...elt
Manipulate Ellipsis Arguments
Description
Counts the number of arguments in ... or evaluates one of the arguments without
unecessarily evaluating any of them.
Usage
...elt(n)
...length()
...names()
chkDots(..., which.call = -1, allowed = character(0))
Arguments
| n |
an integer specifying which ... argument to evaluate. |
| ... |
typically ... from this function's caller.
If this argument is not empty, a warning is given indicating that these
arguments are not used in this function. |
| which.call |
a negative integer indicating how far down
the call stack the possible warning message appears to come from. |
| allowed | a character vector specifying the names of arguments
in ... that should not cause a warning. |
Details
In many cases, you want to evaluate all of the ... arguments.
list(...) is the usual way to do this.
Methods of generic functions can call chkDots(...) to warn
users that this method is ignoring the extra arguments.
Value
- ...length() returns the number of ... arguments given to a function.
- ...elt{n} returns the evaluated n'th of the ... arguments.
- ...names{} returns the names attached to the ... arguments, with NA for the untagged arguments.
- chkDots(...) returns NULL, invisibly.
...elt, ...names, and ...length give an error if there are no ... arguments in scope.
See Also
Examples
f1 <- function(n, ...) ...length()
f1(stop(10), stop(20), stop(30))
f2 <- function(n, ...) ...elt(n)
f2(2, stop(20), 1+2+3)
# find the tags of the ... arguments without evaluating them
f3 <- function(n, ...) {
dots <- substitute(...())
names(dots)
}
f3(n=2, X=stop(X), stop(Y), Z=stop(Z))
f4 <- function(n, ...) {
...names()
}
f4(n=2, X=stop(X), stop(Y), Z=stop(Z))