nargs: The Number of Arguments to a Function
In robertzk/monadicbase: The R Base Package

Description Usage Details References See Also Examples

When used inside a function body, nargs returns the number of arguments supplied to that function, including positional arguments left blank.

nargs()

The count includes empty (missing) arguments, so that foo(x,,z) will be considered to have three arguments (see ‘Examples’). This can occur in rather indirect ways, so for example x[] might dispatch a call to `[.some_method`(x, ) which is considered to have two arguments.

This is a primitive function.

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

args, formals and sys.call.

tst <- function(a, b = 3, ...) {nargs()}
tst() # 0
tst(clicketyclack) # 1 (even non-existing)
tst(c1, a2, rr3) # 3

foo <- function(x, y, z, w) {
   cat("call was ", deparse(match.call()), "\n", sep = "")
   nargs()
}
foo()      # 0
foo(, , 3) # 3
foo(z = 3) # 1, even though this is the same call

nargs()  # not really meaningful