Frequency tables from factors

Recall that a factor defines a partition into groups. Similarly a pair of factors defines a two way cross classification, and so on. The function table() allows frequency tables to be calculated from equal length factors. If there are k factor arguments, the result is a k-way array of frequencies.

Suppose, for example, that statef is a factor giving the state code for each entry in a data vector. The assignment 

> statefr <- table(statef)

gives in statefr a table of frequencies of each state in the sample. The frequencies are ordered and labelled by the levels attribute of the factor. This simple case is equivalent to, but more convenient than, 

> statefr <- tapply(statef, statef, length)

Further suppose that incomef is a factor giving a suitably defined "income class" for each entry in the data vector, for example with the cut() function: 

> factor(cut(incomes, breaks = 35+10*(0:7))) -> incomef

Then to calculate a two-way table of frequencies: 

> table(incomef,statef)
       statef
incomef   act nsw nt qld sa tas vic wa
(35,45]   1   1  0   1  0   0   1  0
(45,55]   1   1  1   1  2   0   1  3
(55,65]   0   3  1   3  2   2   2  1
(65,75]   0   1  0   0  0   0   1  0

Extension to higher-way frequency tables is immediate.