[R] How to use current value of variable in function definition?

[William Dunlap]

> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Allan Engelhardt
> Sent: Thursday, July 02, 2009 9:47 AM
> To: r-help at r-project.org
> Subject: [R] How to use current value of variable in function 
> definition?
> 
> Must be the heat or something but I can't get my brain into gear and 
> figure out how to get something like
> 
> if (1) { c <- 1; foo <- function () print(c); }
> c <- 2
> foo()
> 
> to print 1, not 2.  (The real life example is a little more 
> complex, but 
> you get the idea.  I don't want the variable c in the function 
> definition, I want its value at that time.)
> 
> The only thing I have been able to come up with is something like
> 
> if (1) foo <- (function () { c <- 1; return(function () print(c)) })()
> c <- 2
> foo()
> # [1] 1

You might try local(), as in
   > c<-1 ; foo.local<-local({orig.c <- c ; function()orig.c})
   > foo.local()
   [1] 1
   > c<-3
   > foo.local()
   [1] 1
It is possible for someone to alter the orig.c after you create
foo.local, as in
   > assign("orig.c", 17, env=environment(foo.local))
   > foo.local()
   [1] 17
Looking at the function's code will not make it clear where
orig.c is coming from.  The clue is that its environment is not
one of the standard named ones, but it given by a hex number.
   > foo.local
   function()orig.c
   <environment: 0x02108c54>

You could also use substitute() to change the code in the function.
It can be messy to do but the resulting code may be clearer (although
it won't give a hint as to where that constant came from).  E.g.,
   > foo.substitute<-function()orig.c
   > c<-1 ; functionBody(foo.substitute)<-do.call(substitute,
list(functionBody(foo.substitute), list(orig.c=c)))
   > foo.substitute()
   [1] 1
   > foo.substitute
   function () 
   1

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com 

> but that just hurts.  Please make the pain go away.
> 
> Can someone wake up my brain?
> 
> Allan.
> 
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