[R] changing the value of a variable from inside a function

[Duncan Murdoch]

On 11/15/2005 12:22 PM, Michael Wolosin wrote:
> All -
> 
> I am trying to write R code to implement a recursive algorithm.  I've 
> solved the problem in a klunky way that works, but uses  more memory and 
> computing time than it should.
> 
> A more elegant solution than my current one would require updating the 
> values of a variable that is located in what I will call the "root" 
> environment - that environment from which the original call to the 
> recursive function was issued.  

That's tricky and ugly, but possible in various ways.  However, the 
clean easy way to do this is to wrap your recursive function in a 
non-recursive one, and refer to variables in the non-recursive one using 
lexical scoping.  For example,

wrapper <- function(test) {
    test <- test  # make a copy in the wrapper environment
    blah <- function() {
        # references here to test will see the one in wrapper
        # blah can call itself; each invocation will see the same test

        test[i,] <<- expr  #  use "super-assignment" to modify it
    }
    return(test)
}

This makes one copy of the matrix and works on that.  If you want to 
make zero copies, you need to get tricky.

Duncan Murdoch

Certainly, I could pass the variable into
> the function, update it inside, and return it.  However, the variable I am 
> updating is a large matrix, and the recursion could end up several hundred 
> levels deep.  Passing the matrix around would create a copy in the 
> environment for each call, wasting memory, time, and space.
> 
> I've read the help on the "sys.{}" family of functions, and "eval", and 
> although I can't claim to have absorbed it all, it seems like it is much 
> easier to access the value of a variable in a parent frame than it is to 
> update that value with assignment.
> If you make an assignment inside a function, even if it is to a section of 
> a variable that exists in a parent frame, the variable is only created or 
> updated in the current environment - never in the parent frame.
> 
> For example:
> 
> test <- matrix(NA,nrow=4,ncol=3)
> test[1,] <- c(1,2,3)
> blah <- function(i){
>    test[i,] <- c(0,1,2) + i
>    return(test)
> }
> test
> blah(2)
> test
> 
> So the real question is, how do I write the function like "blah" above that 
> updates "test" in the parent or root frame?
> 
> blah <- function(i){
>    test[i,] <- c(1,2,3) + i  #modify this line somehow
>    return(NULL)
> }
> If done "correctly", we will get:
>  > blah(2)
>  > test
>        [,1] [,2] [,3]
>   [1,]    1    2    3
>   [2,]    2    3    4
>   [3,]   NA   NA   NA
>   [4,]   NA   NA   NA
> 
> And given an example that works from within a single function call, does it 
> have to be modified to work recursively?
> 
> blah <- function(i){
>    if (i<4) {blah(i + 1)}
>    test[i,] <- c(0,1,2) + i  #modify this line somehow
>    return(NULL)
> }
> If written "correctly", the following would be the output:
>  > blah(2)
>  > test
>        [,1] [,2] [,3]
>   [1,]    1    2    3
>   [2,]    2    3    4
>   [3,]    3    4    5
>   [4,]    4    5    6
> 
> One idea would be to write out to a file.  The filename could reside in the 
> root environment, and that is all that is needed.  But  this also seems 
> inelegant (and slow).  If I can read and write to a file, I should be able 
> to read and write to a memory location.
> 
> I suspect that the solution lies somewhere in the "sys" functions, but I 
> was having trouble seeing it.  Any help would be appreciated.
> 
> Thank you in advance,
> 
> Mike
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html