Laboratory: Double-Ended Queue (Dequeue)
CSC 207 Algorithms and Object Oriented Design Spring 2011

Summary: In this lab, you will practice implementing (and testing!) your own specific collection. Namely, a double-ended queue (aka Dequeue).


  1. Create a directory for this lab:
      mkdir somewhere/dequeue
  2. Go to your new directory:
      cd somewhere/dequeue
  3. Copy the files for this lab:
      cp ~weinman/public_html/courses/CSC207/2011S/labs/code/dequeue/ .
      cp ~weinman/public_html/courses/CSC207/2011S/labs/code/dequeue/ .
    This should place a class skeleton and a test suite into your directory.


As you should know from your reading of Weiss 16.5, a double-ended queue allows access at "both ends." In other words, it acts like both a queue (add to end, retrieve from front), and a stack (add to front, retrieve from front). Plus, it allows you to retrieve from the end as well. Thus, it is like a mini-Swiss army knife of data structures. (A list would probably be the full-on Swiss army knife, since it allows arbitrary access.)

Our dequeue implementation will support six core operations,

void addFront( AnyType x ) // Insert x at front
void addRear( AnyType x )  // Insert x at rear
AnyType getFront()         // Return item at front
AnyType getRear()          // Return item at rear
AnyType removeFront()      // Return and remove item at front
AnyType removeRear( )      // Return and remove item at rear


Exercise 1

  1. What will be the results returned by the following sequence of remove operations? Write down your answers.
  2. Check the notes on this exercise to verify your answers.

Exercise 2

  1. Open the file in Emacs.
  2. Briefly examine the public methods
    • Dequeue
    • isEmpty
    • makeEmpty
    • getFront
    • getRear
    • removeFront
    • removeRear
    • addFront
    • addRear
    to be sure you understand what they are to do.
  3. Briefly examine the private methods to be sure you understand what they do or are to do.
  4. Examine the private fields and be sure to understand what they mean. (They are the same as for the dynamic array implementation of a queue used in the book.)
  5. Open the file in Emacs. This contains a unit test suite using a testing library. Briefly scan the methods to see what is tested.
  6. Compile the files and from Emacs (remember the useful short cut C-c C-v C-c).
  7. To run the suite of tests on your current dequeue implementation, you can use the JDE "Run App" command as usual (or the handy shortcut C-c C-v C-r).
  8. Amidst all the output, you should see a summary near the top:
       new TestDequeue:1()
    Ran 51 tests.
    43 tests failed.
    Hey, at least 8 things worked, right?

Exercise 3

  1. Given the private fields front, back, and currentSize, what should the method makeEmpty do? That is, will it differ from the implementation in Figure 16.17 (p. 605)?
  2. Implement makeEmpty in
  3. Given your private fields, will isEmpty differ from the implementation in Figure 16.13 (p. 603)?
  4. Implement isEmpty in
  5. Verify that your code compiles.

Exercise 4

  1. The method getFront is akin to the same method in Weiss's ArrayQueue class. Given your member variables, will the implementation differ substantially?
  2. Implement getFront in Pay careful attention to the specification for the exception (the tester cares what the message is).
  3. Verify that your code still compiles.
  4. The method getRear is new. How will you implement it using your member variables?
  5. Implement getRear in (Again, pay attention to the specification for the exception.)
  6. Verify that your code still compiles.

Exercise 5

  1. The method addRear is akin to enqueue in the original ArrayQueue class. Will the implementation for a dequeue differ substantially?
  2. Implement addRear and compile your code.

Exercise 6

  1. The method removeFront is akin to dequeue in the original ArrayQueue class. Will the implementation for your Dequeue class differ substantially?
  2. Implement removeFront and compile your code.

Exercise 7

The method addFront is new to the dequeue structure. It is essentially the "reverse" of addRear. The original queue has a method increment to implement wrap-around from the end of the array back to the beginning. In order to be able to add to the front of the queue, we'll need to also wrap-around from the beginning of the array back to the end.

  1. Review the method increment and then review the documentation/specification for decrement.
  2. Implement decrement and verify that your code compiles.

Exercise 8

Consider the method addRear, which you have already implemented. As mentioned above, addFront will be similar.
  1. How will front change when you add an item?
  2. How will back change when you add an item?
  3. Implement addFront based on your replies. (Don't forget to use decrement. Of course, by now you should be getting used to compiling your code frequently. So compile already!
  4. Consider the following operations on an empty dequeue:
    This should work correctly and return 1. Consider carefully the status of front, and especially back in your code after the first operation.
  5. Will your implementation function correctly? If you think so, it may be time to run the unit test suite. If both of these methods work correctly, you should pass test number 15.
  6. Make sure you pass tests 15 and 16 before continuing.

Exercise 9

Now it's time for the last piece of the puzzle!
  1. Consider the removeRear method. It acts like the "reverse" of the removeFront method.
  2. How will the size change?
  3. How will front change?
  4. How will back change?
  5. Implement removeRear based on your replies above.
  6. Compile and test your code. You should now fix any outstanding test failures. This may require you to study the test cases more carefully.

For those with Extra Time

  1. Copy the class weiss.nonstandard.ListNode to your current directory and remove the package specification.
  2. Use the ListNode class to implement your own list-backed version of Dequeue called ListDequeue.
  3. Copy to and change all the references to Dequeue objects to ListDequeue objects.
  4. Test your implementation of ListDequeue and fix any bugs.

Notes on the Exercises

Note on Exercise 1

When you add these things to the dequeue, the contents are (front at the left and rear at the right, without wraparound) [6 7 3 5]. Therefore, when you remove the items as shown, the results are 5 3 6 7.
Created: 18 March 2009
Modified: 14 April 2010
Modified: 18 January 2011
Copyright © 2009-2011 Jerod Weinman.
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License .