Re: rand doesn't seem to generate random numbers

golnar wrote:


As others have pointed out, you need to seed the random generator. The
benefit is that saving a particular seed allows you to reproduce the
entire sequence of random numbers that grow from it. That's
particularly useful if you find a sequence of randoms that evinces a bug
in your code. Saving the seed lets you use the sequence in your
regression suite.

I don't think most people like to admit they do this (and will probably
throw rocks at me for suggesting it), but the quick and dirty way to
generate a seed is to use the current time. I also like to generate one
throw-away number right after seeding, because the first number
sometimes doesn't vary much for close seeds.

OK, I've just slapped down some code to help you get a feel for how the
seed affects the random number sequence on your platform. On the
Windows XP box I'm using right now, running five iterations for each of
ten consecutive seeds, I get the following results:

         seed 0 1 2 3 4
------------------------------------------------------------------------
   1157498310 41 18467 6334 26500 19169
   1157498311 15724 11478 29358 26962 24464
   1157498312 5705 28145 23281 16827 9961
   1157498313 491 2995 11942 4827 5436
   1157498314 32391 14604 3902 153 292
   1157498315 12382 17421 18716 19718 19895
   1157498316 5447 21726 14771 11538 1869
   1157498317 19912 25667 26299 17035 9894
   1157498318 28703 23811 31322 30333 17673
   1157498319 4664 15141 7711 28253 6868

Here's the code:

#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iomanip>
#include <string>

template<
     typename seed_type, /* E.g. std::time_t. */
     unsigned count, /* Desired number of calls to rand(). */
     unsigned width> /* Column width of output. */
class srand_test {
public:

     typedef unsigned count_type;
     typedef unsigned width_type;

     /* Print column headers and underline them. */
     static void print_header() {

         std::cout << std::setw(width) << "seed";
         for(count_type i = 0; i < count; ++i) {
             std::cout << std::setw(width) << i;
         }

         std::string underline(width * (count + 1), '-');
         std::cout << '\n' << underline << '\n';
     }

     /* Print the seed, followed by the results of 'count'
      * consecutive calls to std::rand(). */
     void operator()(seed_type seed) {

         std::cout << std::setw(width) << seed;

         for(count_type i = 0; i < count; ++i) {
             std::cout << std::setw(width) << std::rand();
         }

         std::cout << '\n';
     }
};

int main() {

     /* Run the same test ten times, but with a slightly different
      * seed each time. */

     typedef srand_test<std::time_t, 5, 12> test_type;

     test_type::print_header();

     test_type test;
     std::time_t seed = std::time(0);

     for(unsigned i = 0; i < 10; ++i) {
         test(seed + i);
     }
}