# dots: ..., '..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 it 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 `...`. This is the same as `length(list(...))` but without evaluating the expressions in `...` (which happens with `list(...)`).

## Usage

 ```1 2``` ```...length() ...elt(n) ```

## Arguments

 `n` a positive integer, not larger than the number of expressions in ..., which is the same as `...length()` which is the same as `length(list(...))`, but the latter evaluates all expressions in `...`.

`...` and `..1`, `..2` are reserved words in R, see `Reserved`.
 ``` 1 2 3 4 5 6 7 8 9 10``` ```tst <- function(n, ...) ...elt(n) tst(1, pi=pi*0:1, 2:4) ##  0.000000 3.141593 tst(2, pi=pi*0:1, 2:4) ##  2 3 4 try(tst(1)) # -> Error about '...' not containing an element. tst.dl <- function(x, ...) ...length() 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! ```