The Art of R Programming

Common Functions Used with Factors

With factors, we have yet another member of the family of apply functions, tapply. We’ll look at that function, as well as two other functions commonly used with factors: split() and by().

The tapply() Function

As motivation, suppose we have a vector x of ages of voters and a factor f showing some nonumeric trait of those voters, such as party affiliation (Democrat, Republican, Unaffiliated). We might wish to find the mean ages in x within each of the party groups.

In typical usage, the call tapply(x,f,g) has x as a vector, f as a factor or list of factors, and g as a function. The function g() in our little example above would be R’s built-in mean() function. If we wanted to group by both party and another factor, ...