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

[Gabor Grothendieck]

Use eval.parent as shown in example 1.  Note that you might
be tempted to use example 2 but it does not actually fulfill
the letter of the original post since it changes test in the lexical
environment of f, i.e.the environment where f is defined,
rather than the calling frame of f, i.e. the environment from where
f is called.  To get <<- to work with example 2 we must
create a new f that is the same as the original f but whose
lexical environment has been changed to be the caller frame
as shown in example 3.

# example 1.  ok.  test changed in caller frame.
test <- 11:13
f <- function(i) eval.parent(substitute(test[i] <- 99))
g <- function() { test <- 1:3; f(2); print(test) }
g()  # 1 99 3
test # 11 12 13

# example 2.  Same except f has been changed.
# Note that this changes test in the lexical environment
# rather than in the caller frame.
test <- 11:13
f <- function(i) test[i] <<- 99
g <- function() { test <- 1:3; f(2); print(test) }
g()  # 1 2 3
test  # 11 99 13

# example 3. same as example 2 but the lexical environment of f is
# forced to be the caller frame so that it works as in example 1.
# f is the same as in example 1 and g has been changed to
# create a new f like the original f but with the caller frame as its
# lexical environment.
test <- 11:13
f <- function(i) test[i] <<- 99
g <- function() { test <- 1:3; environment(f) <- environment(); f(2);
print(test) }
g()  # 1 99 3
test # 11 12 13


Another possibility, which is similar in effect to example 1, would be
to use defmacro in package gtools.


On 11/15/05, Michael Wolosin <msw10 at duke.edu> 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.  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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