将表达式评估为给定环境

Question

I would like to call a function inside a given environment so that the execution of the function does not mess the global environment. I have something like that:

f <- function() { x <<- 5 }
e <- new.env()
evalq(f(), envir = e)

I want to evaluate f() inside the environment e , but it seems it does not work since the variable x is accessible from the global env instead of the env e . I mean I want to access x by typing e$x , not x .

I tried to build the function f inside e like this:

e <- new.env()
evalq(f <- function() { x <<- 5 }, envir = e) 
evalq(f(), envir = e)

But it still does not work, x is in the global env, not in e .

---

EDIT with more informations:

From the answers I received, I understand that <<- should not be used. The problem is that, in practice, I don't know what f is, since it is obtained by sourcing a file which is downloaded through a shiny app. The purpose of this app is to evaluate a classifier function f written by students. So in case they used the <<- operator in their function, I don't want to pollute my global env (which seem to be impossible from the description of the <<- operator).

I think I could remove the potentially created variables with something like

vars <- ls()
evalq(f(), envir = e)
new_vars <- ls()
rm(list = new_vars[!new_vars %in% vars])

But is there a better solution?

Answer 1

You can use assign to define the environment where the object should be created:

e <- new.env()
f <- function() {
    assign("x", 5, envir = e)
}
f()

x
# Error: object 'x' not found
e$x
# [1] 5

Answer 2

As @lmo says, the simplest way would be to not use <<- to do this. Also, the function runs in its own environment so if you were to use <- you would need to assign the output of the function to a variable (otherwise it is only created in the f 's temporary environment. Something like this would work:

f <- function() { x <- 5 }
e <- new.env()
evalq(x <- f(), envir = e)

e$x
#[1] 5