How to get current seed from C++ rand()?

Question

I generate a few thousand object in my program based on the C++ rand() function. Keeping them in the memory would be exhaustive. Is there a way to copy the CURRENT

Answer 1

Use srand() to set the seed. save the value you used as the seed.

http://cplusplus.com/reference/clibrary/cstdlib/srand/

Answer 2

Does anyone know other random number generators with implemented seed-stealer

All standard C++11 random number generators (also available in TR1 and in Boost) offer this functionality. You can simply copy the generator objects or serialize/deserialize them.

Answer 3

Is there a way to copy the CURRENT seed of rand() at any given time?

What follows is an implementation-specific way to save and restore the pseudo-random number generator (PRNG) state that works with the C library on Ubuntu Linux (tested on 14.04 and 16.04).

#include <array>
#include <cstdlib>
#include <iostream>

using namespace std;

constexpr size_t StateSize = 128;
using RandState = array<char, StateSize>;

void save(RandState& state) {
    RandState tmpState;
    char* oldState = initstate(1, tmpState.data(), StateSize);
    copy(oldState, oldState + StateSize, state.data());
    setstate(oldState);
}

void restore(RandState& state) {
    setstate(state.data());
}

int main() {
    cout << "srand(1)\n";

    srand(1);

    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';

    cout << "srand(1)\n";

    srand(1);

    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';

    cout << "save()\n";

    RandState state;
    save(state);

    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';

    cout << "restore()\n";

    restore(state);

    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
    cout << "  rand(): " << rand() << '\n';
}

This relies on:

1. the same PRNG being used by the C library to expose both rand() and random() interfaces, and
2. some knowledge about the default initialization of this PRNG in the C library (128 bytes state).

If run, this should output:

srand(1)
  rand(): 1804289383
  rand(): 846930886
  rand(): 1681692777
  rand(): 1714636915
  rand(): 1957747793
  rand(): 424238335
  rand(): 719885386
  rand(): 1649760492
srand(1)
  rand(): 1804289383
  rand(): 846930886
  rand(): 1681692777
  rand(): 1714636915
save()
  rand(): 1957747793
  rand(): 424238335
  rand(): 719885386
  rand(): 1649760492
restore()
  rand(): 1957747793
  rand(): 424238335
  rand(): 719885386
  rand(): 1649760492

This solution can help in some cases (code that can't be changed, reproducing execution for debugging purpose, etc...), but it is obviously not recommended as a general one (e.g. use C++11 PRNG which properly support this).

Answer 4

There's no standard way to obtain the current seed (you can only set it via srand), but you can reimplement rand() (which is usually a linear congruential generator) by yourself in a few lines of code:

class LCG
{
private:
    unsigned long next = 1;
public:

    LCG(unsigned long seed) : next(seed) {}

    const unsigned long rand_max = 32767

    int rand()
    {
        next = next * 1103515245 + 12345;
        return (unsigned int)(next/65536) % 32768;
    }

    void reseed(unsigned long seed)
    {
        next = seed;
    }

    unsigned long getseed()
    {
        return next;
    }
};

Answer 5

You could try saving the value that you used to seed right before (or after) the srand.

So, for example:

int seed = time(NULL);
srand(time(NULL));

cout << seed << endl;
cout << time(NULL);

The two values should be the same.

Answer 6

The random number generation classes in C++11 support operator<< to store their state (mostly the seed) and operator>> to read it back in. So, basically, before you create your objects, save the state, then when you need to re-generate same sequence, read the state back in, and off you go.