\HeaderA{switch}{Select One of a List of Alternatives}{switch}
\keyword{programming}{switch}
\begin{Description}\relax
\code{switch} evaluates \code{EXPR} and accordingly chooses one of the
further arguments (in \code{...}).
\end{Description}
\begin{Usage}
\begin{verbatim}
switch(EXPR, ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{EXPR}] an expression evaluating to a number or a character
string.
\item[\code{...}] the list of alternatives, given explicitly.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
If the value of \code{EXPR} is an integer between 1 and
\code{nargs()-1} then the corresponding element of \code{...} is
evaluated and the result returned.

If \code{EXPR} returns a character string then that string is used to
match the names of the elements in \code{...}.  If there is an exact
match then that element is evaluated and returned if there is one,
otherwise the next element is chosen, e.g.,
\code{switch("cc", a=1, cc=, d=2)} evaluates to \code{2}.

In the case of no match, if there's a further argument in \code{switch}
that one is returned, otherwise \code{NULL}.
\end{Details}
\begin{Section}{Warning}
Beware of partial matching: an alternative \code{E = foo} will match
the first argument \code{EXPR} unless that is named. See the examples
for good practice in naming the first argument.
\end{Section}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.
\end{References}
\begin{Examples}
\begin{ExampleCode}
require(stats)
centre <- function(x, type) {
  switch(type,
        mean = mean(x),
        median = median(x),
        trimmed = mean(x, trim = .1))
}
x <- rcauchy(10)
centre(x, "mean")
centre(x, "median")
centre(x, "trimmed")

ccc <- c("b","QQ","a","A","bb")
for(ch in ccc)
    cat(ch,":",switch(EXPR = ch, a=1,     b=2:3), "\n")
for(ch in ccc)
    cat(ch,":",switch(EXPR = ch, a=, A=1, b=2:3, "Otherwise: last"),"\n")

## Numeric EXPR don't allow an 'otherwise':
for(i in c(-1:3,9))  print(switch(i, 1,2,3,4))
\end{ExampleCode}
\end{Examples}

