c++randomfloating-point

Random float number generation


How do I generate random floats in C++?

I thought I could take the integer rand and divide it by something, would that be adequate enough?


Solution

  • rand() can be used to generate pseudo-random numbers in C++. In combination with RAND_MAX and a little math, you can generate random numbers in any arbitrary interval you choose. This is sufficient for learning purposes and toy programs. If you need truly random numbers with normal distribution, you'll need to employ a more advanced method.


    This will generate a number from 0.0 to 1.0, inclusive.

    float r = static_cast <float> (rand()) / static_cast <float> (RAND_MAX);
    

    This will generate a number from 0.0 to some arbitrary float, X:

    float r2 = static_cast <float> (rand()) / (static_cast <float> (RAND_MAX/X));
    

    This will generate a number from some arbitrary LO to some arbitrary HI:

    float r3 = LO + static_cast <float> (rand()) /( static_cast <float> (RAND_MAX/(HI-LO)));
    

    Note that the rand() function will often not be sufficient if you need truly random numbers.


    Before calling rand(), you must first "seed" the random number generator by calling srand(). This should be done once during your program's run -- not once every time you call rand(). This is often done like this:

    srand (static_cast <unsigned> (time(0)));
    

    In order to call rand or srand you must #include <cstdlib>.

    In order to call time, you must #include <ctime>.