changing the value of a variable from inside a function
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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