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Jeff Brown

2010-Apr-01 01:44 UTC

[R] Scope and assignment: baffling

Hi,

The code below creates a value, x$a, which depending on how you access it
evaluates to its initial value, or to what it's been changed to.  The last
two lines should, I would have thought, evaluate to the same value, but they
don't.

f <- function () {
	x <- NULL;
	x$a <- 0;
	x$get.a <- function () {
		x$a;
	};
	x$increment.a <- function () {
		x$a <<- x$a + 5;
	};
	x
};
x <- f();
x$increment.a();
x$get.a();
x$a;

This can be repeated; each time you call x$increment.a(), the value
displayed by x$get.a() changes, but x$a continues to be zero.

Is that totally weird, or what?

Thanks,
Jeff
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Peter Dalgaard

2010-Apr-01 07:48 UTC

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[R] Scope and assignment: baffling

Jeff Brown wrote:> Hi,
> 
> The code below creates a value, x$a, which depending on how you access it
> evaluates to its initial value, or to what it's been changed to.  The
last
> two lines should, I would have thought, evaluate to the same value, but
they
> don't.
> 
> f <- function () {
> 	x <- NULL;
> 	x$a <- 0;
> 	x$get.a <- function () {
> 		x$a;
> 	};
> 	x$increment.a <- function () {
> 		x$a <<- x$a + 5;
> 	};
> 	x
> };
> x <- f();
> x$increment.a();
> x$get.a();
> x$a;
> 
> This can be repeated; each time you call x$increment.a(), the value
> displayed by x$get.a() changes, but x$a continues to be zero.
> 
> Is that totally weird, or what?
In a word, no. It's not the same x. (And R is not Java or C++). It
should be enlightening to try

evalq(x,environment(x$get.a))




-- 
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

William Dunlap

2010-Apr-01 17:00 UTC

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[R] Scope and assignment: baffling

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Jeff Brown
> Sent: Wednesday, March 31, 2010 6:45 PM
> To: r-help at r-project.org
> Subject: [R] Scope and assignment: baffling
> 
> 
> Hi,
> 
> The code below creates a value, x$a, which depending on how 
> you access it
> evaluates to its initial value, or to what it's been changed 
> to.  The last
> two lines should, I would have thought, evaluate to the same 
> value, but they
> don't.
> 
> f <- function () {
> 	x <- NULL;
It would be better to say
      x <- list()
instead of x<-NULL, since the following x$a<-
coerces it to a list.
> 	x$a <- 0;
> 	x$get.a <- function () {
> 		x$a;
> 	};
> 	x$increment.a <- function () {
> 		x$a <<- x$a + 5;
> 	};
> 	x
> };
> x <- f();
Let's make things a tad clearer by naming the output
of f() 'globalX':
   globalX <- f()
globalX is a list containing a [virtual] copy of f's x, as it
stood when f() returned it.  You cannot modify globalX
without doing something like globalX$something<-xxx.
f's x remains in the environment created when you ran
f().  Each time you run f() you create an new environment
(with a new version of f's x).  A function's evaluation
environment generally goes away when the function is done,
but it remains if there are any references to the environment
from f()'s return value.  If the return value contains
functions defined in f() then they refer to f()'s evaluation
environment, so it hangs around.  When you run
    globalX$increment.a()
you are modifying f()'s environment's x$a, not globalX$a.

Usually such function are written so the state variables
are not in the output structure, as in:
    f <- function() {
        fA <- 0
        retval <- list()
        retval$get.a <- function() { fA }
        retval$increment.a <- function() { fA <<- fA + 5 }
        retval
    }
Then you can use it as you did but it is clear that
retval itself doesn't contain any state.  You can find
the possible state variables with
    > globalX <- f()
    > globalX$increment.a()
    > objects(environment(globalX$get.a))
    [1] "fA"     "retval"
    > eval(quote(fA), environment(globalX$get.a))
    [1] 5
    > globalX$increment.a()
    > eval(quote(fA), environment(globalX$get.a))
    [1] 10
    > globalX$get.a()
    [1] 10
The output of objects() suggests you might want
to clean up the code by not naming 'retval', just
returning list(get.a=function()fA, ...).  Then
your environment won't carry around an unused
copy of retval.
    
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
> x$increment.a();
> x$get.a();
> x$a;
> 
> This can be repeated; each time you call x$increment.a(), the value
> displayed by x$get.a() changes, but x$a continues to be zero.
> 
> Is that totally weird, or what?
> 
> Thanks,
> Jeff
> -- 
> View this message in context: 
> http://n4.nabble.com/Scope-and-assignment-baffling-tp1747582p1
747582.html> Sent from the R help mailing list archive at Nabble.com.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Gabor Grothendieck

2010-Apr-01 17:14 UTC

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[R] Scope and assignment: baffling

The code you presented is very close to object oriented in the style
of the proto package.  For example,

library(proto)

# generate a single object p
p <- proto(a = 0,
	geta = function(.) .$a,
	incra = function(.) .$a <- .$a + 5)

p$geta()
p$a # same
p$incra()
p$geta()


# Or if you want to be able to generate objects like that:

Account <- function() proto(a = 0,
	geta = function(.) .$a,
	incra = function(.) .$a <- .$a + 5)

# pp is an Account object
pp <- Account()
pp$geta()
pp$a # same
pp$incra()
pp$geta()

See http://r-proto.googlecode.com for more info.


On Wed, Mar 31, 2010 at 9:44 PM, Jeff Brown <dopethatwantscash at
yahoo.com> wrote:>
> Hi,
>
> The code below creates a value, x$a, which depending on how you access it
> evaluates to its initial value, or to what it's been changed to. ?The
last
> two lines should, I would have thought, evaluate to the same value, but
they
> don't.
>
> f <- function () {
> ? ? ? ?x <- NULL;
> ? ? ? ?x$a <- 0;
> ? ? ? ?x$get.a <- function () {
> ? ? ? ? ? ? ? ?x$a;
> ? ? ? ?};
> ? ? ? ?x$increment.a <- function () {
> ? ? ? ? ? ? ? ?x$a <<- x$a + 5;
> ? ? ? ?};
> ? ? ? ?x
> };
> x <- f();
> x$increment.a();
> x$get.a();
> x$a;
>
> This can be repeated; each time you call x$increment.a(), the value
> displayed by x$get.a() changes, but x$a continues to be zero.
>
> Is that totally weird, or what?
>
> Thanks,
> Jeff
> --
> View this message in context:
http://n4.nabble.com/Scope-and-assignment-baffling-tp1747582p1747582.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Jeff Brown

2010-Apr-06 18:57 UTC

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[R] Scope and assignment: baffling

Thanks, Bill!  That was deep, and took me a long time to work through, but I
get it now.

And Gabor -- r-proto is great!
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