\Header{aggregate}{Compute Summary Statistics of Data Subsets}
\methalias{aggregate.data.frame}{aggregate}
\methalias{aggregate.default}{aggregate}
\methalias{aggregate.ts}{aggregate}
\keyword{category}{aggregate}
\keyword{array}{aggregate}
\begin{Description}\relax
Splits the data into subsets, computes summary statistics for each,
and returns the result in a convenient form.
\end{Description}
\begin{Usage}
\begin{verbatim}
aggregate(x, ...)

## Default S3 method:
aggregate(x, ...)

## S3 method for class 'data.frame':
aggregate(x, by, FUN, ...)

## S3 method for class 'ts':
aggregate(x, nfrequency = 1, FUN = sum, ndeltat = 1,
          ts.eps = getOption("ts.eps"), ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] an R object.
\item[\code{by}] a list of grouping elements, each as long as the variables
in \code{x}.  Names for the grouping variables are provided if
they are not given.  The elements of the list will be coerced to
factors (if they are not already factors).
\item[\code{FUN}] a scalar function to compute the summary statistics which
can be applied to all data subsets.
\item[\code{nfrequency}] new number of observations per unit of time; must
be a divisor of the frequency of \code{x}.
\item[\code{ndeltat}] new fraction of the sampling period between
successive observations; must be a divisor of the sampling
interval of \code{x}.
\item[\code{ts.eps}] tolerance used to decide if \code{nfrequency} is a
sub-multiple of the original frequency.
\item[\code{...}] further arguments passed to or used by methods.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{aggregate} is a generic function with methods for data frames
and time series.

The default method \code{aggregate.default} uses the time series
method if \code{x} is a time series, and otherwise coerces \code{x}
to a data frame and calls the data frame method.

\code{aggregate.data.frame} is the data frame method.  If \code{x}
is not a data frame, it is coerced to one.  Then, each of the
variables (columns) in \code{x} is split into subsets of cases
(rows) of identical combinations of the components of \code{by}, and
\code{FUN} is applied to each such subset with further arguments in
\code{...} passed to it.
(I.e., \code{tapply(VAR, by, FUN, ..., simplify = FALSE)} is done
for each variable \code{VAR} in \code{x}, conveniently wrapped into
one call to \code{lapply()}.)
Empty subsets are removed, and the result is reformatted into a data
frame containing the variables in \code{by} and \code{x}.  The ones
arising from \code{by} contain the unique combinations of grouping
values used for determining the subsets, and the ones arising from
\code{x} the corresponding summary statistics for the subset of the
respective variables in \code{x}.

\code{aggregate.ts} is the time series method.  If \code{x} is not a
time series, it is coerced to one.  Then, the variables in \code{x}
are split into appropriate blocks of length
\code{frequency(x) / nfrequency}, and \code{FUN} is applied to each
such block, with further (named) arguments in \code{...} passed to
it.  The result returned is a time series with frequency
\code{nfrequency} holding the aggregated values.
\end{Details}
\begin{Author}\relax
Kurt Hornik
\end{Author}
\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{SeeAlso}\relax
\code{\Link{apply}}, \code{\Link{lapply}}, \code{\Link{tapply}}.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
data(state)

## Compute the averages for the variables in 'state.x77', grouped
## according to the region (Northeast, South, North Central, West) that
## each state belongs to.
aggregate(state.x77, list(Region = state.region), mean)

## Compute the averages according to region and the occurrence of more
## than 130 days of frost.
aggregate(state.x77,
          list(Region = state.region,
               Cold = state.x77[,"Frost"] > 130),
          mean)
## (Note that no state in 'South' is THAT cold.)

data(presidents)
## Compute the average annual approval ratings for American presidents.
aggregate(presidents, nf = 1, FUN = mean)
## Give the summer less weight.
aggregate(presidents, nf = 1, FUN = weighted.mean, w = c(1, 1, 0.5, 1))
\end{ExampleCode}
\end{Examples}

