xtabs

xtabs is a base R function that creates contingency tables from a formula and a data frame. It builds cross-tabulations of one or more factors and returns the result as an array (class "table"). The function is especially convenient for turning data with a frequency column into multi-dimensional tables or for generating tables directly from a formula interface.

The typical usage uses a formula to specify the variables that define the table. The right-hand side lists the factor variables that form the dimensions of the table, while the left-hand side (if present) can supply observation weights. For example, using xtabs(Freq ~ Class + Sex + Survived, data = Titanic) uses the Freq column as cell counts to produce a three-dimensional table of Class, Sex, and Survived. If the left-hand side is omitted as ~ A + B, xtabs counts each row as one observation, producing a plain two-way table of A and B.

Return value and handling: xtabs returns an array with dimensions corresponding to the levels of the involved factors. The object is typically printed as a table and can be converted to other forms with as.data.frame or aggregated with margin.table or addmargins. Missing values are handled according to the na.action setting, and the function integrates with the standard R formula and data frame workflow.

Related concepts: xtabs is related to table and ftable, and can be used in conjunction with functions like margin.table and addmargins to compute row, column, or higher-dimensional margins. It is a common tool for exploratory data analysis and for preparing data for statistical modeling.