ftable {base}R Documentation

Flat Contingency Tables

Description

Create and manipulate ``flat'' contingency tables.

Usage

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"))

Arguments

... 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 "table" or "ftable".
exclude values to use in the exclude argument of factor when interpreting non-factor objects.
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.
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.
x an arbitrary R object.
file a character string giving the name of the file which the data are to be read from or written to.
sep the field separator string. Values on each line of the file are separated by this string.
quote a character string giving the set of quoting characters for read.ftable; to disable quoting altogether, use quote="". For write.table, a logical indicating whether strings in the data will be surrounded by double quotes.
row.var.names a character vector with the names of the row variables, in case these cannot be determined automatically.
col.vars a list giving the names and levels of the column variables, in case these cannot be determined automatically.
skip the number of lines of the data file to skip before beginning to read data.
digits an integer giving the number of significant digits to use for (the cell entries of) x.

Details

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 row.vars and 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 print.ftable, the print method for objects of class "ftable") is often preferable to showing it as a higher-dimensional array.

ftable is a generic function. Its default method, ftable.default, first creates a contingency table in array form from all arguments except row.vars and col.vars. If the first argument is of class "table", it represents a contingency table and is used as is; if it is a flat table of class "ftable", the information it contains is converted to the usual array representation using 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 table. Then, the arguments row.vars and 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 ftable.formula provides a formula method for creating flat contingency tables.

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 as.table.

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

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 skip, and providing the names of the row variables and the names and levels of the column variable using row.var.names and 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 read.table to read in the data, and create the contingency table from this using xtabs.

Value

ftable returns an object of class "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 "row.vars" and "col.vars".

References

Agresti, A. (1990) Categorical data analysis. New York: Wiley.

See Also

ftable.formula for the formula interface (which allows a data = . argument); table for ``ordinary'' cross-tabulation.

Examples

## 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)