Homework 2 — User-defined Functions

Due: Sunday, November 17, 2008, at 11:59pm

Abstract

Use functions to modify and enhance an existing program that plays the game of Craps.

Outcomes

After successfully completing this assignment, you should be able to:–

·        Understand and modify code written by someone else

·        Design and implement functions for a C program

·        Read mixed data (a mixture of numeric and character data) into a program

·        Use enumerated types to make a program more understandable

Before Starting

Download and study the program that you will modify. This is called craps.c and can be found in the Code Fragments folder of the Course Materials section of the myWPI site for this course. Before attempting to modify it for this assignment, compile and execute it multiple times to see how it works.

Read Chapter 4 of The C Programming Language, 2nd edition, by Kernighan and Ritchie. Details about rand() and srand() can be found in Appendix B, §B5, by reading the Linux man pages for those functions, and by reading the notes at the end of this assignment.

Also review enumerated types. These are introduced on p. 39 of the textbook and defined in §A.8.4 (pp. 214-215).

The Assignment                                                              

Copy the program craps.c to a new file called hw2.c, and modify hw2.c to contain the following enhancements:–

·        the user starts out with a bank balance of $1000

·        the user is allowed to play multiple games of craps

·        the user wagers money on each game played

The program should be modified so that the portion of the program that plays one game of craps is packaged as a function. In addition, separate functions should be defined and added to verify the wager, adjust the bank balance after a game is played, and read in a yes or no answer. Here are the specifications for each of the functions:

·        validWager: This function gets a wager amount and checks to make sure it doesn’t exceed the player’s current bank balance. If the wager is less than or equal to the current bank balance, the function returns 1 (true), otherwise it returns 0 (false).

·        playGame: plays a single game of craps, rolling the dice as many times as necessary and printing the result. It returns to the caller one of the enumerated constants WON or LOST.

·        adjustBalance: This function either adds the wager to or subtracts the wager from the player’s current balance, depending upon whether the last game played was WON or LOST.

·        getYesOrNo: This function asks if the player would like to play another game of craps. The function checks the response to make sure it is either ‘y’ or ‘n’. The function should repeatedly ask for a y/n response until a valid response is entered. The answer (either ‘y’ or ‘n’) is returned to the calling function.

For each function, you should write pre- and post-conditions, design the function as a black box (you don’t have to turn in the black box), define a prototype, write a stub, and, once the program compiles successfully with each stub, fill in the function definitions (one at a time).

Include files

·        stdio.h provides printf, scanf, getchar

·        stdlib.h provides rand, srand

·        time.h provides time, which is used to “seed” the random number generator

Assumptions and Restrictions

The main function should call getYesOrNo only as long as the player’s balance is greater than 0 (once the player’s balance goes to 0, the player is not allowed to continue).

Sample Execution

Balance = $1000.00

Enter wager:  100

Player rolled 5 + 6 = 11

Player wins

 

Balance = $1100.00

Do you want to play another game?  (y or n):  y

Enter wager:  1500

Your wager must not exceed your current balance.

Enter a new wager:  900

Player rolled 3 + 1 = 4

Point is 4

Player rolled 4 + 5 = 9

Player rolled 5 + 5 = 10

Player rolled 5 + 6 = 11

Player rolled 3 + 3 = 6

Player rolled 3 + 5 = 8

Player rolled 6 + 2 = 8

Player rolled 1 + 5 = 6

Player rolled 3 + 5 = 8

Player rolled 5 + 6 = 11

Player rolled 3 + 1 = 4

Player wins

 

Balance = $2000.00

Do you want to play another game?  (y or n):  q

You must answer y or n.

Do you want to play another game?  (y or n):  y

Enter wager:  2000

Player rolled 6 + 5 = 11

Player wins

 

Balance = $4000.00

Do you want to play another game?  (y or n):  n

 

Your final balance is $4000.00

A note about mixed numeric and character input

This program requires a deeper understanding of how C does keyboard input. When reading in single characters (like y or n) it is usually easier to use the C function getchar() instead of using scanf(). getchar() returns the next single character from the input stream. Look at the Sample Execution above. You would use a printf() statement to output the prompt, “Do you want to play another game? (y or n): “. Now think of what the user types in response...not the single character y, but two characters, y and the ENTER key (i.e., the C constant ‘\n’). You can read in the y with the statement

  ch = getchar();

That still leaves the newline character in the input buffer. This becomes a problem if the user had typed in an invalid response like q instead of y or n, because the next time you try to use getchar to read in a valid response it will read in the newline, not the user’s new input value. Similar problems are encountered if you use getchar after reading in a numeric value; scanf reads in the characters for the numeric value, and leaves the next character to be read (the newline character) still in the input buffer. A quick way to fix this problem is to follow each input statement with the loop:

  while (getchar() != ‘\n’);

(Notice the placement of the semi-colon. The loop body is empty.) This loop essentially reads the rest of the characters on the line, and ignores them, until it reaches the end of the line. The next time your program executes an input statement (either scanf or getchar) it will get the input from the next line typed by the user. You may find getYesOrNo the most difficult function to write for this assignment. It is suggested that you get everything else working in your program first, and then add the code that relies on getYesOrNo.

Deliverables

You should write a short text file (i.e., .txt format) called README, to be submitted with your program. Submit your implementation and README files using the following turnin command:–

/cs/bin/turnin submit cs2301 hw2 hw2.c README

Be sure to put your name at the top of ALL files! You would be surprised at how many students forget this.

Programs submitted after 11:59pm on November 16 will be tagged as late, and will be subject to the late homework policy.

Grading

This assignment will be graded on the following areas: documentation (including pre- and post-conditions for all functions, specified as comments in the code), proper choice of parameters and local variables, proper choice of function return types, adherence to specifications for each function, correctness, and robustness (ability to recover from invalid input). Programs must compile successfully in order to receive points for correctness.

Appendix A – the Craps game

Craps is a game of chance in which a player rolls a pair of dice. After the two dice have come to rest, the numbers of spots on the two upward faces are added together. If the sum is 7 or 11, the player wins. If the sum is 2, 3, or 12, the player loses and the “house” wins. If the sum is 4, 5, 6, 8, 9, or 10 on the first throw, then that sum becomes the player’s point, and the player rolls again. The player continues to roll the dice until either he/she gets the point again (in which case the player wins) or a 7 is rolled, in which case the player loses.

Appendix B – pseudo-random generators

Many engineering and scientific problems require programs to use random numbers. For example, random numbers can be used to simulate noise in an electronic circuit or arrival times of events in the external world. For purpose like these, random number generators are used. A random number generator is a function that returns a seemingly random value each time it is called. A large body of mathematical theory exists for random number generators, dealing with their behavior, the quality of their randomness, and other factors.

Linux provides several random number generators. In this assignment, the function

int rand(void);

returns a new random number each time it is called. The random values are in the range

     0 .. RAND_MAX,

where RAND_MAX is as least as large as 215-1 (i.e., 32,767). If you want a random number in the range, say, 0 .. 6, then you can generate one with the expression

rand() % 6

However, random numbers are particularly vexing when trying to debug a program, because they produce a different value each time. Therefore, random number generators are typically designed to accept a seed (i.e., a starting value) and generate a sequence of random numbers from that seed. This way, you can always get the same sequence while you are debugging your program. Once your program is running correctly, you can then replace the seed with something different — for example, the time of day — to generate more realistically random values.

To seed the Linux random number generator, the function

void srand (unsigned int seed);

may be used. The default value for seed is 1.

In the craps.c file, you will notice that srand is called with an argument time(NULL), which returns the time in seconds since an arbitrary beginning used by all Unix and Linux systems. If you have problems during your debugging, it is suggested that you temporarily comment out the call to srand().