ABSTRACT
So far, we have been writing code to handle specific situations, such as subsetting a single data set, often using other people's functions. In cases where you want to reuse the code, it is unwise to simply copy and paste the code and make minor changes to handle the new data. Instead we want something that is able to take multiple values and perform the same action (subset, aggregate, make a plot, webscrape, etc.) on those values. We've used lots of other people's function throughout this book, and in this chapter we'll learn how to create our own.
