missing {base} | R Documentation |
Does a Formal Argument have a Value?
Description
missing
can be used to test whether a value was specified
as an argument to a function.
Usage
missing(x)
Arguments
x |
a formal argument. |
Details
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.
References
Becker RA, Chambers JM, Wilks AR (1988). The New S Language. Chapman and Hall/CRC, London.
Chambers JM (1998). Programming with Data. Springer, New York. ISBN 978-0-387-98503-9.
See Also
substitute
for argument expression;
NA
for missing values in data.
Examples
myplot <- function(x, y) {
if(missing(y)) {
y <- x
x <- 1:length(y)
}
plot(x, y)
}