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 <- and + are both functions that take two arguments. Although they are called binary operators, they are essentially ...

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