weighted random example

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

int main()
{
    // Input
    int n = 4;
    int attempts = 100000;
    char *title[] = {"Red", "Green", "Blue", "Unknown"};
    double weight[] = {0.4, 0.1, 0.2, 0.3};

    srand(time(0));

    // Prefix sum
    double sum[n + 1];
    sum[0] = 0;
    for (int i = 0; i < n; ++i)
        sum[i + 1] = sum[i] + weight[i];

    int hits[n];
    memset(hits, 0, n * sizeof(hits[0]));
    int turn = attempts;
    while (turn--) {
        double p = (double)rand() / RAND_MAX * sum[n];
        int pos;
        for (pos = 0; sum[pos + 1] < p; ++pos); // change to binary search & epsilon compare
        hits[pos]++;
    }

    for (int i = 0; i < n; ++i)
        printf("%s %f\n", title[i], (double)hits[i] / attempts);

    return 0;
}