[R] deciphering help for `attach`

Fox, John
Mon, 27 Apr 2020 14:57:08 -0000

Dear Edward,

Paragraph 4 in the help page goes on to say, "Rather, a new environment is
created on the search path and the elements of a list (including columns of a
data frame) or objects in a save file or an environment are copied into the new
environment."

That seems reasonably clear to me. Here's an example that also illustrates
how attach can lead to confusion:

------- snip --------

> str(cars)
'data.frame':	50 obs. of  2 variables:
 $ speed: num  4 4 7 7 8 9 10 10 10 11 ...
 $ dist : num  2 10 4 22 16 10 18 26 34 17 ...

> attach(cars)

> search()
 [1] ".GlobalEnv"        "cars"
"tools:rstudio"     "package:stats"
 [5] "package:graphics"  "package:grDevices"
"package:utils"     "package:datasets"
 [9] "package:methods"   "Autoloads"
"package:base"

> objects()
character(0)

> objects(pos=2)
[1] "dist"  "speed"

> str(get("dist", pos=2))
 num [1:50] 2 10 4 22 16 10 18 26 34 17 ...

> dist <- 1:10

> head(dist) # shadows dist in copy of cars
[1] 1 2 3 4 5 6

> head(get("dist", pos=2))
[1]  2 10  4 22 16 10

> assign("dist", 10:1, pos=2) # changes dist in objects copied from
cars

> head(get("dist", pos=2))
[1] 10  9  8  7  6  5

------- snip --------

Paragraph 5 also seems clear to me. Here's an example:

------- snip --------

> attach(NULL)

> search()
 [1] ".GlobalEnv"        "NULL"
"cars"              "tools:rstudio"
 [5] "package:stats"     "package:graphics"
"package:grDevices" "package:utils"
 [9] "package:datasets"  "package:methods"
"Autoloads"         "package:base"

> assign("x", 10, pos=2)

> x
[1] 10

------- snip --------

Now that may beg the question of why one would want to do something like this,
which isn't addressed in the help file, but a fair comment is that if you
don't need to store objects in an environment that's accessible on the
path, why worry about it? After all, no one is forcing you to use this trick.

Finally, I too recommended that students use attach() when I first starting
teaching with R, until I noticed that they frequently tied themselves into knots
by attaching different versions of the same data during a session, producing
confusion about where the data were coming from and what version they were
using. It's not hard in R to avoid the use of attach(). Of course, attach()
is still part of the language, so you, and your students, are free to continue
using it if you wish, and perhaps your students avoid the problems that mine
often created.

Best,
 John

  -----------------------------
  John Fox, Professor Emeritus
  McMaster University
  Hamilton, Ontario, Canada
  Web: http::/socserv.mcmaster.ca/jfox

> On Apr 27, 2020, at 9:26 AM, Edward McNeil <edward.m at psu.ac.th>
wrote:
>
> Dear Petr,
> Thanks for your quick reply. Much appreciated. However, you haven't
really answered
> either of my questions, although I don't quite understand your
reference to La Gioconda.
>
> In any case, despite your strong recommendation not to use `attach`, I am
going to keep
> using it, as I have done successfully for the past 16 years, and keep
teaching it, until
> it either kills me or disappears from R. Unfortunately I have to teach R to
students and
> I don't like it when they ask me "tricky" questions to which
I have no answer. ;)
> --
> Edward McNeil
>
> On Mon, April 27, 2020 8:00 pm, PIKAL Petr wrote:
> Hi.
>
> I strongly recommend not to use attach. I agree that mentioned statements
are rather
> contradictory and probably others could give you more insightful answer.
You could
> consider that by attaching some data, you create something like a copy of
original data
> in your system with a feature that you can use column names directly. If
you change
> something in the data after attachment, you change only attached version
and not an
> original.
>
> It is similar as if you take a picture of Gioconda an use some creativity
to add a
> moustache to this picture. In any circumstances moustache does not
propagate to the
> original Louvre painting. Do not perform any tricks, preferably do not
perform attach.
>
> Cheers
> Petr
>
>> -----Original Message-----
>> From: R-help <r-help-bounces at r-project.org> On Behalf Of
Edward McNeil
>> Sent: Monday, April 27, 2020 2:07 PM
>> To: r-help at r-project.org
>> Subject: [R] deciphering help for `attach`
>>
>> Hi,
>> I have two related questions.
>>
>> 1. In the help page for `attach` under "Details" it says in
paragraph 3:
>> "By default the database is attached ..."
>>
>> But then paragraph 4 starts: "The database is not actually
attached."
>>
>> Could somebody explain this contradiction? Is the data(base) attached
or
>> not?
>>
>> 2. What is meant by the 5th paragraph: "One useful ?trick? is to
use what >> NULL (or equivalently a length-zero list) to create a new
environment on the
>> search path into which objects can be assigned by `assign` ... "?
>>
>> I don't understand what this "trick" is or why a
"trick" needs to be performed
>> here.
>>
>> Thanks
>> --
>> Edward McNeil
>>
>> ______________________________________________
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>
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