Project 4: A Linked List simulated in an Array

The goal of this project is help you think through the difference between and array-based way of storing information and a linked manner. This will also prepare you to think in terms of pointers, which we will be studying later this semester when we consider the C programming language.

1. The simulated linked list

Recall from before break how we constructed a linked-style of storage in a structure that is physically an array. The even numbered array positions held data; for each datum, the following odd array position contained the equivalent to a "next" reference: the array position of the next datum. Here's a diagram of an array containing the list 1, 3, 6, 8, 9, 16, 23. 23 is logically the last item in the list, even though it is not the last item physically in the array.

Copy the following file from the class directory:

cp /cslab.all/ubuntu/cs245/proj4/SimulatedLink.java .

Its methods are add(), insert(), delete(), and iterator(). The first and last are written for you. Your assignment is to write insert(), and delete(). These methods are the only part of the code you need to modify (besides uncommenting testing sections in the main() method).

This project requires writing just a little bit of code, but it is very difficult. Do not take the approach of writing a hunk of code you don't fully understand and then fiddle with it until it works. You must think through the problem carefully and understand what you're writing when it's time to code.

The main() method in the file will step you through testing various parts. It is important that you follow the instructions here about the order in which to write part of the program and that you test it as you go along.

I've also provided a method debug() which will show you the physical state of the array, plus the current head and free values. Use them when necessary to figure out what's wrong with your class.

This is technically due on Friday, Mar 20, but I strongly recommend you arrange your schedule to have it finished by Wednesday. We will have several short projects assigned in rapid sequence in the next few weeks.

2. What's already written

Begin by making sure you understand the add() and iterator() methods. Run the program as it is, so you see it working correctly.

3. The insert() method.

The insert() method takes a position and a value (x) and inserts x after the given position. (This class does not insure that the data are in sorted order, like SortList did. However, in the main() method, I happen to add items to the list in such a way that it is always sorted--this makes it easier to trace.)

The special case of inserting when the list is already full is written for you.

To write the insert() method, first run through a couple examples by hand. What should it do? Here's a basic outline:

• You need to have a variable pointing to the given position. So, there will need to be a loop which advances a pointer, following the links and counting the number of steps. If you get to the logical end of the list without finding the correct position, throw a StructureOutOfBoundsException.
• The new value x must be placed in the next available free spot.
• The links must be updated (both the position where we're inserted and the new "node").
• The reference to the first free position must be updated.

Write the code for insert() and then uncomment checkpoint 2 to make sure it works.

4. The delete() method

Now, the method which takes a logical position in the list and deletes it. The special cases where the list is empty and when there is only one item in the list are handled for you.

What this method must accomplish is overwriting the position to be deleted with the physically last position and updating links appropriately.

To do all this, you will need to find three pieces of data. You will need an index to

• the position to be deleted
• the position that precedes the one to be deleted
• the position that is physically last
• the position that precedes the one that is physically last

My recommendation is first to to get pointers to all four of these items. Then perform the updates: overwrite the position to be deleted, update the link, and update free and possibly head.

The order in which you do these updates is tricky. There are several special situations you need to consider. What if the one being deleted itself precedes the item that is physically last? What if the one being deleted is physically last?

Implement the method, and then uncomment and test the checkpoints one at a time:

• Checkpoint 3 checks that it works on items that precede the physically last item.
• Checkpoint 4 checks that it works on items that are physically last.
• Checkpoint 5 checks that it works for deleting the head item
• By the way, does your insert() work for inserting items in the front of the list? Checkpoint 6 checks this. If necessary, go back and fix your insert() method.
• Checkpoint 7 checks that it works when the head is physically last.

5. To turn in

Please copy your file to the turn-in directory I've made:

/cslab.all/ubuntu/cs245/turnin/proj4/#####

where ##### is {daniel, david, johncharles, kendall, lily, michael, suzanne}.

DUE: Friday, March 20, at 5:00 pm.