A function is an object you can call. Basically, it is a machine with internal logic that takes a group of inputs (parameters or arguments) and returns a value as output.
In the previous sections, we encountered some built-in functions of R. For example,
is.numeric() takes an argument that can be any R object and returns a logical value that indicates whether the object is a numeric vector. Similarly,
is.function() can tell whether a given R object is a function object.
In fact, in R environment, everything we use is an object, everything we do is a function, and, maybe to your surprise, all functions are still objects. Even
+ are both functions that take two arguments. Although they are called binary operators, they are essentially ...