modify variable within R function

Question

How do I modify an argument being passed to a function in R? In C++ this would be pass by reference.

g=4
abc <- function(x) {x<-5}
abc(g)

Answer 1

Am I missing something as to why you can't just do this?

g <- abc(g)

Answer 2

I have a solution similar to @Dason's, and I am curious if there are good reasons not to use this or if there are important pitfalls I should be aware of:

changeMe = function(x){
assign(deparse(substitute(x)), "changed", env=.GlobalEnv)
}   

Answer 3

There are ways as @Dason showed, but really - you shouldn't!

The whole paradigm of R is to "pass by value". @Rory just posted the normal way to handle it - just return the modified value...

Environments are typically the only objects that can be passed by reference in R.

But lately new objects called reference classes have been added to R (they use environments). They can modify their values (but in a controlled way). You might want to look into using them if you really feel the need...

Answer 4

There has got to be a better way to do this but...

abc <- function(x){eval(parse(text = paste(substitute(x), "<<- 5")))}
g <- 4
abc(g)
g

gives the output

[1] 5

Answer 5

I think that @Dason's method is the only way to do it theoretically, but practically I think R's way already does it.

For example, when you do the following:

y <- c(1,2)
x <- y

x is really just a pointer to a the value c(1,2). Similarly, when you do

abc <- function(x) {x <- 5; x}
g <- abc(g)

It is not that you are spending time copying g to the function and then copying the result back into g. I think what R does with the code

g <- abc(g)

is:

1. The right side is looked at first. An environment for the function abc is set up.
2. A pointer is created in that environment called x.
3. x points to the same value that g points to.
4. Then x points to 5
5. The function returns the pointer x
6. g now points to the same value that x pointed to at the time of return.

Thus, it is not that there is a whole bunch of unnecessary copying of large options.

I hope that someone can confirm/correct this.