Unit 7: Arrays

Every program so far has handled one object at a time or a handful of separately named ones. This unit gives students one name for many values, including many objects. It covers declaring and filling an array, reading and writing one slot, the index that does not exist, walking an array with a loop and with for-each, totals and counts and highest and lowest, an array of objects with the count you have to keep yourself, searching from the front, searching a sorted array faster, putting an array in order, and then two dimensional arrays, one cell at a time, printed as a grid and totalled by row and column.

The performance task is The Seating Chart. Students model a real physical grid at least four by four that holds objects of a class they wrote, containing at least one empty cell, then print a report a person can read, search it for a case in the interior, a case on an edge and a case that is not there, and produce a sorted list with the criterion stated. The heaviest criterion is correctness at the boundaries, proven by a completed boundary test table that includes the empty cell and one deliberate index one past the end with the exact machine message recorded.

The fixed length problem is confronted in Lesson 2 rather than discovered. Students put a Python list beside a Java array, try to add an eleventh value to an array of ten, record exactly what the machine says, and write in their own words what a Java array will not do. Every later lesson that hits the limit names it again and names Unit 8. The Arrays utility class is named as something that exists and is not used, because a student who calls a sort method has not developed a sorting algorithm.

The unit runs 18 instructional days.