CS 2213/2211  Advanced Programming  Spring 2005  Recitation 2  C I/O and Random Numbers     Week 2: Jan 25-27  Due (on time): 2005-02-01  23:59:59  Due (late):        2005-02-06  23:59:59

Recitation 2 should be submitted following directions at: submissions with deadlines 2005-02-01  23:59:59 (that's Tuesday, 1 February 2005, 11:59:59 pm) for full credit. 2005-02-06  23:59:59 (that's Sunday, 6 February 2005, 11:59:59 pm) for 75% credit.

• C input/output using printf and scanf.
• Random number generators (RNGs) in C, both the library RNG and ones that are written "from scratch".
• The use of "reference" parameters in C.
• Writing some C code similar to that in Recitation 1.

/* craps: simulate play of several
 *   games of craps. Report results.
 */
#include &stdio.h>
#include &stdlib.h> /* for rand */
#include &time.h>   /* for srand */
#define WIN 1
#define LOSE 0

/* prototypes */
int roll(void);
int craps(int* dice);

int main() {
   int dice; /* variable return value */
   int result; /* WIN/LOSE from craps */
   int i; /* loop variable */
   srand(time(NULL)); /* initialize RNG */
   printf("The \"dice value\" below\n"
          "is either:\n" 
          " o immediate win/loss value\n" 
          " o Point value otherwise\n\n");
   for (i = 0; i < 40; i++) {
      int result = craps(&dice);
      if (result == WIN) printf("WIN, ");
      else printf("LOSE,");
      printf(" dice value: %i\n", dice);
   }
}

int roll(void) {
   return 
     (int)(6.0*(rand()/(double)RAND_MAX) + 1.0);
}

int craps(int* dice) {
   int dice2;
   *dice = roll() + roll();
   if (*dice == 2 || *dice == 3 || *dice == 12)
      return LOSE; /* Immediate loss */
   else if (*dice == 7 || *dice == 11)
      return WIN; /* Immediate win */
   else {   /* point: 4, 5, 6, 8, 9, or 10 */
      while (1) {  /* keep rolling */
         dice2 = roll() + roll();
         if (*dice == dice2)
            return WIN;; /* made point, won */
         if (dice2 == 7)
            return LOSE; /* lost with a 7 */
      }
   }
}

% cc -o craps craps.c
% craps
The "dice value" below
is either:
 o immediate win/loss value
 o Point value otherwise

LOSE, dice value: 9
WIN,  dice value: 7
WIN,  dice value: 4
LOSE, dice value: 10
WIN,  dice value: 7
LOSE, dice value: 5
LOSE, dice value: 6
LOSE, dice value: 3
LOSE, dice value: 4
WIN,  dice value: 9
WIN,  dice value: 5
WIN,  dice value: 11
LOSE, dice value: 3
WIN,  dice value: 6
LOSE, dice value: 9
WIN,  dice value: 7
LOSE, dice value: 2
LOSE, dice value: 5
LOSE, dice value: 5
LOSE, dice value: 3
WIN,  dice value: 7
LOSE, dice value: 3
LOSE, dice value: 8
WIN,  dice value: 7
LOSE, dice value: 9
LOSE, dice value: 2
LOSE, dice value: 12
WIN,  dice value: 10
LOSE, dice value: 5
WIN,  dice value: 10
WIN,  dice value: 7
WIN,  dice value: 11
LOSE, dice value: 5
WIN,  dice value: 8
WIN,  dice value: 7
WIN,  dice value: 6
LOSE, dice value: 10
LOSE, dice value: 3
LOSE, dice value: 8
LOSE, dice value: 8

1. Use scanf to read an integer value for the total number of craps games, for example: 36000000.
2. Replace the function roll above with a function that also returns a random integer from 1 to 6 inclusive, but that uses a hand-written RNG, rather than the library one. You can use one of the RNGs presented in the Random Numbers write-up. Initialize this RNG using time(NULL) just as above.
3. Make use of the function craps above without changing it at all. The objective is for you to practice using a reference parameter, such as the one used by this function. (The scanf function also has a reference parameter.) As the main function above illustrates, each call to craps returns either a 1 or a 0 indicating either a win or a loss of the game. In addition, the reference parameter returns a sum of dice which is either the sum that gave an immediate win, or the sum that was a point, either made (win) or not made (loss).
4. Add the following to the main function above:
   • Two arrays of integer counters to count how many times each of rolls 2 through 12 were returned from craps, say, int wins[13] and int losses[13], using the appropriate counter depending on whether it was a win or a loss. (So, for example, 2, 3, or 12 are always losses, 7, 11 are always wins, and the remaining rolls can be either one.)
   • Record the total number of wins and the total number of losses. Of course these are the sum of the entries in the respective arrays, and the sum of these two is the total number of games.
   • Add loops and code to print that will produce output in exactly the following format, assuming that you enter 36000000 as the number of games. With this number of games, it may take your machine several minutes to finish up. Of course your actual numbers will be different, because this is a randomized experiment.

     Output for your program
     craps Enter number of games ---> 36000000 Roll Wins Losses 2 0 1000414 3 0 2001126 4 1001687 1999108 5 1600349 2398699 6 2273472 2727597 7 5997732 0 8 2272140 2727101 9 1600434 2396853 10 1000055 2003161 11 1999775 0 12 0 999991 Games: 36000000, wins: 17745644, losses: 18254050 % wins: 49.2935, % losses: 50.7057

Contents of email submission for Recitation 1: Last Name, First Name; Course Number; Recitation Number. a. Listing of your C program file, with the required changes and additions. b. Result of running this program. c. Answer the question: Consider the second program listing in the write-up about Random Numbers (the listing titled "C RNG Program"). Why does the C++ compiler on the Sun machines produce a different answer than the results using other compilers, even though the program is exactly the same? (This is tricky. Please don't ask anyone for the answer, neither students nor faculty.)