CMPSCI 187: Programming With Data Structures

David Mix Barrington

Fall, 2012

Q&A for Programming Project #1: The StringBag Class

Question text in black, answers in blue.

• Question 1.1, posted 18 September: The project description on the web site says to make a StringBag class, but the exercise mentioned in DJW says to make an interface and then a class implementing the interface. Which should I do?

  Just make the class, as it says on the assignment.

• Question 1.2, posted 18 September: Is it all right if I use a linked list, or an ArrayList, to implement my StringBag class?

  I was thinking you would do it with an array, and if you do you can reuse a lot of code from the ArrayStringLog class in DJW. But you could do it either of those ways if you like. Note that you should still have two constructors, one with a capacity and one without, because the user (our driver program) is expecting that. But since both linked lists and ArrayLists have unlimited capacity, it's ok that your StringBag never becomes "full", no matter what its "capacity" is set to when it is created.

  Note, though, that we tell you exactly what order the elements should be in after a series of inserts and remove(k)'s, and your implementation must respect that. This could take a bit of work to implement remove(k) for a linked list, but that would be good practice.

• Question 1.3, posted 18 September: Does it matter how my output string is formatted by my toString method?

  Yes -- you should have your spaces and "\n"'s exactly where ArrayStringLog's toString method has them -- the difference is that you have no name. The driver we are about to release does not do a string-match on the output of toString, but the grading driver might do this.

• Question 1.4, posted 20 September:

  I have a question about project #1. The issue arises here:

  • "The method called remove (k) should remove and return the k'th element of the StringBag, in the order shown when we call the toString method. The removal of the object creates a gap in the sequence of strings, and we care how you fill the gap because we are going to test for the behavior we expect. You should move the last element of the sequence to fill the gap, unless the gap is already in the last place."

  My question is about the bolded sentence.

  I decided to use a linked list for this project because I would like to dip my toes into unfamiliar territory. Or I just like making my life harder than it needs to be. Anyway, from what I understand about linked lists, removal involves taking the link from the removed node and replacing it with the link of the node before it. ...right? Plucks out the node of interest-- but this process also seems to fill the gap automatically with the nodes that were adjacent to the one removed. Am I right in this assumption? If this is how it works, is that last statement in the project write-up really necessary for linked lists?

  Note that this answer is relevant only to those who are attempting to implement Project 1 with a linked list.

  I tried to address this in lecture but I may not have been clear. If you use a linked list, which is your choice, this particular step leads to some complications. (But doing it to get practice with LL's right away is a great idea for your own development.)

  If you just removed the k'th element on at remove(k) and linked its successor to its predecessor, this would have the same effect as sliding all the later entries one step forward.

  The problem is that I have told you exactly how I want the other elements in the list to move when you do a remove(k), and that simple action is not what I want.

  So what you need to do is take the k'th element out, save it to return when you are done, find the last element in the list, remove it, and then patch the former last element into the gap in the list made by the k'th element.

• Question 1.5, posted 22 September: The project assignment still has the business about using Math.abs and % to get the random index k to use for remove( ). May we get this k by the simpler method used in Discussion #2, using the nextInt(int) method of the Random object?

  Yes, I encourage you to do this, and I have now changed the text on the project assignment page.

• Question 1.6, posted 23 September: Do the remove( ) and the remove(k) methods behave the same way with respect to filling the gap made by the missing element? Do they both return the element that is removed?

  Yes and yes. The most natural way to implement remove( ) is to generate a random k in the correct range with the Random object, then call remove(k). This keeps you from having to write the same code twice.

• Question 1.7, posted 23 September: I wrote a toString method that printed the appropriate lines to System.out. But the compiler demanded that I put in a return statement that I didn't need. I changed the type of the toString method to void, but then when I ran the driver (which prints StringBag objects) I got output like StringBag@b06c6be, which I don't understand. Help!

  First off, the job of the toString method is not to print anything, but to return a String object. The caller of the toString method may or may not choose to print this string. That's why the method is typed String, and why the compiler insists that it return a String. Look at the toString method at the bottom of page 87 of DJW. It assembles a String, which appears on multiple lines when printed because it has "\n"'s in it, and then returns it.

  The reason for your funny output from the driver is worth working through. The method System.out.println expects a String as its parameter. If it gets an object instead, it runs that object's toString method to get a String, then takes that String. Every object has a toString method, because that method is defined in the Object class and every class of objects extends that class. Usually we write our own toString method and thus override the one in Object. But when you defined a void toString method, that no longer overrode the other method because they had different return types. So when the driver tried to print your StringBag object, it called the toString method of Object, which (as you saw) makes a string out of the object's class and its location in memory, which is given in hex and so has digits and letters from a through f.

Last modified 23 September 2012