A new command, append, is used here. The first argument is a vector, and the function adds the following arguments to the end of the vector.

One way to define a method for a class is to use the UseMethod command to define a hierarchy of functions that will react appropriately. The UseMethod command will tell R to look for a function whose prefix matches the current function, and it searches for a suffix in order from the vector of class names. In other words a set of functions can be defined, and the function that is actually called will be determined by the class name of the first object in the list of arguments.

You first have to define a generic function to reserve the function name. The UseMethod command is then used to tell the R system to search for a different function. The search is based on the name of the function and the names of an object’s classes. The name of the functions have two parts separated by a ”.” where the prefix is the function name and the suffix is the name of a class.

That is a lot of verbiage to describe a relatively simple idea. A very basic example is given below:

In particular, we can use the extract function on stanfit objects to obtain the samples. extract extracts samples from the stanfit object as a list of arrays for parameters of interest, or just an array. In addition, S3 functions as.array, as.matrix, and as.data.frame are defined for stanfit objects (using help("as.array.stanfit") to check out the help document in R).