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Groups > comp.lang.c > #147809
| From | Tim Rentsch <tr.17687@z991.linuxsc.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Bart, a tcc "issue" |
| Date | 2019-11-08 20:59 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <86k189ejbd.fsf@linuxsc.com> (permalink) |
| References | <uRIwF.17878$kE3.891@fx36.iad> <bYoxF.132400$Ko9.64189@fx48.iad> |
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.
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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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