missing can be used to test whether a value was specified as an argument to a function.
- a formal argument.
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.
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.
Documentation reproduced from R 3.0.2. License: GPL-2.