# missing

##### Does a Formal Argument have a Value?

`missing`

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

- Keywords
- programming

##### 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, 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.

##### See Also

`substitute`

for argument expression;
`NA`

for missing values in data.

##### Examples

`library(base)`

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

*Documentation reproduced from package base, version 3.0.3, License: Part of R 3.0.3*