with vs. attach

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]]

see

        [[alternative HTML version deleted]]