S4 Classes and Methods

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R has three object oriented (OO) systems: [[S3]], [[S4]] and [[R5]]. This page describes S4.

Compared to S3, the S4 object system is much stricter, and much closer to other OO systems. I recommend you familiarise yourself with the way that [[S3]] works before reading this document - many of underlying ideas are the same, but the implementation is much stricter. There are two major differences from S3:

formal class definitions: unlike S3, S4 formally defines the representation and inheritance for each class

multiple dispatch: the generic function can be dispatched to a method based on the class of any number of argument, not just one

Here we introduce the basics of S4, trying to stay away from the esoterica and focussing on the ideas that you need to understand and write the majority of S4 code.

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S4 Classes and Methods

y ~ normal(x * beta, sigma);

there is no longer an intercept coefficient alpha. Instead, we have assumed that the first column of the input matrix x is a column of 1 values. This way, beta[1] plays the role of the intercept. If the intercept gets a different prior than the slope terms, then it would be clearer to break it out. It is also slightly more efficient in its explicit form with the intercept variable singled out because there’s one fewer multiplications; it should not make that much of a difference to speed, though, so the choice should be based on clarity.

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