Omitted topics in lesson 4

[jrmcgarvey]

Lesson 4 has a series of omissions. Various pandas features are required in the assignment, but nowhere explained in the lesson.

It starts by asking the student to install pandas. This is not necessary for the kaggle notebook. Pandas is also installed when the student sets up their python_homework folder.

Sorting of a dataframe is not explained, although it appears in the assignment. This should be added.

It might be good to mention fillna() again. It is described in lesson 3, but not used in that assignment. It could be illustrated along with the outer join.

There is no explanation of what a pivot table is, or how to create one. This must be added. There is a hint in the assignment, but there should be an explanation of the concept in the lesson, with examples.

There is no explanation of what a cross tabulation is, or how to create one. Again, the concept should be introduced, along with an example.

There isn't a generalized explanation of data transformation. In part, this goes back to issue 14, in that vector operations on series are not explained. But there are several ways to transform a dataframe, and each should be discussed with an example.