Question

I want the random number generation to be controllable with a variable. For example, if I say 100, numbers should be generated with maximum randomness, and if I say 50, the numbers should not be ‘as random’. If I say ‘0’, possibly the random numbers should not be random at all – maybe all numbers generated are same.

Any idea what I can use to generate controlled random numbers like these? Any idea if some C++ inbuilt library, or maybe Boost library does this?

It will be good to have a way to regenerate the same sequence of random numbers, so a way to seed the generator would also be good.

Answer 1

The way you describe your requirement seems to suggest that using a normal distribution (aka Gaussian distribution) may be the way to go. It has two parameters: Mean and standard deviation. If you set the standard deviation very low, you get random values that are probably quite close to the mean. If you set it to a large value, you get them distributed more widely.

In C++11 a normal distribution is available from the standard library. If C++11 is not an option for you, the Boost library has it, too.

Here is some example code:

#include <iostream>
#include <iomanip>
#include <string>
#include <map>
#include <random>
#include <cmath>
#include <iomanip>

int main()
{
  std::random_device rd;
  std::mt19937 gen(rd());

  std::cout << std::fixed << std::setprecision(3);

  /* Mean 5, standard deviation very low (0.002): */
  std::normal_distribution<> d1(5,0.002);
  for (int i = 0 ; i < 20 ; ++i)
    std::cout << std::setw(7) << d1(gen) << "  ";
  std::cout << std::endl;

  /* Mean 5, standard deviation relatively high (2.0): */
  std::normal_distribution<> d2(5,2);
  for (int i = 0 ; i < 20 ; ++i)
    std::cout << std::setw(7) << d2(gen) << "  ";
  std::cout << std::endl;

  return 0;
}

Here is the output:

4.998    5.003    5.001    5.002    5.001    5.001    4.998    5.000    4.999    5.001    5.000    5.003    4.999    5.000    5.001    4.998    5.000    4.999    4.996    5.001  
2.781    3.795    5.669   -0.109    7.831    3.302    3.823    4.439    4.672    4.461    6.626    5.139    6.882    5.406    6.526    5.831    6.759    2.627    3.918    4.617

As you can see, in the first row all numbers are quite close to 5 (i.e., to use your wording, “randomness” is low), while in the second row the numbers are spread much more widely.

(EDIT: Of course, the randomness of these numbers isn’t really affected. It’s just that the standard deviation parameter makes the values more likely to emerge in a smaller (stddev low) or wider (stddev high) range of numbers.)