## Does a Formal Argument have a Value?

## Usage

    missing(x)

## Description

`missing` can be used to test whether a value was specified as an argument to a function. The following 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 indexes.

## See Also

`NA` for ``missing values'' in data.

## Examples

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

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