random_shuffle not really random

Question

I\'m using the random_shuffle on a vector like this:

#include 
vector  deck;
//some code to add cards to the deck h         


        

Answer 1

With current C++ (i.e. C++11) you can use the shuffle algorithm which can take a pseudo random number generator (PRNG) object (which you can seed) as third parameter:

#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;

int main(int argc, char **argv)
{
  vector v;
  for (int i = 1; i(time(0)));
  shuffle(v.begin(), v.end(), g);
  for (auto &x : v)
    cout << x << ' ';
  cout << '\n';
}

(for GCC 4.8.2 you need to compile it via g++ -std=c++11 -Wall -g shuffle.cc -o shuffle)

In the above example, the PRNG is seeded with the current system time.

For pre-C++11 compilers you only have the random_shuffle algorithm in the STL - but even with that you can optionally specify a number generator object/function to it. Note that you can't just pluck in a PRNG object like mtl19937 into it (because it does not provide a operator()(U upper_bound) member).

Thus, you can supply your own adapter like this:

#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;

struct Gen {
  mt19937 g;
  Gen()
   : g(static_cast(time(0)))
  {
  }
  size_t operator()(size_t n)
  {
    std::uniform_int_distribution d(0, n ? n-1 : 0);
    return d(g);
  }
};

int main(int argc, char **argv)
{
  vector v;
  for (int i = 1; i::const_iterator i = v.begin(); i != v.end(); ++i)
    cout << *i << ' ';
  cout << '\n';
}