..., ..1, etc used in Functions

Description

... and ..1, ..2 etc are used to refer to arguments passed down from a calling function. These (and the following) can only be used inside a function which has ... among its formal arguments.

...elt(n) is a functional way to get ..n and basically the same as eval(paste0("..", n)), just more elegant and efficient. Note that switch(n, ...) is very close, differing by returning NULL invisibly instead of an error when n is zero or too large.

...length() returns the number of expressions in ..., and ...names() the names. These are the same as length(list(...)) or names(list(...)) but without evaluating the expressions in ... (which happens with list(...)).

Evaluating elements of ... with ..1, ..2, ...elt(n), etc. propagates visibility. This is consistent with the evaluation of named arguments which also propagates visibility.

Usage

...length()
...names()
...elt(n)

Arguments

See Also

... and ..1, ..2 are reserved words in R, see Reserved.

For more, see the Introduction to R manual for usage of these syntactic elements, and dotsMethods for their use in formal (S4) methods.

Examples

tst <- function(n, ...) ...elt(n)
tst(1, pi=pi*0:1, 2:4) ## [1] 0.000000 3.141593
tst(2, pi=pi*0:1, 2:4) ## [1] 2 3 4
try(tst(1)) # -> Error about '...' not containing an element.

tst.dl  <- function(x, ...) ...length()
tst.dns <- function(x, ...) ...names()
tst.dl(1:10)    # 0  (because the first argument is 'x')
tst.dl(4, 5)    # 1
tst.dl(4, 5, 6) # 2  namely '5, 6'
tst.dl(4, 5, 6, 7, sin(1:10), "foo"/"bar") # 5.    Note: no evaluation!
tst.dns(4, foo=5, 6, bar=7, sini = sin(1:10), "foo"/"bar")
##        "foo"  "" "bar"  "sini"               ""

## From R 4.1.0 to 4.1.2, ...names() sometimes did not match names(list(...));
## check and show (these examples all would've failed):
chk.n2 <- function(...) stopifnot(identical(print(...names()), names(list(...))))
chk.n2(4, foo=5, 6, bar=7, sini = sin(1:10), "bar")
chk.n2()
chk.n2(1,2)