R help - Jul 2009 - Remembering a value in multiple calls of a function

Hello,
I tried this pseudo-generator style experiment (see below). The
"<<-"
operator assigns to in the calling environment which would be the
environment of  "getN".
Yet when the function incr is returned, isn't this environment lost?
Also the print displays GlobalEnv, yet the globalenv does not have any
mention of startgiven.
This code was inspired from and old issue of RJournal , but I'm not
sure how this works.

I'm not sure how this works and an explanation would be helpful.

Thank you
Saptarshi

getN <- function(start){
  startgiven <- start;
  incr <- function(n=NA){
    print(parent.frame())
    if(!is.na(n)) startgiven<<-n
    startgiven<<-startgiven+1
    return(startgiven)
  }
}
a=getN(10)
a() #11
a()#12

On Fri, Jul 17, 2009 at 9:15 AM, Saptarshi Guha<saptarshi.guha at
gmail.com> wrote:
> Hello,
> I tried this pseudo-generator style experiment (see below). The
"<<-"
> operator assigns to in the calling environment which would be the
> environment of ?"getN".
Yes.
> Yet when the function incr is returned, isn't this environment lost?
No.  The function getN returned a function closure so the environment
of getN is preserved as long as the object a is preserved.
> Also the print displays GlobalEnv, yet the globalenv does not have any
> mention of startgiven.
If you want to see the objects in the enclosing environment of a
function you can use

ls(environment(a))

or, perhaps more informatively,

ls.str(environment(a))

Using parent.frame, as you did, will give you the environment from
which the function was called, which in this case is the global
environment.
> This code was inspired from and old issue of RJournal , but I'm not
> sure how this works.
>
> I'm not sure how this works and an explanation would be helpful.
>
> Thank you
> Saptarshi
>
> getN <- function(start){
> ?startgiven <- start;
> ?incr <- function(n=NA){
> ? ?print(parent.frame())
> ? ?if(!is.na(n)) startgiven<<-n
> ? ?startgiven<<-startgiven+1
> ? ?return(startgiven)
> ?}
> }
> a=getN(10)
> a() #11
> a()#12

'startgiven' is in the environment of your function 'incr'
(which is what your function 'a' becomes).  It might be more transparent
to define your function with an enclosing local environment explicitly, for
example:
> b <- local({
+   startgiven <- 0
+   function(n) {
+     if(!missing(n)) {
+       startgiven <<- n
+     } else {
+       startgiven <<- startgiven + 1
+     }
+     startgiven
+   }
+ })
> b(5)
[1] 5
> b()
[1] 6
> b()
[1] 7
> 
You should look carefully at the help information for the superassignment
operator

help("<<-")

to see precisely where assignments made by this operator are located. 


Bill Venables
http://www.cmis.csiro.au/bill.venables/ 


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Saptarshi Guha
Sent: Friday, 17 July 2009 5:16 PM
To: R-help at r-project.org
Subject: [R] Remembering a value in multiple calls of a function

Hello,
I tried this pseudo-generator style experiment (see below). The
"<<-"
operator assigns to in the calling environment which would be the
environment of  "getN".
Yet when the function incr is returned, isn't this environment lost?
Also the print displays GlobalEnv, yet the globalenv does not have any
mention of startgiven.
This code was inspired from and old issue of RJournal , but I'm not
sure how this works.

I'm not sure how this works and an explanation would be helpful.

Thank you
Saptarshi

getN <- function(start){
  startgiven <- start;
  incr <- function(n=NA){
    print(parent.frame())
    if(!is.na(n)) startgiven<<-n
    startgiven<<-startgiven+1
    return(startgiven)
  }
}
a=getN(10)
a() #11
a()#12

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