Question

I’m learning C and find rand() is very strange, maybe due to its randomness :p

I’ve the following code, it always output 1, is there any problem? How would you modify the code to make it do the job?

Cheers,

#include <stdlib.h>

double rand_double()
{
    double ret = (double)rand();
    return ret/(RAND_MAX+1);
}

int sample_geometric_rv(double p)
{
    double q;
    int n = 0;
    do
    {
        q = rand_double();
        n++;
    } while (q >= p);
    return n;
}

int main()
{
    int ans = sample_geometric_rv(0.1);
    printf("Output %d\n", ans);
    return 0;
}

Answer 1

You need to seed the random number generator ONCE. Use srand() with a different value everytime you want a different sequence.

In the absence of a seeding, it is as if you had issued a srand(1);

Tipically, the RNG is seeded in main() with the current time as initialization value. The current time as returned by time() is almost guaranteed to be different in every run of the program (it changes once per second).

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

int main(void) {
    srand(time(0));
    /* rest of program; no more calls to srand() */
    return 0;
}

Note that if you initialize the RNG with the same number, you get the same sequence. This can be interesting, for example, to repeat the data.

Note too that on different computers, the same initialization number does not need to generate the same numbers.