\Header{ftable}{Flat Contingency Tables}
\alias{as.table.ftable}{ftable}
\methalias{ftable.default}{ftable}
\alias{print.ftable}{ftable}
\alias{read.ftable}{ftable}
\alias{write.ftable}{ftable}
\keyword{category}{ftable}
\begin{Description}\relax
Create and manipulate ``flat'' contingency tables.\end{Description}
\begin{Usage}
\begin{verbatim}
ftable(..., exclude = c(NA, NaN), row.vars = NULL, col.vars = NULL)
as.table.ftable(x)
read.ftable(file, sep = "", quote = "\"",
            row.var.names, col.vars, skip = 0)
write.ftable(x, file = "", quote = TRUE, digits = getOption("digits"))
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{...}] \R{} objects which can be interpreted as factors (including
character strings), or a list (or data frame) whose components can
be so interpreted, or a contingency table object of class
\code{"table"} or \code{"ftable"}.
\item[\code{exclude}] values to use in the exclude argument of \code{factor}
when interpreting non-factor objects.
\item[\code{row.vars}] a vector of integers giving the numbers of the
variables, or a character vector giving the names of the variables
to be used for the rows of the flat contingency table.
\item[\code{col.vars}] a vector of integers giving the numbers of the
variables, or a character vector giving the names of the variables
to be used for the columns of the flat contingency table.
\item[\code{x}] an arbitrary \R{} object.
\item[\code{file}] a character string giving the name of the file which the
data are to be read from or written to.
\item[\code{sep}] the field separator string.  Values on each line of the file
are separated by this string.
\item[\code{quote}] a character string giving the set of quoting characters
for \code{read.ftable}; to disable quoting altogether, use
\code{quote=""}.  For \code{write.table}, a logical indicating
whether strings in the data will be surrounded by double quotes.
\item[\code{row.var.names}] a character vector with the names of the row
variables, in case these cannot be determined automatically.
\item[\code{col.vars}] a list giving the names and levels of the column
variables, in case these cannot be determined automatically.
\item[\code{skip}] the number of lines of the data file to skip before
beginning to read data.
\item[\code{digits}] an integer giving the number of significant digits to
use for (the cell entries of) \code{x}.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{ftable} creates ``flat'' contingency tables.  Similar to the
usual contingency tables, these contain the counts of each combination
of the levels of the variables (factors) involved.  This information
is then re-arranged as a matrix whose rows and columns correspond to
unique combinations of the levels of the row and column variables (as
specified by \code{row.vars} and \code{col.vars}, respectively).  The
combinations are created by looping over the variables in reverse
order (so that the levels of the ``left-most'' variable vary the
slowest).  Displaying a contingency table in this flat matrix form
(via \code{print.ftable}, the print method for objects of class
\code{"ftable"}) is often preferable to showing it as a
higher-dimensional array.

\code{ftable} is a generic function.  Its default method,
\code{ftable.default}, first creates a contingency table in array
form from all arguments except \code{row.vars} and \code{col.vars}.
If the first argument is of class \code{"table"}, it represents a
contingency table and is used as is; if it is a flat table of class
\code{"ftable"}, the information it contains is converted to the usual
array representation using \code{as.ftable}.  Otherwise, the arguments
should be \R{} objects which can be interpreted as factors (including
character strings), or a list (or data frame) whose components can be
so interpreted, which are cross-tabulated using \code{\Link{table}}.
Then, the arguments \code{row.vars} and \code{col.vars} are used to
collapse the contingency table into flat form.  If neither of these
two is given, the last variable is used for the columns.  If both are
given and their union is a proper subset of all variables involved,
the other variables are summed out.

Function \code{\Link{ftable.formula}} provides a formula method for
creating flat contingency tables.

\code{as.table.ftable} converts a contingency table in flat matrix
form to one in standard array form.  This is a method for the generic
function \code{as.table}.

\code{write.ftable} writes a flat table to a file, which is useful for
generating ``pretty'' ASCII representations of contingency tables.

\code{read.ftable} reads in a flat-like contingency table from a
file.  If the file contains the written representation of a flat
table (more precisely, a header with all information on names and
levels of column variables, followed by a line with the names of the
row variables), no further arguments are needed.  Similarly, flat
tables with only one column variable the name of which is the only
entry in the first line are handled automatically.  Other variants can
be dealt with by skipping all header information using \code{skip},
and providing the names of the row variables and the names and levels
of the column variable using \code{row.var.names} and \code{col.vars},
respectively.  See the examples below.

Note that flat tables are characterized by their ``ragged'' display of
row (and maybe also column) labels.  If the full grid of levels of the
row variables is given, one should instead use \code{read.table} to
read in the data, and create the contingency table from this using
\code{\Link{xtabs}}.\end{Details}
\begin{Value}
\code{ftable} returns an object of class \code{"ftable"}, which is a
matrix with counts of each combination of the levels of variables with
information on the names and levels of the (row and columns) variables
stored as attributes \code{"row.vars"} and \code{"col.vars"}.\end{Value}
\begin{References}\relax
Agresti, A.  (1990)
\emph{Categorical data analysis.}
New York: Wiley.\end{References}
\begin{SeeAlso}\relax
\code{\Link{ftable.formula}} for the formula interface (which allows a
\code{data = .} argument);
\code{\Link{table}} for ``ordinary'' cross-tabulation.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
## Start with a contingency table.
data(Titanic)
ftable(Titanic, row.vars = 1:3)
ftable(Titanic, row.vars = 1:2, col.vars = "Survived")
ftable(Titanic, row.vars = 2:1, col.vars = "Survived")

## Start with a data frame.
data(mtcars)
x <- ftable(mtcars[c("cyl", "vs", "am", "gear")])
x
ftable(x, row.vars = c(2, 4))

## Agresti (1990), page 157, Table 5.8.
## Not in ftable standard format, but o.k.
file <- tempfile()
cat("             Intercourse\n",
    "Race  Gender     Yes  No\n",
    "White Male        43 134\n",
    "      Female      26 149\n",
    "Black Male        29  23\n",
    "      Female      22  36\n",
    file = file)
file.show(file)
ft <- read.ftable(file)
ft
unlink(file)

## Agresti (1990), page 297, Table 8.16.
## Almost o.k., but misses the name of the row variable.
file <- tempfile()
cat("                      \"Tonsil Size\"\n",
    "            \"Not Enl.\" \"Enl.\" \"Greatly Enl.\"\n",
    "Noncarriers       497     560           269\n",
    "Carriers           19      29            24\n",
    file = file)
file.show(file)
ft <- read.ftable(file, skip = 2,
                  row.var.names = "Status",
                  col.vars = list("Tonsil Size" =
                      c("Not Enl.", "Enl.", "Greatly Enl.")))
ft
unlink(file)
\end{ExampleCode}
\end{Examples}

