R help - Mar 2009 - changing function argument

Hi,

I wonder if the following is possible in R:

Suppose a function takes an argument, and wants to modify this argument 
so that the change is visible _at the call side_. It would be what is 
for example known as pass-by-reference-to-non-const in C++.

test <- function(x)
{
  x <- 10
  ...
  return (somethingElse) # I do NOT want having to return x
}

number = 5
test(number)
stopifnot(number == 10)


Is there some trick with scoping rules and maybe <<- to achieve this ?

thanks,
Thomas

Thomas Mang <thomas.mang <at> fiwi.at> writes:
> I wonder if the following is possible in R:
> 
> Suppose a function takes an argument, and wants to modify this argument 
> so that the change is visible _at the call side_. It would be what is 
> for example known as pass-by-reference-to-non-const in C++.
> 
> test <- function(x)
> {
>   x <- 10
>   ...
>   return (somethingElse) # I do NOT want having to return x
> }
> 
> number = 5
> test(number)
> stopifnot(number == 10)
> 
> Is there some trick with scoping rules and maybe <<- to achieve this
?
Yes, it is possible. No, don't do it (coming from the c++ world, I
understand
your temptation). Try to look for "assign" to see the attached flames.


http://r-project.markmail.org/search/?q=assign

Dieter

On Fri, 13 Mar 2009, Thomas Mang wrote:
> Hi,
>
> I wonder if the following is possible in R:
>
> Suppose a function takes an argument, and wants to modify this argument so
>that  the change is visible _at the call side_. It would be what is for
>example known  as pass-by-reference-to-non-const in C++.
>
> test <- function(x)
> {
> x <- 10
> ...
> return (somethingElse) # I do NOT want having to return x
> }
>
> number = 5
> test(number)
> stopifnot(number == 10)
>
It's possible in your example

test<-function(x){
    eval(substitute(x<-10),parent.frame())
    return(42)
}

However, it is still likely to be a bad idea.  What do you want to happen for
  test(5)
  test(x+y)
  test(test(y))

  retest<-function(z) {z<-5; test(z); print(z)}
  retest(x)


R really doesn't have pointer or reference types, and there's no way to
ensure that the arguments to a function are lvalues.


      -thomas

Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle