For this assignment, you will not write an application, but just a module that can be used as part of a larger application, as you did for assignment 4.

Create a module called pqueue.cpp that implements priority queues, with a header file called pqueue.h that describes what pqueue.cpp exports. The interface includes the following, and nothing else. Types ItemType and PriorityType are discussed below under the refinement plan.

A type, PriorityQueue. Creating a PriorityQueue object with line

 PriorityQueue q; 

makes q be an initially empty priority queue.

Function isEmpty(q) returns true if q is an empty priority queue. Its prototype is

 bool isEmpty(const PriorityQueue& q); 

Function insert(q, x, p) inserts item x with priority p into q. Its prototype is

 void insert(PriorityQueue& q, ItemType x, PriorityType p); 

Function remove(q, x, p) removes the item that has the smallest priority from q. Its prototype is

 void remove(PriorityQueue& q, ItemType& x, PriorityType& p); 

Parameters x and p are out-parameters. 'Remove' sets x to the item that was removed and sets p to the priority associated with that item.

If q is empty, then remove(q, x, p) writes a message on the standard output explaining the problem and stops the program. Statement

 exit(1); 

stops the program and returns exit status 1 to the operating system. You will need to include to use exit.

Development plan

1. Create a directory for assignment 6 and create files pqueue.h and pqueue.cpp.

Use the template for both. Modify the templates with your name and the separation between tab stops, if you use tabs.

Add a comment to pqueue.cpp telling its purpose.

2. Items and Priorities: Types ItemType and PriorityType

Your module is intended to support an application, not to be an application. But it is not clear right now what types of items or priorities the application will need. For example, should the priorities be integers or real numbers or something else? Should the items be strings or numbers or widgets or something else?

To handle that, define two types, ItemType and PriorityType, in pqueue.h. For this assignment, use definitions

 typedef const char* ItemType; typedef double PriorityType; 

Later, when you want to change the definitions of ItemType and PriorityType, you should only need to change those two lines. Not a single other line should need to be touched. (You might need to add one or more #include lines to pqueue.h to make some types available.)

That means you need to write the entire implementation of priority queues using ItemType for the type of an item and PriorityType for the type of a priority. Do not assume that ItemType is const char* or that PriorityType is double. They might be changed.

3. Representing Priority Queues: Types PriorityQueue and PQCell

You are required to store the information in a priority queue using a linked list, kept in nondecreasing order by priority. That means items with smaller priorities are closer to the beginning of the linked list. You will need the following types. Provide documentation for each type.

In pqueue.cpp, define a structure type, PQCell, that is used as a cell in a linked list. It holds an item, a priority, and a pointer to the next cell in the list. Document type PQCell. Be sure to say that an object of type PQCell is a cell in a linked list.

In pqueue.h, write a forward declaration of PQCell, as follows.

 struct PQCell; 

That allows your definition of PriorityQueue to use type PQCell*.

In pqueue.h, define a structure type, PriorityQueue, that holds only a pointer to a linked list made of PQCells. This pointer must be initially set to NULL by a parameterless constructor in the definition of PriorityQueue.

pqueue.h

4. Testing Whether a Priority Queue Is Empty

In pqueue.cpp, document and define function isEmpty(q) with the following prototype.

 bool isEmpty(const PriorityQueue& q); 

isEmpty(q) must return true if q is empty, false if q is not empty.

Put a prototype for isEmpty into pqueue.h and a definition of isEmpty in pqueue.cpp.

5. Insertion into a Priority Queue

In pqueue.cpp, document and define function insert(q, item, pri) with the following prototype.

 void insert(PriorityQueue& q, ItemType x, PriorityType p); 

'Insert' inserts item x with priority p into PriorityQueue object q.

Put a prototype for this 'insert' into pqueue.h.

You will find it convenient to write a helper function

 void insertCell(PQCell*& L, ItemType x, PriorityType p) 

that inserts item x with priority p into linked list L. It assumes that L is in nondescending order by priority, and it inserts the new item into the correct spot so that the list is still in nondescending order.

The reason is that 'insertCell' can be recursive, and that makes it easier to write. The body of 'insert' should just make a call to 'insertCell'. That is, 'insert' should have a one-line body.

Do not put a prototype for 'insertCell' into pqueue.h since 'insertCell; is not being exported; it is only to be used in pqueue.cpp.

6. Debugging: Printing the Priority Queue

In pqueue.cpp, write a function for debugging that prints the content of the priority queue in a readable format.

There is an important issue here. Remember that the definitions of types ItemType and PriorityType might be changed. You cannot assume that ItemType is const char* or that PriorityType is double. But then how do you know how to print the items and priorities?

A solution is to require the module that uses priority queues to say how to print items and priorities by providing two functions, one for printing an item and another for printing a priority. C++ allows you to pass a function as a parameter of a function. (Think of lending a tool to a friend. You don't use the tool, your friend does.)

Add the following type definitions to pqueue.h.

 typedef void (*ItemPrinter)(ItemType); typedef void (*PriorityPrinter)(PriorityType); 

They define types ItemPrinter and PriorityPrinter. The function to print an item has type ItemPrinter; it takes a parameter of type ItemType and has a void return type. The function to print a priority has type PriorityPrinter. It takes a parameter of type PriorityType and has a void return type.

In pqueue.cpp, document and define a function printPriorityQueue with the following prototype.

 void printPriorityQueue(const PriorityQueue& q, ItemPrinter pi, PriorityPrinter pp); 

It must show a clear and readable representation of what is in priority queue q, including each value and its priority. In the body of printPriorityQueue, use statement

 pi(x); 

to print item x and

 pp(y); 

to print priority y.

PrintPriorityQueue should assume that pi and pp do not write any spaces or newlines. PrintPriorityQueue should print those. It is not acceptable to write out the entire priority queue on one line.

Function printPriorityQueue is only intended to be used for debugging. Put a prototype for printPriorityQueue into pqueue.h so that another module can use printPriorityQueue for debugging.

7. Testing insert

Add a stub for 'remove'. It does nothing, but it allows you to run the tester to test 'insert'. Here is a suitable stub.

 void remove(PriorityQueue& q, ItemType& x, PriorityType& p) { *x = "nothing"; *p = 0.0; } 

Put a prototype for remove into pqueue.h. (It is just the heading followed by a semicolon.)

Now use the automated tester to test insert. Don't be surprised that errors are reported about 'remove' not working. But don't move on to the next step until 'insert' is working.

8. Removing an Item

In pqueue.cpp, replace the stub for 'remove' by a correct definition. 'Remove' must remove the item from q that has the smallest priority. (If two or more items have the same priority, remove one of them.)

Parameters x and p are out-parameters. 'Remove' must store the item that is removed into variable x and store its priority into p.

Be sure that remove deletes the list cell that is removed so that you do not have a memory leak.

9. Test remove

Rerun the automated tester. This time, everything should look right. If not, fix any errors.

10. Proofread pqueue.cpp and pqueue.h.

Check contracts for correctness, spelling and grammar. Are standards followed? Are all parameters described and referred to by name? Are contracts written in complete sentences?

11. Submit your work.

12. Summary of the priority queue interface

A module that uses priority queues can do the following, and nothing more. Be sure that testpq.cpp adheres to this.

It can create a variable of type PriorityQueue, as follows.

 PriorityQueue q; 

(Of course, you can call the priority queue whatever you like.) The priority queue is initially empty.

It can use the following functions.

 bool isEmpty(const PriorityQueue& q); void insert(PriorityQueue& q, ItemType item, PriorityType pri); void remove(PriorityQueue& q, ItemType& item, PriorityType& pri); 

Strictly for debugging, it can use

 void printPriorityQueue(const PriorityQueue& q, ItemPrinter pi, PriorityPrinter pp);