Project 0: C Programming warm-up

Skeleton Code

project0.tar.gz (Updated: 11:00am, 3/29)

Background

Despite incredible advances in programming languages over the last 30 years, most serious systems programming is still done in C.

Why is this? Because C gives the programmer more control and power over the code's execution than do other, higher-level languages like Java or even C++. Also, C typically has less runtime overhead than higher-level languages, which can translate into increased performance. Suppose you have a function that takes an integer and returns a double. In a strongly typed language, all you can do with this function is call it while passing an integer and treat the result as a double. Of course, you can do this in C. But you can also call it with no parameters, call it with 5 parameters, take the result and store it in an integer. Even better, you could treat the function as an array and read each instruction as an integer if you like. Or, you could call not the first instruction in the function, but maybe the second, or the third, or ... there is a reason why C is sometimes referred to as a "high-level assembly language".

What's bad about this freedom? Bugs. Forgot a parameter? Maybe you did it on purpose. Or maybe (and probably) not. In Java, the compiler shows you your mistake. In C, the compiler is very easy to please, but when you run the program, it fails, generally in a very cryptic way - "segmentation violation," and "core dumped" are the principle error messages. Both mean you made a mistake.

All of this means that you need to program carefully and deliberately in C. If you do this, you can write programs that are as well structured and clear as you can in languages like Java. But, if you don't, you'll quickly have a big mess on your hands. Hard to debug. Hard to read. Hard to modify.

There are lots of good references for programming in C. The primary one is "C Programming Language (2nd Edition)" by Kernighan and Ritchie.

Assignment

You should do this assignment on a Unix machine, which will provide you not only with the compiler (cc, or gcc, depending on the installation) but also a debugger (gdb). You can use any editor you like, but I recommend that you check out emacs, which has support for C programming and debugging.

We recommend the use of the CSE home Linux virtual machine for this assignment. It's handy to set up, and comes pre-configured with everything you need. (It's sufficient for all projects in this course, except for Project 1, which is done on a CSE Linux box devoted to this course and configured in an odd way.)

Part 1. The Basic Queue

The queue is one of the most important data structures you'll be dealing with in this class. Consequently, it's a good one to start working with early.

For this part of the assignment, I will be providing you with a complete interface and mostly complete implementation for a queue. Starting with this, you will:

1. Find and fix two critical bugs in the implementation. (I've put these bugs in after getting the code working). You will need to build a much more intensive test infrastructure than what you will find in main.c. You should include comments that make it clear what problems you fixed.

2. Implement the two declared, but not implemented, methods: queue_reverse() and queue_sort(). Both methods must work in-place: they can't create a new queue and move or copy elements from the original queue to build the result. The time efficiency of the sorting algorithm is not important, as long as it's something reasonable (not worse than O(n^2), not better than O(nlog(n)). queue_reverse() should execute in O(n) time. queue_sort() sorts in non-descending order.

You must follow the coding style (indentation, naming, etc) that you find inside queue.c and queue.h

Grading Criteria
2 points	Perfect. Two critical bugs found and fixed. New methods implemented cleanly and correctly.
1 point	Slightly less than perfect.
0 points	Seriously flawed.

Part 2. The Hash Table

Following the same style for the queue, write the code that defines (.h) and implements (.c) a hash table.

The hash table allows you to store and retrieve values of any kind (pointers) based on a key value.

You're free to use any hash table implementation you like. You learned several variations, such as linear probing or separate chaining, in your data structures course.

The operations (in English) you need to define are:

1. Create. Creates and returns a new hash table.

2. Set hash function. Establishes the function to be used when computing a hash value based on a key.

3. Set key comparison function. Establishes the function to be used when comparing two keys for equality. (A common scheme is to return 0 for equal, -1 for less than, and 1 for greater than, although all you actually need for this assignment is an indication of equal or not equal.)

   Note: You can combine the last two, or three, functions into one if you want.

4. Add [key, item] pair. Inserts the given pair into the table.

5. Lookup. Given a key, returns the associated item. Indicates failure in the event that the key is not present.

6. IsPresent. Given a key, returns a boolean indicating that the key is present.

7. Remove. Given a key, deletes the associated [key, item] pair from the hash table.

Grading Criteria
2 points	Clean code. Works. Clearly demonstrates an understanding of the coding style laid out in Part 1.
1 point	Code not so clean. Or, maybe doesn't work so well.
0 points	Problems abound.

Part 3: Tables of functions

This section describes the goals/requirements of Part 3. It uses familiar C constructs to explain those goals. Your actual implementation will use some new constructs, not the ones shown here. The actual implementation strategy is described on another page, linked at the bottom of this description. You should understand the requirements before moving on the implementation details page.

It is quite common to put functions into a table and then rely on indirection to make the call. This allows functions to be dynamically bound to their names, yet provides a not too intolerable calling syntax. In addition, it provides the module implementing the functions with a single point of mediating control.

As a simple example, let's suppose that we have a module which implements a set of mathematical functions (add, subtract, multiply, slope, etc).

That is, somewhere in the system the actual functions are defined as follows:

int add(int x, int y);
int sub(int x, int y);
int mult(int x, int y);
int slope(int x1, int y1, int x2, int y2);

but these functions are not directly callable by clients.

Rather than declare these functions directly in a header file, the module instead declares them symbolically, for example, using #defines.

#define ADD 1
#define SUB 2
#define MULT 3
#define SLOPE 4

Then, for each of these functions, the header file defines a structure appropriate for the arguments. For example, add and subtract each take two arguments and so would require that the header file define a structure as per:

struct args2 {
   int a1;
   int a2;
};

The slope function takes four arguments (x1, y1, x2, y2) and would require a comparable definition.

Lastly, mean takes two arguments, the first of which is simply the address of the zeroth element in an array of integers, and the second of which is the number of integers to be "meaned." This too would need to be expressed explicitly within the header file.

To facilitate invocation, the header file exports a single callable entry function which takes three arguments: the identifier of the function to call, a pointer to the structure containing the arguments to be passed to the function, and a pointer to where the results should be stored.

int math_call(int function_name, void *args, int *result)

Thus, a client wanting to add two numbers might say:

void main() 
{
   struct args2 a2;
   int result;
   a2.a1 = 100;
   a2.a2 = 200;
   if (math_call(ADD, &a2, &result) < 0) {
      printf("Error");
   } else
      printf("%d\n", result);
}

The implementation of math_call would then:

1. Verify that the requested function is legitimate (eg, in the table). If not, the function should return -1.

2. Based on the number of arguments expected by the called function, invoke through table indirection the appropriate function in its native form with the appropriate arguments.

3. Store the result of the invocation in the result parameter specified by the caller.

4. Return 0 indicating success.

For part 3, you will implement the math module. Put it in a file called mathtable.c. Your implementation must conform to the interface defined in mathtable.h. In addition, implement a main module (main.c) that shows how to use all of the methods defined indirectly in mathtable.h.

Details on how to actually implement this part of the assignment are described in this assignment addendum.

Grading Criteria
2 points	Clean code that works.
1 point	code maybe not so clean, or maybe doesn't work so well.
0 points	seriously flawed.

Turn-in

Your submission should be lastname.firstname.tar.gz and should decompress to the directory lastname.firstname which contains all your files for this project, which are:

• All files needed to build your solutions
• The Makefiles you used so the TA knows how to compile your code