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

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)
        }