At any R Q&A site, you’ll frequently see an exchange like this one:
Q: How can I use a loop to […insert task here…] ?
A: Don’t. Use one of the apply functions.
So, what are these wondrous apply functions and how do they work? I think the best way to figure out anything in R is to learn by experimentation, using embarrassingly trivial data and functions.
If you fire up your R console, type “??apply” and scroll down to the functions in the base package, you’ll see something like this:
Let’s examine each of those.
Description: “Returns a vector or array or list of values obtained by applying a function to margins of an array or matrix.”
OK – we know about vectors/arrays and functions, but what are these “margins”? Simple: either the rows (1), the columns (2) or both (1:2). By “both”, we mean “apply the…