For new objects in R, how to assign values through subscripting? - r

Question

For new objects in R, how can I specify assignments to subscripted elements? As in object[3] <- new value. Here is a specific example of the problem I have.
# Rectangle example:
Rectangle <- function(a, b,...){
R <- list(a=a, b=b, others=list(...))
structure(R, class="Rectangle")
}#
`[.Rectangle` <- function(R,ind){
if(ind==1) return(R$a)
if(ind==2) return(R$b)
if(ind>=3) return(R$others[[ind-2]])
}#
R <- Rectangle(2,3,"other1","other2")
> R[1]; R[2]; R[3]; R[4];
[1] 2
[1] 3
[1] "other1"
[1] "other2"
> R[4] <- "new.other";
> R[1]; R[2]; R[3]; R[4];
[1] 2
[1] 3
[1] "other1"
[1] "other2"
Clearly, the assignment to the subscripted object hasn't worked. I would like to know the syntax to define such assignments properly. That is, I would need an example for the following:
`[<-.Rectangle` <- function(){ }
Thank you very much.

Answer 1

To override subset-assign, your function needs to accept three arguments (x, index, value) and return the modified object. It is important that the third parameter is called exactly value, since R internally calls the function using that name (rather than positionally).
Here’s an example:
`[<-.Rectangle` = function (x, index, value) {
if (index == 1L) {
x$a = value
}
else if (index == 2L) {
x$b = value
}
else {
x$others[[index - 2L]] = value
}
x
}
It probably goes without saying that this is a pretty convoluted logic, I’m not convinced that real-world code should have objects with such an API.

Answer 2

Maybe this help you:
> str(R)
List of 3
$ a : num 2
$ b : num 3
$ others:List of 2
..$ : chr "other1"
..$ : chr "other2"
- attr(*, "class")= chr "Rectangle"
> R[4]='hello'
> str(R)
List of 4
$ a : num 2
$ b : num 3
$ others:List of 2
..$ : chr "other1"
..$ : chr "other2"
$ : chr "hello"
- attr(*, "class")= chr "Rectangle"
> R[4]
[1] "other2"
> R[[4]]
[1] "hello"