From ggrothendieck at gmail.com  Thu Jul  8 13:02:31 2010
From: ggrothendieck at gmail.com (Gabor Grothendieck)
Date: Thu, 8 Jul 2010 07:02:31 -0400
Subject: [Rd] pass-by-reference
In-Reply-To: <1278564749228-2281802.post@n4.nabble.com>
References: <1278564749228-2281802.post@n4.nabble.com>
Message-ID: <AANLkTikVa4qG4pQl1mRkTOacMZfCk10K0E_j71nAjwtB@mail.gmail.com>

On Thu, Jul 8, 2010 at 12:52 AM, Paul Bailey <pdbailey at umd.edu> wrote:
>
> I'm working with a large object that I want to modify slightly in a function.
> Pass-by-reference would make a lot of sense, but I don't know how to do it.
>
> I've searched this archive and thought that I can do something like
>
> f <- function(x) {
> ?v1 <- list(a=x,b=3)
> ?g(x)
> ?v1
> }
> g <- function(x) {
> ?frame <- parent.frame()
> ?assign("v1",list(a=x,b=x),frame)
> }
> f(4)
> returns list(a=4,b=4)
>
> but what if I wanted to make v1[[1]] = v1[[1]] + v1[[2]] without creating a
> copy of v1?
>
> f2 <- function(x) {
> ?v1 <- list(a=x,b=3)
> ?g2(x)
> ?v1
> }
> g2 <- function(x) {
> ?frame <- parent.frame()
> ?v1 <- get("v1",envir=frame)
> ?v1[[1]] <- v1[[1]] + v1[[2]]
> }
> f2(4)

Try:

g2 <- function(x, env = parent.frame()) {
   with(env, v1[[1]] <- v1[[1]] + v1[[2]])
}

or

g2 <- function(x, env = parent.frame()) {
   env$v1[[1]] <- env$v1[[1]] + env$v1[[2]]
}

Sometimes the reason people want pass by reference is not so much for
efficiency is but that they are trying to recreate an object oriented
structure without realizing it.  In that case the packages referred to
by Henrik would be useful.



>
> but this fails. (it returns list(a=4,b=3) because v1 was copied into g2, not
> passed by reference) Is there a way to do this?
> --
> View this message in context: http://r.789695.n4.nabble.com/pass-by-reference-tp2281802p2281802.html
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>
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[Rd] pass-by-reference
Gabor Grothendieck