missing can be used to test whether a value was specified
as an argument to a function.
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
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.
substitute for argument expression;
NA for missing values in data.