| % File src/library/base/man/missing.Rd |
| % Part of the R package, https://www.R-project.org |
| % Copyright 1995-2009 R Core Team |
| % Distributed under GPL 2 or later |
| |
| \name{missing} |
| \alias{missing} |
| \title{Does a Formal Argument have a Value?} |
| \usage{ |
| missing(x) |
| } |
| \description{ |
| \code{missing} can be used to test whether a value was specified |
| as an argument to a function. |
| } |
| \arguments{ |
| \item{x}{a formal argument.} |
| } |
| \details{ |
| \code{missing(x)} is only reliable if \code{x} has not been altered |
| since entering the function: in particular it will \emph{always} |
| be false after \code{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 \code{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 \code{local} call. This may change in the future. |
| |
| This is a \sQuote{special} \link{primitive} function: it must not |
| evaluate its argument. |
| } |
| \references{ |
| Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) |
| \emph{The New S Language}. |
| Wadsworth & Brooks/Cole. |
| |
| Chambers, J. M. (1998) |
| \emph{Programming with Data. A Guide to the S Language}. |
| Springer. |
| } |
| \seealso{ |
| \code{\link{substitute}} for argument expression; |
| \code{\link{NA}} for missing values in data. |
| } |
| \examples{ |
| myplot <- function(x, y) { |
| if(missing(y)) { |
| y <- x |
| x <- 1:length(y) |
| } |
| plot(x, y) |
| } |
| } |
| \keyword{programming} |