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

[Feng Tai]

You could use 'ref' package.

Feng


>From: Michael Wolosin <msw10 at duke.edu>
>To: r-help at stat.math.ethz.ch
>Subject: [R] changing the value of a variable from inside a function
>Date: Tue, 15 Nov 2005 12:22:15 -0500
>
>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.  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
>
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