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Path: utzoo!linus!philabs!seismo!ut-sally!riddle
From: riddle@ut-sally.UUCP (Prentiss Riddle)
Newsgroups: net.games.rogue
Subject: Re: Nonexistent patterns in random numbers
Message-ID: <627@ut-sally.UUCP>
Date: Wed, 14-Dec-83 16:22:47 EST
Article-I.D.: ut-sally.627
Posted: Wed Dec 14 16:22:47 1983
Date-Received: Fri, 16-Dec-83 02:38:12 EST
References: <461@seismo.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 150

Not only do people insist on seeing patterns in random numbers, but the
converse is also true:  when people try to create a sequence of random
numbers, they usually insist on building patterns into it.

Years ago, the magazine "Creative Computing" published a rather simple-
minded program to demonstrate this.  It asked the user to pretend that
s/he was repeatedly flipping a coin.  The program would try to antici-
pate each "h" or "t" entered by the user.  The number of correct guesses
was expressed as a percentage; with a large number of guesses, the score
would almost always stabilize well above 50%.

For what it's worth, here is my re-creation of that "Creative Computing"
program.  If you work at it, it's quite easy to outwit the program, but
if you stick with the premise that it's trying to test (i.e., just imagine
that you're flipping a coin), you'll probably find that your flips are
anything but random.

--------------------------------------------------------------------------

/*  guess - heads/tails guesser		12/14/83
 *
 *  Guess is intended to demonstrate that human beings are poor random
 *  number generators.  The model for this program appeared in an old
 *  (ca. 1975?) issue of "Creative Computing".
 *
 *  Compilation: "cc -o guess guess.c"
 *
 *  Author:  Prentiss Riddle  (...{ihnp4,seismo,ctvax}!ut-sally!riddle)
 */

#include <signal.h>

#define CBREAKON  "stty cbreak"  /* system command to turn on cbreak mode */
#define CBREAKOFF "stty -cbreak" /* system command to turn off cbreak mode */

#define HEADS    1
#define TAILS    0


main ()

{
    int  flipit();           /* flip a coin */
    int  quit();             /* exit routine */

    static
    int  history[2][2][2] = {{{0,0},{0,0}},{{0,0},{0,0}}};
                             /* history -- three flips of heads/tails */
    int  f1, f2, f3;         /* three flips determining current state */
    int  flip;               /* user's flip */
    int  guess;              /* computer's guess */
    int  fcount = 0;         /* flip count */
    int  score = 0;          /* flips correctly guessed by computer */
    char c;                  /* user input */
    long seed;               /* seed for random number generator */

    /* set up interrupt handling and cbreak mode */
    signal (SIGINT, quit);
    system (CBREAKON);

    /* randomize initial state */
    seed = getpid() + 1984;
    f1 = flipit(&seed);
    f2 = flipit(&seed);
    f3 = flipit(&seed);

    while (1) {

        /* perform guess */
        if (history[f1][f2][f3] > 0)
            guess = HEADS;
        else if (history[f1][f2][f3] < 0)
            guess = TAILS;
        else guess = flipit(&seed);

        /* read user's flip */
        do {
            printf ("h(eads),t(ails),q(uit):");
            c = getchar();
            switch (c) {
                case 'h':
                    flip = HEADS;
                    break;
                case 't':
                    flip = TAILS;
                    break;
                case 'q':
                case '\0': /* control D */
                    putchar('\n');
                    quit();
                    break;
                default:
                    putchar('\n');
                    flip = -1;
                    break;
            }
        } while (flip == -1);

        /* update score */
        ++fcount;
        if (flip == guess)
            ++score;

        /* update history */
        switch (flip) {
            case HEADS:
                ++history[f1][f2][f3];
                break;
            case TAILS:
                --history[f1][f2][f3];
                break;
        }
        f1 = f2;
        f2 = f3;
        f3 = flip;

        printf ("    my guess:%c  my score: %d out of %d (%2d%%)\n",
                ((guess == HEADS) ? 'h' : 't'), score, fcount,
                (int) (((float) score / (float) fcount) * 100.0 + 0.5) );
    }

}


int flipit (seed)       /* flip a coin */

/* random number generator taken from Grogono p. 119 */

long *seed;	/* seed for random number generation */
{
    *seed = ( 25173 * *seed + 13849 ) % 65536;
    return (((((double) *seed) / 65536) > 0.5) ? HEADS : TAILS) ;
}


quit ()             /* fix terminal back up and exit */
{
    /* turn off further interrupts */
    signal (SIGINT, SIG_IGN);

    /* turn off cbreak mode */
    system (CBREAKOFF);

    /* bye-bye */
    putchar ('\n');
    exit (0);
}
--------------------------------------------------------------------------
Prentiss Riddle
{ihnp4,seismo,ctvax}!ut-sally!riddle