Creating an S3 method (2)
If no suitable method is found for a generic, then an error is thrown. For example, at the moment, get_n_elements() only has a method available for data.frames. If you pass a matrix to get_n_elements() instead, you'll see an error.
> get_n_elements(matrix())
Error: no applicable method for 'get_n_elements' applied to an object of class "c('matrix', 'array', 'logical')"
Rather than having to write dozens of methods for every kind of input, you can create a method that handles all types that don't have a specific method. This is called the default method; it always has the name generic.default. For example, print.default() (docs will print any type of object that doesn't have its own print() (docs) method.
Deze oefening maakt deel uit van de cursus
Object-Oriented Programming with S3 and R6 in R
Oefeninstructies
The generic get_n_elements() function and a method for data.frames have been defined in your workspace.
- Use
ls.str()(docs) to explore your workspace. - Write a S3 default method for
get_n_elements().- The name of the function should be the name of the generic, then a
.thendefault. - The input arguments should be
xand.... - The body of the function should be a single line, returning the number of elements in an arbitrary object. Use the same technique as with the chess game in the Make it Classy (1) exercise.
- The name of the function should be the name of the generic, then a
- Call your method on the
ability.cov(docs) dataset and assign the result to the variablen_elements_ability.cov.
Praktische interactieve oefening
Probeer deze oefening eens door deze voorbeeldcode in te vullen.
# View predefined objects
___
# Create a default method for get_n_elements
___ <- ___
# Call the method on the ability.cov dataset
n_elements_ability.cov <- ___