Let's start by reviewing the data format of the MovieLens dataset, the u.data file.
As you might recall, the u.data file on each line, consists of a user ID, a movie ID, a rating, and a timestamp. Each line says, "this user watched this movie, gave it this rating, and did it at this time":

Our task is to just figure out which movie was watched most often or which movie ID appears most frequently in the entire dataset. This isn't a very hard thing to do; in fact, if you want to go give it a crack yourself, feel free. In this section we'll take a look at the implementation that I came up with, get that to run, and see what we come up with