missing: Does a Formal Argument have a Value?
In robertzk/monadicbase: The R Base Package

Description Usage Arguments Details References See Also Examples

missing can be used to test whether a value was specified as an argument to a function.

1	missing(x)

`x`	a formal argument.

missing(x) is only reliable if x has not been altered since entering the function: in particular it will always be false after x <- match.arg(x).

The example shows how a plotting function can be written to work with either a pair of vectors giving x and y coordinates of points to be plotted or a single vector giving y values to be plotted against their indices.

Currently missing can only be used in the immediate body of the function that defines the argument, not in the body of a nested function or a local call. This may change in the future.

This is a ‘special’ primitive function: it must not evaluate its argument.

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

Chambers, J. M. (1998) Programming with Data. A Guide to the S Language. Springer.

substitute for argument expression; NA for missing values in data.

myplot <- function(x, y) {
                if(missing(y)) {
                        y <- x
                        x <- 1:length(y)
                }
                plot(x, y)
        }