normal_prng.c

/* Copyright (C) 2016 Salvatore Sanfilippo. All Rights Resereved.
 * This code is released under the BSD 3 clause license. */

#include <math.h>
#include <stdlib.h>

/* Return a random number with normal distribution and the specified
 * mean and variance. It uses the "polar method" but does not cache one
 * of the previously generated random numbers, it just returns a single
 * one per iteration in order for the function to be completely stateless. */
double normrand(double mean, double variance) {
    double v1, v2, s;
    do {
        /* Generate two uniform random numbers in the -1,1 range. */
        v1 = (double)rand()/RAND_MAX * 2 - 1;
        v2 = (double)rand()/RAND_MAX * 2 - 1;
        s = v1*v1+v2*v2;
    } while(s >= 1);
    /* Obtain a number with unit normal distibution. */
    v1 = sqrt(-2*log(s)/s)*v1;
    /* Convert it to requested mean and variance. */
    return mean + sqrt(variance) * v1;
}

#include <stdio.h>
#include <time.h>
int main(void) {
    int histogram[21] = {0};

    srand(time(NULL));
    for (int j = 0; j < 5000; j++) {
        double r = normrand(10.5,10);
        int idx = r;
        if (idx < 0 || idx > 20) continue;
        histogram[idx]++;
    }

    for (int j = 0; j < 21; j++) {
        for (int i = 0; i < histogram[j]/10; i++) printf("%c",'o');
        printf("\n");
    }
    return 0;
}

$ ./a.out

o
oo
oooooo
ooooooooo
oooooooooooooooooo
ooooooooooooooooooooooooooo
ooooooooooooooooooooooooooooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooooooooooo
ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
ooooooooooooooooooooooooooooooooooooooooooooooooooo
ooooooooooooooooooooooooooooooooooooooo
oooooooooooooooooooooooooooo
ooooooooooooooooo
ooooooooo
oooo
oo