My purpose is indeed to write a '[' method.
I will go for the `[.data.frame` solution then.
Thank you.
On 25/01/2011 12:53, Prof Brian Ripley wrote:
On Tue, 25 Jan 2011, Renaud Gaujoux wrote:
Hi,
is there an easy, robust, and/or recommended way to distinguish a
missing argument from an empty argument as in:
An empty argument is a missing argument when argument matching is
done, e.g.
foo <- function(i,j) match.call()
foo(i)
foo(i = i)
foo(i,)
foo(i = i)
foo(,j)
foo(j = j)
It is rather against the spirit of R to use the actual call rather
than the matched call. Unless you are doing this to write a '['
method I would suggest you find a different convention, e.g.
distinguish f(i) and f(i, NULL). For the exception, look at
`[.data.frame`, which does use nargs().
(NB: what I have said does not apply to primitives like '[' itself,
which do not do standard argument matching.)
foo <- function(i, j){
print(missing(j))
print(nargs())
}
foo(i) # TRUE, 1
foo(i,) # TRUE, 2
I know I can work around with nargs, the list of arguments and the
names of the passed arguments, but I wish there is something already
in place for this.
This is specially important for '['-like methods where x[i,] is not
the same as x[i].
What I am looking for is a function that tells me if an argument has
actually been passed empty:
foo <- function(i, j, k){
print( empty.arg(j) )
print(nargs())
}
would result in:
foo(i) # FALSE, 1
foo(i, ) # TRUE, 2
foo(i, j) # FALSE, 2
foo(i, k=2) # FALSE, 2
foo(i, k=2, ) # TRUE, 3
Thank you for any help or pointer.
Bests,
Renaud
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