Just for the record, following Bill Dunlap's advice, I think this is
the best answer to the question as originally posed is.
myfun <- function(vec, i=stop("'i' must be supplied")){
vec[i]
}
> myfun(1:40,10)
[1] 10
> myfun(1:10)
Error in myfun(1:10) : 'i' must be supplied
>
--
Paul E. Johnson
Professor, Political Science
1541 Lilac Lane, Room 504
University of Kansas
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