[R] deciphering help for `attach`

Bert Gunter
Mon, 27 Apr 2020 15:25:47 -0000

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.

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 <jfox at 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 <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
> >>
> >> ______________________________________________
> >> 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.
>
>
>
>
> ______________________________________________
> 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