What does "S3 methods" mean in R?

Question

Since I am fairly new to R, I do not know what the S3 methods and objects are. I found that there are S3 and S4 object systems, and some recommend to use S3 over S4 if possible (See Google's R Style Guide at http://google-styleguide.googlecode.com/svn/trunk/google-r-style.html)*. However, I do not know the exact definition of S3 methods/objects.

Update: As of 2019, Google's R Style Guide hyperlink is now here.

Answer 1

Most of the relevant information can be found by looking at ?S3 or ?UseMethod, but in a nutshell:

S3 refers to a scheme of method dispatching. If you've used R for a while, you'll notice that there are print, predict and summary methods for a lot of different kinds of objects.

In S3, this works by:

• setting the class of objects of interest (e.g.: the return value of a call to method glm has class glm)
• providing a method with the general name (e.g. print), then a dot, and then the classname (e.g.: print.glm)
• some preparation has to have been done to this general name (print) for this to work, but if you're simply looking to conform yourself to existing method names, you don't need this (see the help I refered to earlier if you do).

To the eye of the beholder, and particularly, the user of your newly created funky model fitting package, it is much more convenient to be able to type predict(myfit, type="class") than predict.mykindoffit(myfit, type="class").

There is quite a bit more to it, but this should get you started. There are quite a few disadvantages to this way of dispatching methods based upon an attribute (class) of objects (and C purists probably lie awake at night in horror of it), but for a lot of situations, it works decently. With the current version of R, newer ways have been implemented (S4 and reference classes), but most people still (only) use S3.