generates same number in Linux, but not in Windows

Question

The code below is meant to generate a list of five pseudo-random numbers in the interval [1,100]. I seed the default_random_engine with time(0), which returns the system time in unix time. When I compile and run this program on Windows 7 using Microsoft Visual Studio 2013, it works as expected (see below). When I do so in Arch Linux with the g++ compiler, however, it behaves strangely.

In Linux, 5 numbers will be generated each time. The last 4 numbers will be different on each execution (as will often be the case), but the first number will stay the same.

Example output from 5 executions on Windows and Linux:

      | Windows:       | Linux:        
---------------------------------------
Run 1 | 54,01,91,73,68 | 25,38,40,42,21
Run 2 | 46,24,16,93,82 | 25,78,66,80,81
Run 3 | 86,36,33,63,05 | 25,17,93,17,40
Run 4 | 75,79,66,23,84 | 25,70,95,01,54
Run 5 | 64,36,32,44,85 | 25,09,22,38,13

Adding to the mystery, that first number periodically increments by one on Linux. After obtaining the above outputs, I waited about 30 minutes and tried again to find that the 1st number had changed and now was always being generated as a 26. It has continued to increment by 1 periodically and is now at 32. It seems to correspond with the changing value of time(0).

Why does the first number rarely change across runs, and then when it does, increment by 1?

The code. It neatly prints out the 5 numbers and the system time:

#include <iostream>
#include <random>
#include <time.h>

using namespace std;

int main()
{
    const int upper_bound = 100;
    const int lower_bound = 1;

    time_t system_time = time(0);    

    default_random_engine e(system_time);
    uniform_int_distribution<int> u(lower_bound, upper_bound);

    cout << '#' << '\t' << "system time" << endl
         << "-------------------" << endl;

    for (int counter = 1; counter <= 5; counter++)
    {
        int secret = u(e);
        cout << secret << '\t' << system_time << endl;
    }   

    system("pause");
    return 0;
}

Answer 1

Here's what's going on:

• default_random_engine in libstdc++ (GCC's standard library) is minstd_rand0, which is a simple linear congruential engine:

  typedef linear_congruential_engine<uint_fast32_t, 16807, 0, 2147483647> minstd_rand0;
  

• The way this engine generates random numbers is x_i+1 = (16807x_i + 0) mod 2147483647.

• Therefore, if the seeds are different by 1, then most of the time the first generated number will differ by 16807.

• The range of this generator is [1, 2147483646]. The way libstdc++'s uniform_int_distribution maps it to an integer in the range [1, 100] is essentially this: generate a number n. If the number is not greater than 2147483600, then return (n - 1) / 21474836 + 1; otherwise, try again with a new number.

  It should be easy to see that in the vast majority of cases, two ns that differ by only 16807 will yield the same number in [1, 100] under this procedure. In fact, one would expect the generated number to increase by one about every 21474836 / 16807 = 1278 seconds or 21.3 minutes, which agrees pretty well with your observations.

MSVC's default_random_engine is mt19937, which doesn't have this problem.

Answer 2

The std::default_random_engine is implementation defined. Use std::mt19937 or std::mt19937_64 instead.

In addition std::time and the ctime functions are not very accurate, use the types defined in the <chrono> header instead:

#include <iostream>
#include <random>
#include <chrono>

int main()
{
    const int upper_bound = 100;
    const int lower_bound = 1;

    auto t = std::chrono::high_resolution_clock::now().time_since_epoch().count();

    std::mt19937 e;
    e.seed(static_cast<unsigned int>(t)); //Seed engine with timed value.
    std::uniform_int_distribution<int> u(lower_bound, upper_bound);

    std::cout << '#' << '\t' << "system time" << std::endl
    << "-------------------" << std::endl;

    for (int counter = 1; counter <= 5; counter++)
    {
        int secret = u(e);

        std::cout << secret << '\t' << t << std::endl;
    }   

    system("pause");
    return 0;
}

Answer 3

In Linux, the random function is not a random function in the probabilistic sense of the way, but a pseudo random number generator. It is salted with a seed, and based on that seed, the numbers that are produced are pseudo random and uniformly distributed. The Linux way has the advantage that in the design of certain experiments using information from populations, that the repeat of the experiment with known tweaking of input information can be measured. When the final program is ready for real-life testing, the salt (seed), can be created by asking for the user to move the mouse, mix the mouse movement with some keystrokes and add in a dash of microsecond counts since the beginning of the last power on.

Windows random number seed is obtained from the collection of mouse, keyboard, network and time of day numbers. It is not repeatable. But this salt value may be reset to a known seed, if as mentioned above, one is involved in the design of an experiment.

Oh yes, Linux has two random number generators. One, the default is modulo 32bits, and the other is modulo 64bits. Your choice depends on the accuracy needs and amount of compute time you wish to consume for your testing or actual use.