Shuffling in C/C++

gistfile1.txt

#include <iostream>
#include <cstdlib>

using namespace std;

// From http://stackoverflow.com/questions/2509679/how-to-generate-a-random-number-from-within-a-range
long random_at_most(long max) {
    unsigned long
    // max <= RAND_MAX < ULONG_MAX, so this is okay.
    num_bins = (unsigned long) max + 1,
    num_rand = (unsigned long) RAND_MAX + 1,
    bin_size = num_rand / num_bins,
    defect   = num_rand % num_bins;
    
    long x;
    do {
        x = random();
    }
    // This is carefully written not to overflow
    while (num_rand - defect <= (unsigned long)x);
    
    // Truncated division is intentional
    return x/bin_size;
}

void swap (int& a, int& b) {
    int t = a;
    a = b;
    b = t;
}

void shuffle (int* values, size_t length) {
    for (size_t i = 0; i < length; i++) {
        size_t idx = random_at_most(length - i);
        swap(values[i], values[idx]);
    }
}

void demo () {
    int values [] = {1,2,3,4,5,6,7,8,9};
    shuffle(values, 9);
    for (int i = 0; i < 9; i++) {
        printf("%d ", values[i]);
    }
}

int main(int argc, const char * argv[]) {
    // insert code here...
    for (int i = 0; i < 10; i++) {
        demo();
        printf("\n");
    }
    return 0;
}