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 <edwar...@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-boun...@r-project.org> On Behalf Of Edward McNeil
>> Sent: Monday, April 27, 2020 2:07 PM
>> To: r-help@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
>>
>> ______________________________________________
>> R-help@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@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@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.
