[Phys-L] noise question(s)

[Bernard Cleyet <bernard@cleyet.org>]

I wish to add noise to the escapement (drive) of my clock pendulum simulation. The below works “sort of”. 

Either it’s bad code or a rather bad random number generator.  I verified the system is C++11, which is supposed to contain a better generator.  However, the sum of the noise output is varyingly ~ -2561 (instead of zero) with 1e6 points for four runs.    

[Later I found code for <random> (is next)]

==========================

#include <iostream>
#include <random>

int main()
{
    std::random_device seed;
    std::mt19937 gen(seed());
    std::uniform_int_distribution<int> dist(0, 100);

    for (int i = 0; i < 10; i++)
    {
        std::cout << dist(gen) << '\n';
    }
}
============================
My use of above:


#include <iostream>
#include <random>

int main()
{
    std::random_device seed;
    std::mt19937 gen(seed());
    std::uniform_int_distribution<int> dist(-100, 100);
    
    for (int i = 0; i < 100; i++)
    {
        std::cout << dist(gen) << '\n';
    }
}

,

which didn’t appear better than my first try. (which is next)



    /* initialize random seed: */
    //srand (time(NULL));
    srand((unsigned)time(0));  

……………………………….

      /* generate number between 0 and 9: */
      
      inoise10 = rand() % 10 ;

      /* generate number between 0 and 1: */

    
      inoise2 = rand() % 2 ;
      
      if (inoise2 == 0) {
          inoise10 = inoise10;
      } else {
          inoise10 = -inoise10;
      }
===================================      
      
I later replaced the second random (directly above) with:  (May be rewritten w/o the else, I think.)

/////////////////////////
      
      if ( N % 2 == 0 ) {
          inoise10 = inoise10;

      } else {
          inoise10 = -inoise10;

      }
   ////////////////////
    
With ~ same result.  

=================================


      noise = inoise10/2;
      
————————


      dr = dr*(1+noise);

——————

  
      ///////////////////////////////////  [
      
      //Euler-Richardson method with quadratic dissipation (drag) and "circular error":
      dr = dr*(1+noise);
      // acceleration at beginning of interval:
      
      double A = - ((4*pi*pi)/(period*period)) * sin(theta)  - r*thetadot*abs(thetadot) +dr;  // was (g/l)*(1+g increment) * sin(theta), ff.  note: r (gamma) includes the mass

etc.  

———————— ——

bc  assumes “white” noise;  will later try 1/f, and random walk.
And is amazed at how much c++ “ tuff” is on the internet.