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 indices.
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.
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