In this section, you are going to learn about two very useful functions to apply an operation on the subsets of data. The two functions,
apply, along with a few others, form a collection of functions called
apply functions. The functions in the collection are used to apply (hence the name) a function we choose over subsets of an object, and then join the results to form a single object once again. The
apply functions are a defining feature of R; they replace the necessity to write explicit loops in many common situations in data analysis, which makes the code shorter and more elegant.
tapply function is used to apply a function ...