我的c ++随机数不是随机的

Question

I am running this

#include <boost/mpi.hpp>
#include <iostream>
#include <vector>
#include <cstdlib>
#include <time.h>
namespace mpi = boost::mpi;

int main()
{
    mpi::environment env;
    mpi::communicator world;



    srand (time(NULL));
    std::srand(time(0) + world.rank());
    int my_number = std::rand();
    if (world.rank() == 0) {
        std::vector<int> all_numbers;
        gather(world, my_number, all_numbers, 0);
        for (int proc = 0; proc < world.size(); ++proc)
            std::cout << "Process #" << proc << " thought of "
            << all_numbers[proc] << std::endl;
    } else {
        gather(world, my_number, 0);
    }

    return 0;
}

to distributively generate random number, however, it gives me the number around the same magnitude everytime....

dhcp-18-189-66-216:ising2 myname$ make
mpic++ -I/usr/local/include/boost -L/usr/local/lib -lboost_mpi -lboost_serialization main.cpp -o main
mpirun -n 4 main
Process #0 thought of 238772362
Process #1 thought of 238789169
Process #2 thought of 238805976
Process #3 thought of 238822783
dhcp-18-189-66-216:ising2 myname$ make
mpic++ -I/usr/local/include/boost -L/usr/local/lib -lboost_mpi -lboost_serialization main.cpp -o main
mpirun -n 4 main
Process #0 thought of 238805976
Process #1 thought of 238822783
Process #2 thought of 238839590
Process #3 thought of 238856397
dhcp-18-189-66-216:ising2 myname$ make
mpic++ -I/usr/local/include/boost -L/usr/local/lib -lboost_mpi -lboost_serialization main.cpp -o main
mpirun -n 4 main
Process #0 thought of 238856397
Process #1 thought of 238873204
Process #2 thought of 238890011
Process #3 thought of 238906818
dhcp-18-189-66-216:ising2 myname$ 

In the website, http://www.boost.org/doc/libs/1_55_0/doc/html/mpi/tutorial.html , others said they get:

Process #0 thought of 332199874
Process #1 thought of 20145617
Process #2 thought of 1862420122
Process #3 thought of 480422940
Process #4 thought of 1253380219
Process #5 thought of 949458815
Process #6 thought of 650073868

I am very confused.... Any help? Thank you.

Answer 1

You Problem is the rand() function of the cstdlib. It is no good random number generator. If you want to use proper random numbers in c++ use some from the header from c++-11 or external random number generators (eg mersenne twister). But nonetheless, using random numbers in for parallel programs is no easy task. You should use random number generators, which are specialised on that (eg r250_omp.h).

Answer 2

The problem is likely to be caused by your rand . See the discussion and answers for this question: First random number is always smaller than rest

It seems that the generated numbers for neighboring seeds (your case) can be quite heavily correlated. rand's implementations might vary, and for some implementations it seems to be a much more pronounced phenomena.

Answer 3

I think your random gen should be like this:

int max=100, min=0;
srand(time(NULL));
int random = (rand() % (max-min)) + min;