R help - Jan 2011 - Global variables

Dear R-users,

Is there a way I can prevent global variables to be visible within my 
functions?

Sebastien

On 06/01/2011 4:45 PM, Sebastien Bihorel wrote:
> Dear R-users,
>
> Is there a way I can prevent global variables to be visible within my
> functions?

Yes, but you probably shouldn't.  You would do it by setting the 
environment of the function to something that doesn't have the global 
environment as a parent, or grandparent, etc.  The only common examples 
of that are baseenv() and emptyenv().  For example,

x <- 1
f <- function() print(x)

Then f() will work, and print the 1.  But if I do

environment(f) <- baseenv()

then it won't work:

 > f()
Error in print(x) : object 'x' not found

The problem with doing this is that it is not the way users expect 
functions to work, and it will probably have weird side effects.  It is 
not the way things work in packages (even packages with namespaces will 
eventually search the global environment, the namespace just comes 
first).  There's no simple way to do it and yet get access to functions 
in other packages besides base without explicitly specifying them (e.g. 
you'd need to use stats::lm(), not just lm(), etc.)

Duncan Murdoch

On Fri, May 4, 2012 at 3:06 PM, Luis Goncalves <lgoncalves at gmail.com>
wrote:
>
>
> On May 2 2011, 3:02?pm, Kenn Konstabel <lebats... at gmail.com>
wrote:
>> On Mon, May 2, 2011 at 2:19 PM, abhagwat <bhagwatadi... at
gmail.com> wrote:
>> > Well, what would be really helpful is to restrict the scope of all
>> > non-function variables, but keep a global for scope of all
function
>> >variables. Then, you still have access to all loaded functions, but
you
>> > don't mix upvariables.
>>
>> > How would one do that?
>>
>> But what's the real motivation for this? It could be useful for
>> ensuring that there are no unexpectedglobalvariablesin your code
>> but you can do it using findGlobals in codetools package.
>>
>> fun <- function() mean(x)
>> findGlobals(fun, merge=FALSE)
>>
>> Kenn
>>
>
> Kenn,
>
> I tried your method :
>
> ? ? ?library(codetools)
>
> ? ? ?square <- function (x) {
> ? ? ? ? ?y^2
> ? ? ?}
>
> ? ? ?y <- c(1, 3, 5, 9)
>
> ? ? ?square_without_globals <- function (x) {
> ? ? ? ? ?y^2
> ? ? ?}
>
> ? ? ?findGlobals(square_without_globals, merge=FALSE)
>
> but still get global variables in ?square_without_globals():
>
> ? ? ?> source('R_test_block_global_variables.R')
> ? ? ?> y
> ? ? ?[1] 1 3 5 9
> ? ? ?> square(7)
> ? ? ?[1] ?1 ?9 25 81
> ? ? ?> square_without_globals(7)
> ? ? ?[1] ?1 ?9 25 81
>
> What have I done wrong?
  findGlobals helps you find the global variables in a function but it
does nothing with them. That is, it shows you something *about* a
function but does nothing *with* a function.

  findGlobals(square_without_globals, merge=FALSE)

shows you that you use 3 global variables in your function:

$functions
[1] "^" "{"

$variables
[1] "y"

Now it's up 2 you how you use this information. You would probably
like to use ^ and { (global variables) in your function but maybe not
y. So you can edit your function and leave y out or add y as an
argument.
> (PS: I am an R novice, coming from the Matlab world. In Matlab, you
> have to declare global variables to be used in a function explicitly.
> Isn't it good programming practice (in ANY language) to NOT allow
> global variable visibility as a default? Leads to a lot less hard-to-
> find bugs!!!)
Yes but ... in R, functions are also "variables", and you would
probably like some "global" functions (`+` or `(`) to be visible. You
can modify your function's environment so that it would access only
functions from certain packages (e.g base) and/or nothing from the
global workspace; that is what is done when functions are included in
packages.

Otherwise, I agree that it is best to avoid global variables and when
you use them, they should be "declared":

      square <- function (x) {
          # beware! y is used here as a "global variable"
          y^2
      }