Using two lists in parallel has long been a way to represent associative data. where one set of data is closely related to another. We associate the number 28 with the month February by putting them in matching positions in two different lists.

I could do this with any kind of association -- such as one list has student names, one has semester standing, one has exam grades, and so on. Unfortunately, a serious problem may arise if I were to try to add or remove a student from the roster, and forget to change the other lists. What might that problem be?