Dear Bert,
> -----Original Message-----
> From: Bert Gunter <bgunter.4...@gmail.com>
> Sent: Monday, April 27, 2020 11:26 AM
> To: Fox, John <j...@mcmaster.ca>
> Cc: edwar...@psu.ac.th; r-help@r-project.org
> Subject: Re: [R] deciphering help for `attach`
>
> What is the use case for attach? As the Help says, I find that with() or
> sometimes within() handles the situations where I would use it.
I don't believe that I was making a general case for using attach(), and in
fact was arguing to the contrary that its use can produce confusion. I was
simply trying to clarify what the help page says, which was the focus of the
original question.
I've only once encountered a situation where I wanted to attach an environment
(not a data frame) to the search path.
Best,
John
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along and
> sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>
> On Mon, Apr 27, 2020 at 7:58 AM Fox, John <j...@mcmaster.ca> wrote:
> >
> > 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.
______________________________________________
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.
