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 functions.
When we do casual, interactive data analysis, at times, we won't have to write any function on our own since the built-in functions and those provided by thousands of packages are usually enough.
However, if you need to repeat your...