How do I generate a random number within a range consisting of negative and positive values in C++?

Question

I have created a cube in Visual Studio using C++ and I am trying to give it a random direction to move along. From what I have seen online, the equations I'm using for random numbers should be generating a random number between -1 and 1 or more formally (-1, 1). But for some reason, the random numbers being generated are the exact same.

I feel like there is something wrong with the division but I just cannot figure it out. Any help would be appreciated!

// Give the cube a random direction
float x = 2*((float)rand()/(float)(RAND_MAX + 1.0)) - 1; 
float y = 2*((float)rand()/(float)(RAND_MAX + 1.0)) - 1;
float z = 2*((float)rand()/(float)(RAND_MAX + 1.0)) - 1;

m_directionVector = XMFLOAT3(x, y, z);

All those casts are unnecessary, `rand() / (RAND_MAX + 1.0)` already performs fractional division. — fredoverflow, Mar 02 '14 at 10:05

score 2 · Answer 1 · answered Mar 02 '14 at 08:06

2

you should change seed for new random numbers. use:

srand(time(NULL));

answered Mar 02 '14 at 08:06

Behnam Safari

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score 1 · Answer 2 · answered Mar 02 '14 at 11:54

    #include <iostream>
    #include <chrono>
    #include <random>

    int main()
    {
        // use current time as a seed
        unsigned nSeed = static_cast<unsigned>
                 (std::chrono::system_clock::now().time_since_epoch().count());

        // choose your favourite random engine
        std::default_random_engine gen(nSeed);

        // chose a distribution: uniform, normal, etc.
        double dLo = -10.0;
        double dHi = 10.0;
        std::uniform_real_distribution<double> dist(dLo, dHi);

        for (int i=0; i<10; ++i)
            std::cout << dist(gen) << std::endl;

        return 0;
    }

.

Note: The above code is pinched (with minor alterations) from http://www.cplusplus.com.

How do I generate a random number within a range consisting of negative and positive values in C++?

2 Answers2