Why is Thrust uniform random distribution generating wrong values?

Question

I want to fill a device vector with random values in the range [-3.2, 3.2). Here is the code I wrote to generate this:

#include <thrust/random.h>
#include <thrust/device_vector.h>

struct RandGen
{
    RandGen() {}

    __device__
    float operator () (int idx)
    {
        thrust::default_random_engine randEng(idx);
        thrust::uniform_real_distribution<float> uniDist(-3.2, 3.2);
        return uniDist(randEng);
    }
};

const int num = 1000;
thrust::device_vector<float> rVec(num);
thrust::transform(
                thrust::make_counting_iterator(0),
                thrust::make_counting_iterator(num),
                rVec.begin(),
                RandGen());

I find that the vector is filled with values like this:

-3.19986 -3.19986 -3.19971 -3.19957 -3.19942 -3.05629 -3.05643 -3.05657 -3.05672 -3.05686 -3.057

In fact, I could not find a single value that is greater than zero!

Why is this not generating random values from the range I set? How do I fix this?

Answer 1

You have to call randEng.discard() function to make the behavior random.

__device__ float operator () (int idx)
{
    thrust::default_random_engine randEng;
    thrust::uniform_real_distribution<float> uniDist(-3.2, 3.2);
    randEng.discard(idx);
    return uniDist(randEng);
}

P.S: Refer to this answer by talonmies.