with vs. attach

On May 6, 2016, at 5:47 AM, Spencer Graves <spencer.graves at effectivedefense.org> wrote:



On 5/6/2016 6:46 AM, peter dalgaard wrote:

On 06 May 2016, at 02:43 , David Winsemius <dwinsemius at comcast.net> wrote:

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)

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

I liked eval.expression and tested it with a bquote(...) argument to see if that would succeed. It did, but it didn't return what you wanted for `a/2`, so I tried seeing if a "double eval wuold deliver both yours and my desired results:

 eval.w.sub <- function(a, Dat){
  eval( eval(substitute(a),Dat), env=Dat)
 }
x=2
 eval.w.sub( a/2, dat)
[1] 0.5 1.0
 eval.w.sub( bquote(2*a*.(x) ), dat)
[1] 4 8

We are here retracing the path the Hadley took in some of his ggplot2 design decsions. Unfortunately for me those NSE rules often left me confused about what should and shouldn't be 'quoted' in the as-character sense and what should be quote()-ed or "unquoted" in the bquote() sense.
--

     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

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David Winsemius
Alameda, CA, USA

David Winsemius
Alameda, CA, USA