[vox-tech] help: random number problem in C (yet again!)

[Rod Roark]

On my bleeding-edge Gentoo machine, "man 3 rand" says "The rand()=20
function returns a pseudo-random integer between 0 and RAND_MAX."

Perhaps this is _inclusive_ of RAND_MAX, since an example in the same man=
=20
page includes the term 10.0*rand()/(RAND_MAX+1.0).  This is also consiste=
nt=20
with the notion that "MAX" stands for "maximum".

-- Rod
   http://www.sunsetsystems.com/

On Saturday 20 April 2002 10:49 am, Peter Jay Salzman wrote:
> hello all,
>
> given the code:
>
>    #include <stdio.h>
>    #include <stdlib.h>
>    int Irand(int low, int high);
>
>
>    int main(void)
>    {
>       unsigned int N =3D 4;
>       unsigned int M =3D 4;
>       unsigned int count =3D 0;
>       int x, y;
>       srand(2844604396);
>
>       while (1) {
>          ++count;
>          x =3D Irand(0, N-1);
>          if (x<0 || x>3) {
>             printf("count: %u, x: %u\n", count, x);
>             exit(0);
>          }
>       }
>
>    }
>
>
>    int Irand(int low, int high)
>    {
>       return low + (int)( (double)(high-low+1) * rand()/RAND_MAX );
>    }
>
>
> when count =3D 109595656, the random number generator (which should onl=
y
> return integers between 0 and 3) returns a 4.
>
> clearly, my random number generator is faulty, but i can't figure out
> why i should return a 4.   and after 109595656 calls, this is a very
> uncommon occurance!
>
> does anyone know why this subroutine should return a 4 under any
> circumstances?
>
> can someone suggest a better subroutine to calculate an integer between
> low and high?
>
> btw, the seed is set to a particular value because i was tracking down =
a
> problem.  normally, my seed is obtained through a call to:
>
>    unsigned int SeedRandomGenerator(void)
>    {
>       unsigned int seed;
>       FILE *fp;
>
>       if( (fp =3D fopen("/dev/random", "r")) =3D=3D NULL)
>             die("Can't open /dev/random for reading.");
>       fread(&seed, sizeof(unsigned int), 1, fp);
>       fclose(fp);
>       srand(seed);
>       return seed;
>    }
>
> this is a show stopper for me.  any help greatly appreciated!!
>
> pete
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