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R Scoping Rules

5 messages · Stephen G. Eick, Gabor Grothendieck, Thomas Lumley

Original
Gabor Grothendieck
# 
R uses lexical scope. That means that when a variable is not found in
a function it next looks where the function was _defined_ and not
where the function was called from.   Since simple is defined in the
global environment it looks there for height if can`t find it within
simple.  The parent environment is where the function was defined and
the parent frame is where it was called from.  To force it to look in
the parent frame rather than the parent environment try this:

simple <- function(env = parent.frame()) {
   if(exists("height", env)) cat("height exists\n")
   else cat("height does not exist\n")
}
foo <- function() { height <- 3; simple() }
foo() # height exists
On Sun, Apr 18, 2010 at 8:27 PM, Stephen G. Eick <eick at vistracks.com> wrote:
Stephen G. Eick
# 
Thanks.  This makes sense.  Is there any way to make R search over every
frame all the way back to the global frame?  

simple <- function(env = parent.frame()) {
   if(exists("height", env)) cat("height exists\n")
   else cat("height does not exist\n")
}
a<-function()simple()
foo <- function() { height <- 3; a() }
foo() # height does not exist

-----Original Message-----
From: Gabor Grothendieck [mailto:ggrothendieck at gmail.com] 
Sent: Sunday, April 18, 2010 8:00 PM
To: eick at cs.uic.edu
Cc: r-sig-geo at stat.math.ethz.ch
Subject: Re: [R-sig-Geo] R Scoping Rules

R uses lexical scope. That means that when a variable is not found in
a function it next looks where the function was _defined_ and not
where the function was called from.   Since simple is defined in the
global environment it looks there for height if can`t find it within
simple.  The parent environment is where the function was defined and
the parent frame is where it was called from.  To force it to look in
the parent frame rather than the parent environment try this:

simple <- function(env = parent.frame()) {
   if(exists("height", env)) cat("height exists\n")
   else cat("height does not exist\n")
}
foo <- function() { height <- 3; simple() }
foo() # height exists
On Sun, Apr 18, 2010 at 8:27 PM, Stephen G. Eick <eick at vistracks.com> wrote:
variable
Gabor Grothendieck
# 
You can use parent.frame(n), see ?parent.frame and ?get,  but its
probably not a good idea since you will wind up with a function that
does not work in the way the rest of R works.
On Sun, Apr 18, 2010 at 9:17 PM, Stephen G. Eick <eick at vistracks.com> wrote:
Thomas Lumley
#
On Sun, 18 Apr 2010, Stephen G. Eick wrote:

            

      
      
      
    

        
'height' is neither a local variable, nor visible in the environment where 'simple()' was defined.  That's the point of static lexical scope -- the scope depends on what was visible where the function was defined.

You seem to want dynamic scope, where variables in the calling environment are visible.  This can be faked with eval.parent(), but isn't how R naturally works.

            

      
      
      
    

        
R *does* use scoping rules like python (since python 2.1).  Your example doesn't work in python, for exactly the same reason.

            

      
      
      
    

        
...     print height
...

            

      
      
      
    

        
...     height = 10
...     simple()
...

            

      
      
      
    

        
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "<stdin>", line 3, in foo
   File "<stdin>", line 2, in simple
NameError: global name 'height' is not defined

      -thomas

Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle