[Rd] length of `...`

[Duncan Murdoch]

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.

Duncan Murdoch