CODEWITHSUNDEEP
rfunctionpass-by-referencearguments
Alex
Alex

Reputation: 19873

modify variable within R function

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)

I would like g to be set to 5.

Upvotes: 16

Views: 19512

Answers (5)

John R.B. Palmer
John R.B. Palmer

Reputation: 2092

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)
}

Upvotes: 3

Xu Wang
Xu Wang

Reputation: 10607

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.

Upvotes: 2

Tommy
Tommy

Reputation: 40871

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...

Upvotes: 14

Rory
Rory

Reputation: 49

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

g <- abc(g)

Upvotes: 0

Dason
Dason

Reputation: 61983

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

Upvotes: 4

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