Generate random numbers uniformly over an entire range

Question

I need to generate random numbers within a specified interval, [max;min].

Also, the random numbers should be uniformly distributed over the interval, not located to

Answer 1

This should provide a uniform distribution over the range [low, high) without using floats, as long as the overall range is less than RAND_MAX.

uint32_t rand_range_low(uint32_t low, uint32_t high)
{
    uint32_t val;
    // only for 0 < range <= RAND_MAX
    assert(low < high);
    assert(high - low <= RAND_MAX);

    uint32_t range = high-low;
    uint32_t scale = RAND_MAX/range;
    do {
        val = rand();
    } while (val >= scale * range); // since scale is truncated, pick a new val until it's lower than scale*range
    return val/scale + low;
}

and for values greater than RAND_MAX you want something like

uint32_t rand_range(uint32_t low, uint32_t high)
{
    assert(high>low);
    uint32_t val;
    uint32_t range = high-low;
    if (range < RAND_MAX)
        return rand_range_low(low, high);
    uint32_t scale = range/RAND_MAX;
    do {
        val = rand() + rand_range(0, scale) * RAND_MAX; // scale the initial range in RAND_MAX steps, then add an offset to get a uniform interval
    } while (val >= range);
    return val + low;
}

This is roughly how std::uniform_int_distribution does things.