\HeaderA{groupGeneric}{S3 Group Generic Functions}{groupGeneric}
\aliasA{.Group}{groupGeneric}{.Group}
\aliasA{Complex}{groupGeneric}{Complex}
\aliasA{group generic}{groupGeneric}{group generic}
\aliasA{Math}{groupGeneric}{Math}
\methaliasA{Math.data.frame}{groupGeneric}{Math.data.frame}
\aliasA{Ops}{groupGeneric}{Ops}
\methaliasA{Ops.data.frame}{groupGeneric}{Ops.data.frame}
\aliasA{S3groupGeneric}{groupGeneric}{S3groupGeneric}
\aliasA{Summary}{groupGeneric}{Summary}
\methaliasA{Summary.data.frame}{groupGeneric}{Summary.data.frame}
\keyword{methods}{groupGeneric}
\begin{Description}\relax
Group generic methods can be defined for four pre-specified groups of
functions, \code{Math}, \code{Ops}, \code{Summary} and \code{Complex}.
(There are no objects of these names in base \R, but there are in the
\pkg{methods} package.)

A method defined for an individual member of the group takes
precedence over a method defined for the group as a whole.
\end{Description}
\begin{Usage}
\begin{verbatim}
## S3 methods for group generics have prototypes:
Math(x, ...)
Ops(e1, e2)
Complex(z)
Summary(..., na.rm = FALSE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x, z, e1, e2}] objects.
\item[\code{...}] further arguments passed to methods.
\item[\code{na.rm}] logical: should missing values be removed?
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
There are four \emph{groups} for which S3 methods can be written,
namely the \code{"Math"}, \code{"Ops"}, \code{"Summary"} and
\code{"Complex"} groups.  These are not \R{} objects, but methods can be
supplied for them and base \R{} contains \code{\LinkA{factor}{factor}},
\code{\LinkA{data.frame}{data.frame}} and \code{\LinkA{difftime}{difftime}} methods for
the first three groups.  (There is also a \code{\LinkA{ordered}{ordered}}
method for \code{Ops}, \code{\LinkA{POSIXt}{POSIXt}} and \code{\LinkA{Date}{Date}}
methods for \code{Math} and \code{Ops}, \code{\LinkA{package\_version}{package.Rul.version}}
methods for \code{Ops} and \code{Summary}, as well as a
\code{\LinkA{ts}{ts}} method for \code{Ops} in package \pkg{stats}.)

\Enumerate{
\item Group \code{"Math"}:
\Itemize{
\item
\code{abs}, \code{sign}, \code{sqrt}, \\
\code{floor}, \code{ceiling}, \code{trunc},\\
\code{round}, \code{signif}

\item \code{exp}, \code{log}, \\
\code{cos}, \code{sin}, \code{tan},\\
\code{acos}, \code{asin}, \code{atan}

\code{cosh}, \code{sinh}, \code{tanh},\\
\code{acosh}, \code{asinh}, \code{atanh}

\item
\code{lgamma}, \code{gamma}, \code{gammaCody},\\
\code{digamma}, \code{trigamma}

\item \code{cumsum}, \code{cumprod}, \code{cummax}, \code{cummin}
}
Members of this group dispatch on \code{x}.  Not all members will
accept more than one argument: most members have default methods
expecting one argument.  (Specifically, \code{atan} and the gamma
functions accept only one argument and \code{log}, \code{round} and
\code{signif} accept one or two.)

\item Group \code{"Ops"}:
\Itemize{
\item
\code{"+"}, \code{"-"}, \code{"*"}, \code{"/"},
\code{"\textasciicircum{}"}, \code{"\%\%"}, \code{"\%/\%"}

\item \code{"\&"}, \code{"|"}, \code{"!"}

\item \code{"=="}, \code{"!="},
\code{"<"}, \code{"<="}, \code{">="}, \code{">"}
}
This group contains both binary and unary operators (\code{+},
\code{-} and \code{!}): when a unary operator is encountered the
\code{Ops} method is called with one argument and \code{e2} is
missing.

The classes of both arguments are considered in dispatching any
member of this group.  For each argument its vector of classes is
examined to see if there is a matching specific (preferred) or
\code{Ops} method.  If a method is found for just one argument or
the same method is found for both, it is used.
If different methods are found, there is a warning about
\sQuote{incompatible methods}: in that case or if no method is found
for either argument the internal method is used.

\item Group \code{"Summary"}:
\Itemize{
\item \code{all}, \code{any}
\item \code{sum}, \code{prod}
\item \code{min}, \code{max}
\item \code{range}
}
Members of this group dispatch on the first argument supplied.

\item Group \code{"Complex"}:
\Itemize{
\item \code{Arg}, \code{Conj}, \code{Im}, \code{Mod}, \code{Re}
}
Members of this group dispatch on \code{z}.
}

Note that a method will used for either one of these groups or one of
its members \emph{only} if it corresponds to a \code{"class"}
attribute, as the internal code dispatches on \code{\LinkA{oldClass}{oldClass}}
and not on \code{\LinkA{class}{class}}.  This is for efficiency: having to
dispatch on, say, \code{Ops.integer} would be too slow.

The number of arguments supplied for primitive members of the
\code{"Math"} group generic methods is not checked prior to dispatch.

There is no lazy evaluation of arguments for group-generic functions.
\end{Details}
\begin{Section}{Technical Details}
The details of method dispatch and variables such as \code{.Generic}
are discussed in the help for \code{\LinkA{UseMethod}{UseMethod}}.  There are a
few small differences:
\Itemize{
\item For the operators of group \code{Ops}, the object
\code{.Method} is a length-two character vector with elements the
methods selected for the left and right arguments respectively.  (If
no method was selected, the corresponding element is \code{""}.)
\item Object \code{.Group} records the group used for dispatch (if
a specific method is used this is \code{""}).
}
\end{Section}
\begin{References}\relax
Appendix A, \emph{Classes and Methods} of\\
Chambers, J. M.  and Hastie, T. J. eds (1992)
\emph{Statistical Models in S.}
Wadsworth \& Brooks/Cole.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{methods}{methods}} for methods of non-Internal generic functions.

\LinkA{S4groupGeneric}{S4groupGeneric} for group generics for S4 methods.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
d.fr <- data.frame(x=1:9, y=rnorm(9))
class(1 + d.fr) == "data.frame" ##-- add to d.f. ...

methods("Math")
methods("Ops")
methods("Summary")
methods("Complex")  # none in base R
\end{ExampleCode}
\end{Examples}

