[R] with vs. attach

Jeff Newmiller jdnewmil at dcn.davis.ca.us
Mon May 9 01:02:22 CEST 2016 

The lazyeval package addresses the problem of how to delay evaluation even when the function you want to do the evaluation in is buried two or more function calls below where the original call was made. If you are not building nested function calls with delayed evaluation then you probably don't need that package. 
-- 
Sent from my phone. Please excuse my brevity.

On May 8, 2016 3:30:16 PM PDT, Spencer Graves <spencer.graves at effectivedefense.org> wrote:
>Hi, Hadley et al.:
>
>
>       Hadley's link requires his development version of "lazyeval", 
>which can be obtained as follows:
>
>
>library(devtools)
>install_github("hadley/lazyeval")
>
>
>       Hadley's link describes real problems with elegant solutions.
>
>
>       However, David's solution solved my immediate problem, and it's 
>not immediately obvious to me how his "expr_text" function (or other 
>functions in "lazyevel") to produce a better solution.
>
>
>       Thanks again to David, Peter and Hadley for their replies.
>
>
>       Spencer Graves
>
>
>On 5/6/2016 5:08 PM, Hadley Wickham wrote:
>> 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.
>>
>>
>
>______________________________________________
>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.

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