I suspect you've attach()'ed `DF' multiple times in your
code (possibly inside a loop, or perhaps a function that
was called several times). Note that if it were a
`package', it would show up in search() as `package:DF'
rather than just `DF'. Also, R Core folks took care to
avoid attaching the same package multiple times:
> library(MASS)
> search()
[1] ".GlobalEnv" "package:MASS"
"package:methods"
"package:stats"
[5] "package:graphics" "package:grDevices"
"package:utils"
"package:datasets"
[9] "Autoloads" "package:base"
> library(MASS)
> search()
[1] ".GlobalEnv" "package:MASS"
"package:methods"
"package:stats"
[5] "package:graphics" "package:grDevices"
"package:utils"
"package:datasets"
[9] "Autoloads" "package:base"
Notice how trying to load a package that's already on the
search path has no effect.
This is not true for R objects, though.
When you attach a data frame, say, `DF', (or a list), it
places a _copy_ on the search path, so you can access
the variables in the data frame (or components of the
list) directly. When you make modifications to the
variables (such as x[i] <- something, rather than
DF$x[i] <- something), the modifications are applied to
the _copy_ on the search path, not the original.
HTH,
Andy
> From: Fernando Saldanha
>
> I have noticed that after I ran a batch script multiple times I get
> multiple copies of a package's name when I call search(). Is this a
> problem?
>
> > search()
> [1] ".GlobalEnv" "DF" "DF"
> [4] "DF" "DF" "DF"
>
> multiple copies here ...
>
> [13] "DF" "DF" "DF"
>
> other packages here ...
>
> [28] "package:quadprog" "package:car"
"package:methods"
> [31] "package:stats" "package:graphics"
"package:grDevices"
> [34] "package:utils" "package:datasets"
"Autoloads"
> [37] "package:base"
>
> The following strange (to me) behavior that may be related. Suppose I
> have a variable x that is in the global environment, and also there is
> an 'x' in a dataframe called DF. Then I remove the variable x from
the
> Global Environment, with
>
> remove('x', pos = 1)
> At this point if I call remove again in the same way I get an error:
> the variable x does not exist anymore. However, at this point I also
> can check that DF$x exists. So far so good.
>
> Further down in my code I have an assignment of the type
>
> x[i] <- something (*)
>
> which works fine, except that then if I look at x[i] and DF$x[i] they
> are different. So it looks like x was recreated in the Global
> Environment, which I actually can check by typing
>
> .GlobalEnv$x.
>
> On the other hand, if I put in my code something like
>
> newvar[i] <- something
>
> where newvar was never defined, then I get an error. I was hoping that
> the statement (*)
> above would assign to the variable DF$x. But it looks like (although
> that is probably not the correct explanation) that the interpreter
> somehow "remembers" that once there was a variable x in the
global
> environment and accepts the assignment to x[i], recreating that
> variable.
>
> Any insights on this?
>
> Many thanks,
>
> Fernando
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
> http://www.R-project.org/posting-guide.html
>
>
>