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Re: Bart, a tcc "issue"

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From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: Bart, a tcc "issue"
Date Sun, 10 Nov 2019 03:04:57 +0000
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Richard Damon <Richard@Damon-Family.org> writes:

> On 11/9/19 6:16 PM, Ben Bacarisse wrote:
>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>> <cut>
>>> 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.
>> 
>> Are you considering edge cases in the C standard?  If so, one of those
>> has RAND_MAX == UINT_MAX.
>
> Since rand() returns an int, the only way for RAND_MAX to be UINT_MAX is
> if UINT_MAX == INT_MAX (which generally means a Signed-Magnitude machine
> or some other odd architecture). And yes, if you want to deal with that
> sort of machine a lot of care needs to be taken, as a lot of things we
> often take for granted aren't true, like that an unsigned can count the
> number of values between any two signed numbers!!

Yes, it's a gruesome corner case for some types of code.

-- 
Ben.

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Thread

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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