| dots {base} | R Documentation |
..., ..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
n |
a positive integer, not larger than the number of expressions
in ..., which is the same as |
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)