CMPS 144L
Spring 2019 Lab #3 (and Intersession 2020 Lab #2):
Red-White-Blue Partitioning of int[]

Resources

• RWB_PartitionerOfInt: abstract (and unfinished) class having a partition() method that rearranges the elements in a given array of type int[] into segments containing RED, WHITE, and BLUE values, respectively. A concrete child class must provide the body of the isRed(int), isWhite(int), and isBlue(int) methods, which decide into which category each int value falls.

  Your major task is to complete the partition() method of this class.

• Neg0PosPartitioner: concrete child class of RWB_PartitionerOfInt, it partitions arrays in accord with the classification of negative numbers as RED, zero as WHITE, and positive numbers as BLUE.
• Even_3Mult_Partitioner: concrete (and unfinished) child class of RWB_PartitionerOfInt, it partitions arrays in accord with the classification of even numbers as RED, non-even multiples of three as WHITE, and the remaining numbers as BLUE.

  Your secondary task is to complete this class.

• RWB_PartTester.java: Java application having as its purpose to test the RWB_PartitionerOfInt class and its child classes.

These classes are analogous to ones that were discussed in lecture (and to which you can find links starting at the course web page).

Major Task: Completing the partition() Method

An annoying byproduct of having three categories rather than only two is that, after an array has been partitioned, there are two boundaries between adjacent segments (the RED-WHITE and WHITE-BLUE boundaries) rather than only one (RED-BLUE). For the partition() method to return the locations of both boundaries would require that it return some kind of composite object (e.g., an array of two int's, or an OrderedPairOfInt object, perhaps) rather than a value of type int, as was the case with the partition() method discussed in lecture. Rather than having the method return a composite object, your instructor decided to introduce two new methods by which a client could obtain this information, one of which returns the location of the RED-WHITE boundary and the other of which returns the location of the WHITE-BLUE boundary.

To support this approach, it was necessary to introduce new instance variables, the names of which are rw and wb, intended to suggest "red-white" and "white-blue", respectively.

As the comments in the partition() method indicate, the code you supply must be consistent with the loop invariant depicted by this figure:

0 rw q wb N +-------------+-------------+--------------+--------------+ b | all RED | all WHITE | ? | all BLUE | +-------------+-------------+--------------+--------------+

Remember that rw and wb are instance variables, so they are not declared as local variables in the partition() method. On the other hand, q is declared inside partition(), as its value is meaningful only during execution of that method. (Its name is intended to suggest that it marks the starting location of the ?-segment of the array.)

You will notice that there are two assert statements in the partition() method; their purpose is to verify that the loop invariant is true immediately before and immediately after each iteration of the loop.

Before you run the "tester" program to test your work, make sure that you enable assertions (or else they will be ignored during execution).

Enabling Assertions

To enable assertions when running a program from the command line (using the java command), you need to include the -ea (or the more verbose -enableassertions) option. For example, you would enter

To enable assertions in jGrasp:

• Click on "Settings" in the bar of menu items.
• Hover over "Compiler Settings" on the dropdown menu, and click on "Workspace".
• On the window that appears, click on "Flags/Args".
• On the form that appears, go to the row labeled "Run" and in the text box in the second column (labeled "FLAGS2 OR ARGS2"), enter "-ea" (not including the quotes). (If you prefer, you can enter the longer version, which is "-enableassertions".) However, before you can enter text into the box, you'll have to click on the smaller box next to it, which has a black dot inside. Doing so should make the little black dot disappear, at which point you can enter text into the text box.
• Click on "Apply" (at bottom), and then click on "OK".

To verify that assertion testing is turned on, try running the AssertTest program. It should abort due to an AssertionError.

Secondary Task: Even_3Mult_Partitioner

Tertiary Task: partition2()

It should look much like the first version of the method, but it should be based upon the loop invariant depicted here:

0 rw wb q N +-------------+-------------+--------------+-----------+ b | all RED | all WHITE | all BLUE | ? | +-------------+-------------+--------------+-----------+

You should find that the loop body necessary to maintain this loop invariant is a little bit more complicated than that needed in your first version of partition(). Indeed, you will probably find that, during certain loop iterations, two swaps of array elements are called for rather than only one.

Of course, to test your work you should also introduce and make use of a new loopInvariant2() method to check that the loop invariant holds before and after each loop iteration.

Submitting Your Work