Change a data frame in an outer scope in a function - r

Question

In fun_outer I created an empty data frame df, and I want to add a new row to df via the inner function fun_inner:
fun_outer <- function() {
df <- data.frame()
fun_inner()
return(df)
}
fun_inner <- function(){
tmp <- data.frame(x = 1 ,y = 2)
df <<- rbind(df, tmp)
}
I would expect that executing fun_outer() could return a df like:
x y
1 2
But I actually got an error:
Error in rep_len(xi, nvar) (temp.R#355): attempt to replicate non-vector
Then I tried another approach:
fun_outer <- function() {
df <- data.frame()
fun_inner(df)
return(df)
}
fun_inner <- function(x){
tmp <- data.frame(x = 1 ,y = 2)
df <<- rbind(x, tmp)
}
And this time by executing fun_outer() I got another error:
Error in fun_inner(df) (temp.R#344): cannot change value of locked binding for 'df'
How can I create a data frame in an outer function, and bind row(s) to it using an inner scope function?
My intention was to use an iterator function inside a function A to append new data from each iteration to a data frame created inside function A

Answer 1

If a variable cannot be found in the current function it is looked up in the environment where the function was defined, not the environment from which it was called. <<- works the same way. What you want is the parent frame which is the caller.
fun_outer <- function() {
df <- data.frame()
fun_inner()
return(df)
}
fun_inner <- function(envir = parent.frame()){
tmp <- data.frame(x = 1 ,y = 2)
envir$df <- rbind(envir$df, tmp)
}
fun_outer()
## x y
## 1 1 2