[R] NULL or NA for missing function arguments?

[Duncan Murdoch]

On 10/16/2006 5:06 PM, Brahm, David wrote:
> Hans-Peter <gchappi at gmail.com> wrote:
>> I am troubled by the use of NULL or NA to indicate
>> missing/non-specified function arguments.
> 
> I suggest using NULL for arguments which are vectors or lists of
> unspecified length, and NA for "scalars" (arguments whose length
> should always be one, such as na.rm).  I rarely use missing arguments,
> as they are harder to pass down to other functions.

To be a little more precise:  they are easy to pass down, but hard to do 
anything else with.  For example:

> f <- function(x) if (missing(x)) print("missing!") else print(x)
> g <- function(x) f(x)
> h <- function(x) { y <- x; f(y) }
> g()
  [1] "missing!"
> h()
  Error in h() : argument "x" is missing, with no default

Duncan Murdoch