Assign element to list in R - r

Question

We can use append function to add element to list. For example like blow.
a_list <- list()
a_list <- append(a_list, "a")
But I want do to like this. The append_new don't return but change the a_list.
a_list <- list()
append_new(a_list, "a")
It can be used by eval function to do this.
a_list <- list()
eval(parse(text="a_list[[1]]<-a"))
a_list
But if I want to write the function add_element_to_list.
a_list <- list()
add_element_to_list(a_list, "a")
a_list ## same as list("a")
How to write the function? This function like assign but more powerful.
The post use eval(parse(text="")) but it can not write in the custom function append_new.

Answer 1

Simpler:
`append<-` <- function(x, value) {
c(x, value)
}
x <- as.list(1:3)
y <- as.list(1:3)
append(x) <- y
append(x) <- "a"
print(x)
[[1]]
[1] 1
[[2]]
[1] 2
[[3]]
[1] 3
[[4]]
[1] 1
[[5]]
[1] 2
[[6]]
[1] 3
[[7]]
[1] "a"

Answer 2

Using evil parse:
append_new <- function(x, y){
eval(parse(text = paste0(x, "[ length(", x, ") + 1 ]<<- '", y, "'")))
}
a_list <- list()
append_new(x = "a_list", y = "a")
a_list
# [[1]]
# [1] "a"
append_new(x = "a_list", y = "b")
a_list
# [[1]]
# [1] "a"
#
# [[2]]
# [1] "b"

Answer 3

Perhaps something like this?
add_element_to_list <- function(this, that)
{
if(typeof(this) != "list") stop("append_new requires a list as first argument")
assign(deparse(substitute(this)),
append(this, that),
envir = parent.frame(),
inherits = TRUE)
}
a_list <- list()
add_element_to_list(a_list, "a")
a_list
#> [[1]]
#> [1] "a"
add_element_to_list(a_list, "b")
a_list
#> [[1]]
#> [1] "a"
#>
#> [[2]]
#> [1] "b"
I would be very cautious in using something like this in a package though, since it is not idiomatic R. In general, R users expect functions not to modify existing objects but to return new objects.
Of course there are some notable exceptions...