[4] "package:datasets" "package:methods" "Autoloads"
[7] "package:base"
Thanks again to Peter and David. Spencer
Peter D.
--
David.
eval.w.with(a/2, dat) # Error ... 'a' not found
Thanks, Spencer Graves
[[alternative HTML version deleted]]
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David Winsemius
Alameda, CA, USA
______________________________________________
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______________________________________________
R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
