Homework 4
User-defined functions: Modifying the craps game program

Due: Tuesday, November 20, 2012 at 5pm

Outcomes

• Create arrays of data
• Pass arrays as arguments to functions
• Allow functions to modify the contents of arrays
• Build a multi-file application in C
• Use makefiles to correctly build, rebuild, and clean your working directory

Before Starting

The Assignment

• Copy the functions playGame and rollDice from Homework 3 into a new file named, for example, playcraps.c. Create a new header file, playcraps.h, that contains the function prototype of playGame and anything else needed by a caller of playGame. (For example, playGame needs to initialize (i.e., "seed") the random number generator.)

• Modify playGame so that (a) it does not print anything, and (b) it returns an integer reflecting both the number of throws of the dice and whether the game was won or lost. In particular, if the player wins a game after n throws, it should return the integer value n. If the player loses after n throws, it should return the integer value -n.

• In one or more new .c files, create the following functions:
  • A function that prompts the user for the number of games of craps to play and then plays that number of games without human intervention (i.e., by calling playGame). It stores the results of the games in an appropriately-sized array, one element per game. In particular, the number of elements in the array should be exactly the number of games entered by the user.

  • A function to print out the number of games played and compute the mean and median numbers of throws required for a game to be won or lost.

  • A function to analyze the array of results and print out answers to the questions listed below.

  • The main() function, that controls all of the others. In particular, main or one of the functions it calls must free any arrays that were allocated.

• Construct a makefile patterned after Lab #4 to build the executable file (call it statistics) and all of its components. Make sure that $(CFLAGS) is specified in the commands to build individual modules and that it defaults to -g. Be sure that your makefile rebuilds only those files that are necessary and that it can make your directory clean.

1. What is the probablility that the player wins a game of craps?

2. What percentage of games are won on the first throw, second throw, third throw, ..., twentieth throw, and after the twentieth throw? (The sum of these percentages should total to the probability in Question 1, expressed as a percentage.)

3. What percentage of games are lost on the first throw, second throw, third throw, ..., twentieth throw, and after the twentieth throw? (The sum of these percentages, when added to the probability in Question 1 expressed as a percentage, should total 100%.)

Test and debug your program with at least 1000 games. After you have debugged your program, modify it to seed the random number generator with the time of day and test it again. Be sure that the program you turn in is seeded with the time.

Include files

• stdio.h provides printf(), scanf(), getchar()
• stdlib.h provides rand(), srand(), and abs()
• time.h provides time()

Means and Medians

To calculate the median, you must sort the array. If a sorted array A has n elements, the median is defined as A[n/2] if n is odd and as (A[n/2-1] + A[n/2])/2 if n is even. (This particular definition of the median is based on the fact that arrays in C are indexed from zero.)

You may sort the array using the bubble sort algorithm, shown here:

void bubbleSort (int A[], const int arraySize){
  int i, j;
  for (i=0; i < arraySize; i++)
      for (j=0; j < arraySize-1; j++)
          if (abs(A[j]) > abs(A[j+1]))
             swap(A+j, A+j+1);
}

The swap function is

void swap (int *a, int *b){
  int temp = *a;
  *a = *b;
  *b = temp;
}

Deliverables

Grading

• Correct compilation without warnings using make, and correct operation of make for individual files and for clean - 5 points

• README.txt submitted as specified - 2 points

• Program documentation and formatting, including pre- and post-conditions for each function - 3 points

• Program organization into three or more .c files, two or more .h files, and a makefile - 5 points

  Note: The .c files should include at least playgame.c (or whatever you wish to call it), the new .c file that sets up the array and plays the requested number of games, and your sort function. The final target of the makefile should be an executable called statistics.

• Correctly allocating an array of n elements, correctly playing the game n times, correctly storing the results in the array, correctly freeing the array at the end - 5 points

• Correctly sorting the array and obtaining the mean and median number of throws - 5 points

• Building a table or other suitable data structure to answer the analysis questions, printing out those results, and getting answers consistent with the TAs' test cases - 5 points