## Kernel Class

Similarly to implemented probability distributions, implemented kernels all inherit from a single class ‘kernel’, which in turn inherits from Distribution (see the uml diagram). But this is a much simpler class with reduced methods and many of these return the same result and are therefore defined in the kernel parent class. For every kernel the following is true

• kernel$mean() == 0 • kernel$median() == 0
• kernel$mode() == 0 • kernel$type() == Reals$new() • kernel$valueSupport() == continuous
• kernel$variateForm() == univariate And for all kernels the sampling method uses inverse transform sampling, so this is also defined in the parent class. Hence the only methods required to add are • pdfSquared2Norm: This is new for kernels (i.e. not given for probability distributions) • variance And then the d/p/q functions are given in the constructor just like probability distributions. ## Creating a Kernel ### Kernel Variables These are identical to the SDistribution public variables: • name - Full (unique) name of kernel • short_name - Short name (unique) id for kernel • description - Short description, usually just the name • package - The package in which the d/p/q functions are written For the Epanechnikov kernel, the above all looks like Epanechnikov <- R6::R6Class("Epanechnikov", inherit = Kernel, lock_objects = F) Epanechnikov$set("public","name","Epanechnikov")
Epanechnikov$set("public","short_name","Epan") Epanechnikov$set("public","description","Epanechnikov Kernel")

Note:

1. Again note the use of lock_objects = F which ensures correct usage with decorators.
2. The package variable is omitted as it defaults to ‘distr6’

### Kernel Methods

As stated above, there are fewer methods that need to be implemented in kernels than probability distributions. These include

• variance
• pdfSquared2Norm: the squared 2-norm of the kernel’s pdf over the full support

Again the d/p/q methods are implemented in the constructor. So for the Epanechnikov kernel,

Epanechnikov$set("public","pdfSquared2Norm",function(){ return(3/5) }) Epanechnikov$set("public","variance",function(){
return(1/5)
})

Note:

1. These methods will always return constants, they are defined as methods not variables for consistency
2. If no constant numeric is available, omit the method so a numeric one can be added after decoration
3. As well as the methods listen above, the following methods are also included by default and don’t need to be defined
• prec
• stdev
• iqr

### The Constructor

The constructor for kernels is much more simple than that of probability distributions and only need include the d/p/q methods and properties. For the Epanechnikov kernel:

Epanechnikov$set("public","initialize",function(decorators = NULL){ pdf <- function(x1){ return(0.75 * (1-x1^2)) } cdf <- function(x1){ return(3/4*x1 - 1/4*x1^3 + 1/2) } super$initialize(decorators = decorators, pdf = pdf, cdf = cdf,
support = set6::Interval$new(-1, 1), symmetric = TRUE) invisible(self) }) Note: 1. We omit the quantile method as no closed form analytic expression was found 2. The support will generally either be Reals$new() or Interval\$new(-1,1)
3. Kernels are all symmetric
4. The constructor always takes one argument only, decorators which is passed to the parent-class constructor

## Summary

Kernels are much simpler to extend than SDistributions. Just remember the following

1. The 4 public variables: name, short_name, description, package
2. The 2 public methods: variance, pdfSquared2Norm
3. The constructor: Includes d/p/q, the only argument is decorators, and the properties are generally identical