[R] with vs. attach
Hadley Wickham
h.wickham at gmail.com
Sat May 7 00:08:54 CEST 2016
You may want to read http://rpubs.com/hadley/157957, which captures my
latest thinking (and tooling) around this problem. Feedback is much
appreciated.
Hadley
On Fri, May 6, 2016 at 2:14 PM, David Winsemius <dwinsemius at comcast.net> wrote:
>
>> 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
>>>>>
>>>>> [[alternative HTML version deleted]]
>>>>>
>>>>> ______________________________________________
>>>>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>>>> and provide commented, minimal, self-contained, reproducible code.
>>>> David Winsemius
>>>> Alameda, CA, USA
>>>>
>>>> ______________________________________________
>>>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>>> and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius
> Alameda, CA, USA
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
--
http://hadley.nz
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