[R] Multiple copies of attached packages

[Liaw, Andy]

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
> 
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