[R] Forwarding missing arguments to the `[` method

[Bert Gunter]

I haven't followed this thread, so this may be stupid, but have you
looked at the do.call(`[`,...) idiom? Something like:

 x <- array(1:8, c(2,2,2))
> x
, , 1

     [,1] [,2]
[1,]    1    3
[2,]    2    4

, , 2

     [,1] [,2]
[1,]    5    7
[2,]    6    8

> l <- as.list(rep(TRUE, 2))
> do.call(`[`, c(list(x), l, list(1, drop = TRUE)))
     [,1] [,2]
[1,]    1    3
[2,]    2    4

Feel free to ignore without comment if this is unhelpful.


Bert Gunter

Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Wed, Dec 8, 2021 at 5:52 AM Ivan Krylov <krylov.r00t using gmail.com> wrote:
>
> Got some progress on this, but still not sure how to continue. First, a
> much more simple script reproducing the behaviour that confuses me:
>
> foo <- function(x, ...) structure(x, class = 'foo')
> `[.foo` <- function(x, i, ..., drop = TRUE) {
>         print(sys.call())
>         print(match.call())
>         foo(NextMethod()) # x[] should return an object of class foo
> }
> `[<-.foo` <- function(x, i, ..., value) {
>         print(sys.call())
>         print(match.call())
>         x[i, ..., drop = FALSE]
>         # imagine that we need to perform some checks on the
>         # subset of x that we're about to replace
>         NextMethod()
> }
>
> x <- foo(42)
> x[]      # this works
> x[] <- 1 # this doesn't
>
> Now, the crucial difference between the x[] and x[] <- 1 calls is that
> in the former case, the `i` argument isn't even specified:
>
> x[]
> # `[.foo`(x, )
> # `[.foo`(x = x)
> # [1] 42
> # attr(,"class")
> # [1] "foo"
>
> While in the latter case, `i` is specified in the x[] call (but
> missing):
>
> x[] <- 1
> # `[<-.foo`(`*tmp*`, , value = 1)
> # `[<-.foo`(x = `*tmp*`, value = 1)
> # x[i, ..., drop = FALSE]
> # `[.foo`(x = x, i = i, drop = FALSE)
> Error in NextMethod() : argument "i" is missing, with no default
>
> What's the best way to forward the arguments from [<-.class methods to
> the [.class methods in order to handle cases like x[] properly?
>
> --
> Best regards,
> Ivan
>
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