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