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Re: Bart, a tcc "issue"

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On 11/8/2019 8:59 PM, Tim Rentsch wrote:
> Richard Damon <Richard@Damon-Family.org> writes:
> 
>> On 11/6/19 6:40 PM, DFS wrote:
>>
>>> I wrote a distribution of characters counter, and on tcc (0.9.26) on
>>> Windows it always undercounts the last character in the set (9).  If you
>>> have tcc on Windows maybe you could test it.  Doesn't happen on
>>> gcc/MSYS2/Windows.
>>
>> This is a fundamental problem.  A very simple version of it would be to
>> try to take 12 apples and divide them into 5 (or 7) equal piles (without
>> cutting any of them).
>>
>> The 12 apples are the RAND_MAX possible values.
>>
>> The 5 piles are the buckets you are putting them into.
>>
>> If the number of buckets doesn't exactly divide into the number of
>> results you can get from the random number generator, the output won't
>> be equally likely.  This most noticeable when the number of possible
>> numbers is smaller.
>>
>> The simplest solution would be to throw away a few of the random
>> numbers, so that the number of 'usable' random numbers is a multiple of
>> the number of buckets.
>>
>> The simple modification to the code is rather than using:
>> (low + rand() / (RAND_MAX / (high - low + 1) + 1))
>> use
>> (low + rand() / (RAND_MAX / (high - low + 1)))
>> and redraw if the result is > high.
> 
> It's interesting -- three different people posted corrections or
> improvements to the uniform interval code, and all of the revisions
> have been a little bit off in one way or another.  Care is needed to
> deal with edge cases correctly.
> 
>      int
>      uniform_less_than( unsigned k ){
>          unsigned d, r;
> 
>          if(  k-1 > RAND_MAX  ||  k == 0  )  oops_someone_messed_up();
> 
>          d = (RAND_MAX - (k-1)) / k + 1;
>          do  r = rand()/d;  while(  r >= k  );
> 
>          return  r;
>      }
> 
>      int
>      uniform_on_closed_interval( int a, int b ){
>          if(  a > b  )  return  uniform_on_closed_interval( b, a );
> 
>          return  a + uniform_less_than( b + 1u - a );
>      }
> 
> A good test for a uniform closed-interval RNG is to check that
> it handles the interval 0..RAND_MAX, including both endpoints,
> correctly.
> 

Fwiw, off topic wrt integers, a while ago I just used an epsilon to deal 
with random intervals wrt rand(). Sometimes RAND_MAX does not get hit 
often enough for my needs, so I adjusted epsilon. Heck, is rand() even 
guaranteed to return RAND_MAX at all? Well, I used this code for a 
fractal, so it was good enough, wrt 0...1 being a unit interval:
__________________________________
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>


double ct_rand()
{
     return rand() / (RAND_MAX - 0.);
}


int main(void)
{
     {
         unsigned long n = 2000000;
         double eps = .00000666;

         for (unsigned long i = 0; i < n; ++i)
         {
             double rnum = ct_rand();

             //printf("i[%lu]:rnum = %lf\n", i, rnum);

             if (1. - rnum < eps)
             {
                 double dif = eps - (1. - rnum);
                 printf("i[%lu]:rnum = %lf, %lf\n", i, rnum, dif);

                 rnum = 1.; // artificially set high...
             }
         }
     }

     printf("\n\nfin...\n");

     return 0;
}
__________________________________


Pretty nasty... ;^)

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Bart, a tcc "issue" DFS <nospam@dfs.com> - 2019-11-06 18:40 -0500
  Re: Bart, a tcc "issue" Bart <bc@freeuk.com> - 2019-11-07 00:03 +0000
    Re: Bart, a tcc "issue" Bart <bc@freeuk.com> - 2019-11-07 00:06 +0000
    Re: Bart, a tcc "issue" Bart <bc@freeuk.com> - 2019-11-07 01:44 +0000
  Re: Bart, a tcc "issue" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-07 10:05 -0500
    Re: Bart, a tcc "issue" DFS <nospam@dfs.com> - 2019-11-07 13:22 -0500
    Re: Bart, a tcc "issue" DFS <nospam@dfs.com> - 2019-11-08 09:19 -0500
      Re: Bart, a tcc "issue" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-08 11:03 -0500
  Re: Bart, a tcc "issue" Manfred <noname@add.invalid> - 2019-11-07 18:22 +0100
    Re: Bart, a tcc "issue" Manfred <noname@add.invalid> - 2019-11-07 18:30 +0100
  Re: Bart, a tcc "issue" Richard Damon <Richard@Damon-Family.org> - 2019-11-08 20:51 -0500
    Re: Bart, a tcc "issue" Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-08 20:59 -0800
      Re: Bart, a tcc "issue" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-09 23:16 +0000
        Re: Bart, a tcc "issue" Richard Damon <Richard@Damon-Family.org> - 2019-11-09 20:27 -0500
          Re: Bart, a tcc "issue" Bart <bc@freeuk.com> - 2019-11-10 01:38 +0000
            Re: Bart, a tcc "issue" Richard Damon <Richard@Damon-Family.org> - 2019-11-09 20:59 -0500
              Re: Bart, a tcc "issue" Keith Thompson <kst-u@mib.org> - 2019-11-09 19:22 -0800
          Re: Bart, a tcc "issue" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-10 03:04 +0000
            Re: Bart, a tcc "issue" Richard Damon <Richard@Damon-Family.org> - 2019-11-09 23:04 -0500
              Re: Bart, a tcc "issue" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-10 13:15 +0000
        Re: Bart, a tcc "issue" Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-10 06:04 -0800
        Re: Bart, a tcc "issue" Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-10 07:49 -0800
          Re: Bart, a tcc "issue" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-10 17:49 +0000
      Re: Bart, a tcc "issue" "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-11 02:44 -0800
        Re: Bart, a tcc "issue" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-11 13:57 -0500
          Re: Bart, a tcc "issue" "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 16:51 -0800
        Re: Bart, a tcc "issue" Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 16:03 -0800

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