missing                 package: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 indexes.

     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.

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)
             }

