Producing random float from negative to positive range?

Question

I am trying to produce a random float within the range -50.0 and 50.0 inclusively, using rand(). I've looked everywhere for an answer but it deals with ints and % operator.

Answer 1

Try this:

1. rand() gives you a number between 0 and RAND_MAX
2. so divide by RAND_MAX to get a number between 0 and 1
3. you desire a range of 100 from -50 to 50, so multiply by 100.0
4. finally shift the center from 50 (between 0 and 100 per point 3) to zero by subtracting 50.0

Answer 2

Try this:

float RandomNumber(float Min, float Max)
{
    return ((float(rand()) / float(RAND_MAX)) * (Max - Min)) + Min;
}

Answer 3

Honestly, all present answers don't explain that there is a change in distribution in their solutions(I am assuming that rand() follows the uniform distribution! correct me if I am wrong please). Use a library please, and my recommendation is using the new facilities in C++0x:

#include <random>
#include <functional>

int main()
{
    std::mt19937 generator;
    std::uniform_real_distribution<float> uniform_distribution(-50.0, 50.0);
    auto my_rand = std::bind(uniform_distribution, generator);
}

If you can't, Boost is a perfect choice. That way, you can use my_rand() just like good ol' rand():

std::vector<float> random_numbers(1000);
std::generate(random_numbers.begin(), random_numbers.end(), my_rand);

Answer 4

((float)rand())/RAND_MAX * 100.0 - 50.0