Andrew Treadway covers a few of the lesser-known apply functions in R:
rapply
Let’s start with rapply. This function has a couple of different purposes. One is to recursively apply a function to a list. We’ll get to that in a moment. The other use of rapply is to a apply a function to only those elements in a list (or columns in a data frame) that belong to a specified class. For example, let’s say we have a data frame with a mix of categorical and numeric variables, but we want to evaluate a function only on the numeric variables.
Click through for some examples of rapply as well as vapply and eapply. I’ve used rapply to get cardinality of each feature in a data frame but the other two are new to me. H/T R-bloggers