[R] using object reference

[Spencer Graves]

      Yes, but use sparingly, because any use of "<<-", "assign", etc., 
from within a function to change something other than what appears in 
the standard return from a function call generates "spaghetti code" that 
is difficult to maintain.  A month or a year from now, someone (the 
developer or someone else) may have difficulty understanding how "z" got 
created or changed. 

      Consider the following: 

 > x <- 1
 >
 > f <- function(z) {
+ attr(z,'a') <<- 'some new text'
+ }
 >
 > f(x)
 >
 > z
[1] 1
attr(,"a")
[1] "some new text"
 > x
[1] 1

      If you intend to create an object "z" this way using the function 
"f", that's fine.  However, if you subsequently forget, it can be very 
confusing: 
 > z <- 3
 > f(2)
 > z
[1] 2
attr(,"a")
[1] "some new text"

      Why is "z" no longer "3"?  I didn't obviously "assign" to "z" the 
results of f(2). 

 > z <- "x"
 > g <- function(a){
+  w <- f(a)
+  w2 <- paste("2", w)
+  list(w=w, w2=w2)
+ }
 > g(2)
$w
[1] "some new text"

$w2
[1] "2 some new text"

 > z
[1] 2
attr(,"a")
[1] "some new text"
 >
      Moreover, the resulting code will not transfer to S-Plus 6.2. 

 > f(2)
Problem in f(2): "<<-" not allowed in replacements: see ?"<<-"
Use traceback() to see the call stack

      I suggest you use the standard function return mechanism, and not 
try to melt gracefully through the wall. 

      hope this helps.  spencer graves

Gabor Grothendieck wrote:

>The double arrow in the example you cite is essential;
>you can't leave it out since that's what tells R to 
>search through its parents' environments:
>
>attr(z,'a') <<- 'some new text' 
>
>
>---
>Date:   Tue, 02 Mar 2004 16:52:20 -0500 
>From:   Rajarshi Guha <rxg218 at psu.edu>
>To:   R <r-help at stat.math.ethz.ch> 
>Subject:   [R] using object reference 
>
> 
>Hi,
>I have read the previous thread on using references to objects in a
>function but the solution suggested does'nt seem to be working.
>
>basically I have an object x which has an attribute a containing some
>text. I would like to pass x to a function which will change the
>attribute a with some new text and have the change visible when the
>function exits.
>
>something like
>
>attr(x,'a') <- 'some text'
>f <- function(z) {
>attr(z,'a') <- 'some new text'
>}
>
>So that when I call f(x)
>attr(x,'a')
>
>gives
>
>'some new text'
>
>I went by the example below 
>g <- function(z) eval(eval(substitute(expression(z[1] <<- z[1]+1))))
>a <- 1:5
>g(a) # increments first element of a by 1
>a # c(2,2,3,4,5)
>
>replcing the innermost bracket with attr(z,'a') <- 'some new text' but
>the the after returning from the function the attribute of x does not
>get changed.
>
>Could anybody point out how I could achieve this? Do I need to use the
>R.oo package or can this be done without external packages?
>
>Thanks,
>
>-------------------------------------------------------------------
>Rajarshi Guha <rxg218 at psu.edu> <http://jijo.cjb.net>;
>GPG Fingerprint: 0CCA 8EE2 2EEB 25E2 AB04 06F7 1BB9 E634 9B87 56EE
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