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Multiple copies of attached packages

2 messages · Liaw, Andy, Brian Ripley

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:
[1] ".GlobalEnv"        "package:MASS"      "package:methods"
"package:stats"    
 [5] "package:graphics"  "package:grDevices" "package:utils"
"package:datasets" 
 [9] "Autoloads"         "package:base"
[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
Brian Ripley
#
On Thu, 14 Apr 2005, Liaw, Andy wrote:

            

      
      
      
    

        
Not quite.  The correct description is in ?attach (and apart from 
mentioning attach was used, reading the help before posting is de rigeur).
Here is the version from 2.1.0 beta (which has been expanded):

      The database is not actually attached.  Rather, a new environment
      is created on the search path and the elements of a list (including
      columns of a dataframe) or objects in a save file are _copied_
      into the new environment.  If you use '<<-' or 'assign' to assign
      to an attached database, you only alter the attached copy, not the
      original object.  (Normal assignment will place a modified version
      in the user's workspace: see the examples.) For this reason
      'attach' can lead to confusion.

Examples:

      summary(women$height)   # refers to variable 'height' in the data frame
      attach(women)
      summary(height)         # The same variable now available by name
      height <- height*2.54   # Don't do this. It creates a new variable
                              # in the user's workspace
      find("height")
      summary(height)         # The new variable in the workspace
      rm(height)
      summary(height)         # The original variable.
      height <<- height*25.4  # Change the copy in the attached environment
      find("height")
      summary(height)         # The changed copy
      detach("women")
      summary(women$height)   # unchanged

Notice the difference between <- and <<- .

Assigning to an element follows the same rules, and in addition is an 
error unless an object exists that can be suitably subscripted.