Gaussian distribution

/* utils.h  begins */

#include <stdlib.h>

extern int d (int n);                  /* n-sided die. */

/* utils.h  ends */



/* utils.c  begins */

#include "utils.h"

int
d (int n) {                 /* Given n, throw an n-sided die. */
    return 1 + (rand () / (RAND_MAX / n + 1));
}

/* utils.c  ends */



/* gauss.h  begins */

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#include "utils.h"


#define NDICE    3              /* The number of dice to be rolled. */
#define SIDES    6              /* Each die has these many sides. */
#define MAXSUM   SIDES * NDICE  /* The maximum possible sum of NDICE
                                   dice.
                                */
#define LIMIT    (MAXSUM) + 1   /* This constant is used:
                                   0) to declare the array of sums
                                   1) as a loop limit
                                */
#define ROLLS    10000          /* The number of throws of the dice */
#define SCALE    100            /* Used to fit the histogram into the
                                   screen width.  A lower value
                                   implies longer bars.
                                */

extern void initsums (int sums [], int limit);
extern int sumdice (int ndice);
extern void fillsums (int sums [], int maxrolls);
extern void printsums (int sums [], int limit);
extern void histogram (int sum, int n);
extern void histsums (int sums [], int limit);

/* gauss.h  ends */



/* gauss.c  begins */

#include "gauss.h"

/* Initialize only the relevant indices to 0.  So, care must be taken
   not to try using indices 0 and NDICE - 1.
*/
void
initsums (int sums [], int limit) {
    int i;

    for (i = NDICE; i < limit; i++)
        sums [i] = 0;
}

/* Roll the given number of dice and return their sum. */
int
sumdice (int ndice) {
    int i;
    int sum = 0;

    for (i = 0; i < ndice; i++)
        sum += d (SIDES);

    return sum;
}

/* For the specified number of dice throws: calculate the sum of the
   given number of dice, and increment the array slot whose index is
   the sum.
*/
void
fillsums (int sums [], int maxrolls) {
    int sum;                    /* Sum of the dice. */
    int i;

    for (i = 0; i < maxrolls; i++) {
        sum = sumdice (NDICE);
        ++sums [sum];
    }
}

/* Print the number of occurrences of the various sums. */
void
printsums (int sums [], int limit) {
    int i;

    for (i = NDICE; i < limit; i++)
        printf ("%2d: %d\n", i, sums [i]);
}

/* Print a horizontal bar histogram for a given sum. */
void
histogram (int sum, int n) {
    int i;

    printf ("%2d: ", sum);
    for (i = 0; i < n / SCALE; i++)
        putchar ('=');
    putchar ('\n');
}

/* Print a histogram of the occurrences. */
void
histsums (int sums [], int limit) {
    int i;

    if ((NDICE > 0) && (SIDES > 1)) {
        for (i = NDICE; i < limit; i++)
            histogram (i, sums [i]);
    }
    else {
        if (NDICE < 1) {
            fputs ("NDICE set to insane value\n", stderr);
            fputs ("So, no histogram.\n", stderr);
        }
        if (SIDES < 2) {
            fputs ("SIDES set to insane value\n", stderr);
            fputs ("So, no histogram.\n", stderr);
        }
    }
}

/* gauss.c  ends */



/* main.c  begins */

#include "gauss.h"

int
main (void) {
    int sums [LIMIT];           /* We use only indices
                                   NDICE to LIMIT - 1
                                   for indexing the sums of the given
                                   number of dice.  We waste
                                       sizeof (int) * NDICE
                                   bytes.
                                */

    srand (time (NULL));        /* Seed the RNG using the current
                                   time.
                                */

    initsums (sums, LIMIT);     /* Initialize the (relevant) indices
                                   to 0.
                                */
    fillsums (sums, ROLLS);     /* Count the sums. */
    printsums (sums, LIMIT);    /* Print how many of each sum
                                   counted. */
    putchar ('\n');
    histsums (sums, LIMIT);     /* Print a histogram of the same. */

    exit (EXIT_SUCCESS);        /* Done.  So, exit. */
}

/* main.c  ends */