[Rd] length of `...`

[Hadley Wickham]

On Thu, May 3, 2018 at 9:50 AM, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:
> On 03/05/2018 11:18 AM, Duncan Murdoch wrote:
>>
>> On 03/05/2018 11:01 AM, William Dunlap via R-devel wrote:
>>>
>>> In R-3.5.0 you can use ...length():
>>>     > f <- function(..., n) ...length()
>>>     > f(stop("one"), stop("two"), stop("three"), n=7)
>>>     [1] 3
>>>
>>> Prior to that substitute() is the way to go
>>>     > g <- function(..., n) length(substitute(...()))
>>>     > g(stop("one"), stop("two"), stop("three"), n=7)
>>>     [1] 3
>>>
>>> R-3.5.0 also has the ...elt(n) function, which returns
>>> the evaluated n'th entry in ... , without evaluating the
>>> other ... entries.
>>>     > fn <- function(..., n) ...elt(n)
>>>     > fn(stop("one"), 3*5, stop("three"), n=2)
>>>     [1] 15
>>>
>>> Prior to 3.5.0, eval the appropriate component of the output
>>> of substitute() in the appropriate environment:
>>>     > gn <- function(..., n) {
>>>     +   nthExpr <- substitute(...())[[n]]
>>>     +   eval(nthExpr, envir=parent.frame())
>>>     + }
>>>     > gn(stop("one"), environment(), stop("two"), n=2)
>>>     <environment: R_GlobalEnv>
>>>
>>
>> Bill, the last of these doesn't quite work, because ... can be passed
>> down through a string of callers.  You don't necessarily want to
>> evaluate it in the parent.frame().  For example:
>>
>> x <- "global"
>> f <- function(...) {
>>     x <- "f"
>>     g(...)
>> }
>> g <- function(...) {
>>     firstExpr <- substitute(...())[[1]]
>>     c(list(...)[[1]], eval(firstExpr, envir = parent.frame()))
>> }
>>
>> Calling g(x) correctly prints "global" twice, but calling f(x)
>> incorrectly prints
>>
>> [1] "global" "f"
>>
>> You can get the first element of ... without evaluating the rest using
>> ..1, but I don't know a way to do this for general n in pre-3.5.0 base R.
>
>
> Here's a way to do that:
>
> eval(as.name(paste0("..", n)))
>
> I was surprised this worked for n > 9, but it does.  Looking at the source,
> I think the largest legal value for n is huge; you'd hit other limits long
> before n was too big.

Maybe just get(paste0("..", n)) ?

Hadley

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