Problem with srand() and rand()

[Saeraph]

Googled around and could not find a straightforward solution.

When outputting the result of rand(), the value is not random, but only gradually increases every second.

#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

int main()
{
   srand(time(NULL));

   cout << rand() << endl;

   return 0;
}

Running the program once every second gives these results:

21339
21346
21350
21354
etc.

I'm using Code::Blocks with the GNU GCC Compiler and think the problem may lie with that. When I tried running the code on codepad.org, it ran as it should have.

[Dy7SLyTq]

yeah i think its that cause i see no problem. try testing it in a while loop

[yelnatz]

rand() is a PRNG. It means pseudorandom number generator.

The thing is, you run your program and then exit it right away. So every time you run it, the seed ( srand(time(NULL)); ) is reset. The seed is using time as you can see.

It should give out a better random generation if you do something like:

srand(time(NULL));

while( true )
{
   cout << rand() << endl;
   sleep(1000); // wait for one second and then loop back
}

[modoran]

That sleep() call it absolutely useless there, it will not influence program output in any way, you will get exactly the same results with or without it.

[Peter87]

yelnatz's code will probably give better random numbers compared to running Saeraph's code once every second.

[S G H]

modoran wrote:

That sleep() call it absolutely useless there, it will not influence program output in any way, you will get exactly the same results with or without it.

+1

Peter87 wrote:

yelnatz's code will probably give better random numbers compared to running Saeraph's code once every second.

+1

You should do an empty rand() before the real rand() so it gets re-pseudorandomly-seeded, like:

#include <iostream>
#include <ctime>
#include <cstdlib>

int main()
{
    srand(time(NULL));
    rand();
    cout << rand() << endl;
    return 0;
}

Not sure, but that should work.

[Saeraph]

Thank you EssGeEich, your solution worked. I can't figure out why that worked though.

[S G H]

Because each time you call rand(), its seed gets re-randomized, mostly the algorithm is a linear multiplication of the seed, so summing up multiplications and capping the result to RAND_MAX ( as done inside rand() )will give a more random value.

Pseudocode Explaination:

int main()
{
    srand(time(0)); // seed is now, say... (40100*4)+2
    rand(); // returns 160402, then seed is, say... (160402*4)+2
    rand(); // now this was to return 641610 but if it exceeds RAND_MAX
// it gets modulo'ed to RAND_MAX so the number will lower down a lot,
// looking then random. again seed goes multiplied by 4 and added by 2
}

The "(x*4)+2" is a theorical part, but that should be the "issue".

Another solution is to multiply the result from time(0) like:

srand(time(NULL)*436);
and a single call to rand() should be enough.