`glm::linearRand(-1.0f, 1.0f)`, gives more negative numbers than positive. Why is that? `rand` seems ok

Question

I am using glm::linearRand(-1.0f, 1.0f) to generate random floating point numbers between -1 and 1 . Afterwards, I output the percentage of numbers that are positive ( 0.0f or above).

std::srand(time(0)); // Give glm a new seed

uint32_t samples = 1000000000;
uint32_t positive = 0;
uint32_t negative = 0;

for (uint32_t i = 0; i < samples; i++) {
    float rand = glm::linearRand(-1.0f, 1.0f);

    if (rand >= 0.0f) {
        positive++;
    } else {
        negative++;
    }
}

std::cout << "positive %: " << std::setprecision(6) << ((float)positive / samples) * 100 << std::endl;

The percentage of positive numbers always ends up around 49.6% , no matter how often I run the program (with different seeds.), If I understand floating point numbers correctly, there are equally many between -1.0f and 0.0f as there are between 0.0f and 1.0f .

So why does this program always generate more negative numbers than positive numbers?

Note this test code provided by Bob__ (result of comments exchange between him, IkarusDeveloper and Marek R ). This proves that rand is not problem this time, but there is some problem with floats rounding inside of glm::linearRand .

Answer 1

This is a bug in GLM. While the usual admonition about using % with rand is that the range doesn't evenly divide RAND_MAX , this code opts for the more straightforward approach of reducing rand() modulo UINT8_MAX , so that 255 is never produced. Every random value is ultimately derived from combining several such bytes, so 127/255=49.8% of the values will be in the upper half (here, positive).