Programming Forums - rand() isn't so random

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- - rand() isn't so random (http://www.programmingforums.org/showthread.php?t=10573)

Zap	Jun 29th, 2006 2:14 PM

rand() isn't so random

I'm having a problem with the rand() function in the C standard library. I'm sure I'm not the first to complain about this. Every time I use this function, it generates a seemingly random number. But the number(s) is the same each time the program is ran. Is there a way around this? Or a better random number generator somewhere, that doesn't use seeds to generate random numbers? (like a generator based on the time and date)

Prm753

Jun 29th, 2006 2:20 PM

Take a look at this.

nemesis

Jun 29th, 2006 2:22 PM

You need to call srand and give it a seed value which you can get from the time function

#include <stdlib.h>

#include <stdio.h>

#include <time.h>



int main()

{

     long seed = time(NULL); //gets a seed from time

     srand((int)seed); //sets the seed

     for(int i = 0; i != 10; i++)

     {

           printf("%d ", rand());

     }

     return 0;

}

Zap	Jun 29th, 2006 2:30 PM

I thought someone would bring this up. I've tried this. But I still get the same results each time the program is run. I read an article about rand() saying I should seed it with the time. It still yields the same results...

King	Jun 29th, 2006 2:37 PM

Maybe post some code so we can have a look.

nemesis

Jun 29th, 2006 2:37 PM

Well i tested my own code, and it definatly gives different results each time i run it.

Zap	Jun 29th, 2006 2:42 PM

Like even this for example:

#include <stdio.h>

#include <stdlib.h>

#include <time.h>



int main()

{



    int i;



    /* Set seed (initial seed) */

    srand( (unsigned)time( NULL ) );



    for (i = 0; i < 5; i++) {

        printf("%d\n",rand()%50);

    }



    return 0;

}

Keeps giving me:
25
10
33
16
45

nemesis

Jun 29th, 2006 2:45 PM

Well i compiled you code and got 31 0 13 28 39 on the first run and 29 42 13 8 35 on the second run. So i guess it something up with your compiler, but i have no idea.

Arevos

Jun 29th, 2006 3:11 PM

I got 29 32 20 20 29, then 5 32 23 28 30, so it's randomises correctly for me as well. I'm using GCC.

jim mcnamara

Jun 29th, 2006 3:12 PM

Zap -

what C compiler and OS?

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