Quick rand() question:
I know you're not supposed to use "rand() % 1024" for instance,
because it focuses on the lower bits. However, it seems to me that
given that the argument is not a power of two (or near a power of
two), that this is not an issue. The upper bits will participate
equally in the result with the lower. Am I missing something?
Thanks!
-- Rich --
15  3945 
On Oct 22, 6:04 pm, Rich Fife <rf...@amug.org wrote:
Quick rand() question:
I know you're not supposed to use "rand() % 1024" for instance,
because it focuses on the lower bits. However, it seems to me
that given that the argument is not a power of two (or near a
power of two), that this is not an issue. The upper bits will
participate equally in the result with the lower. Am I missing
something?
Yes, you are missing some mathematical analysis to back up what you
just said. If you do (rand() % 1023) on Microsoft Visual C++ or
WATCOM C/C++, 32 of the possible outputs will have an extra 3% bias no
matter how good your random number generator is. No C compiler's
rand() that I have ever seen has, by itself, a worse effect on random
output than that.
The C.L.C. FAQ about this gives extremely misleading advice on this
point and it should seriously be ignored. If you want to seriously
deal with random numbers just read my page about it: http://www.pobox.com/~qed/random.html
I build up a *REAL* ranged random number generator with reasonable
performance characteristics , culminating in the randrange() function
that removes all the primary problems with ranged random numbers. If
you want something with pure random bit quality you can always use the
Mersenne Twister or Fortuna as a base for my generator function.
--
Paul Hsieh http://www.pobox.com/~qed/ http://bstring.sf.net/
Paul Hsieh <we******@gmail .comwrote:
No C compiler's
rand() that I have ever seen has, by itself, a worse effect on random
output than that.
Then you've never seen the truely bad BSD rand() that fostered most of
the paranoia about rand. With it, rand() % 1 generated the sequence 0,
1, 0, 1, 0, 1, ....
--
Larry Jones
This game lends itself to certain abuses. -- Calvin
On Thu, 23 Oct 2008 10:46:39 -0400, lawrence.jones wrote:
Paul Hsieh <we******@gmail .comwrote:
>No C compiler's
rand() that I have ever seen has, by itself, a worse effect on random
output than that.
Then you've never seen the truely bad BSD rand() that fostered most of
the paranoia about rand. With it, rand() % 1 generated the sequence 0,
1, 0, 1, 0, 1, ....
That would be bad, as rand() % 1 should only ever produce 0.
Kelsey Bjarnason <ke*****@lgisp. netwrites:
On Thu, 23 Oct 2008 10:46:39 -0400, lawrence.jones wrote:
>Paul Hsieh <we******@gmail .comwrote:
>>No C compiler's
rand() that I have ever seen has, by itself, a worse effect on random
output than that.
Then you've never seen the truely bad BSD rand() that fostered most of
the paranoia about rand. With it, rand() % 1 generated the sequence 0,
1, 0, 1, 0, 1, ....
That would be bad, as rand() % 1 should only ever produce 0.
Obviously Larry is using a font that doesn't distinguish clearly
enough between '%' and '&'. Yeah, that's it.
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister" la************@ siemens.com wrote:
Paul Hsieh <we******@gmail .comwrote:
>No C compiler's rand() that I have ever seen has, by itself, a
worse effect on random output than that.
Then you've never seen the truely bad BSD rand() that fostered
most of the paranoia about rand. With it, rand() % 1 generated
the sequence 0, 1, 0, 1, 0, 1, ....
FYI the value of "rand() % 1" is identically 0. Except it may be
considerably slower than just writing "0".
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.
Keith Thompson <ks***@mib.orgw rites:
Kelsey Bjarnason <ke*****@lgisp. netwrites:
>On Thu, 23 Oct 2008 10:46:39 -0400, lawrence.jones wrote:
>>Paul Hsieh <we******@gmail .comwrote:
No C compiler's
rand() that I have ever seen has, by itself, a worse effect on random
output than that.
Then you've never seen the truely bad BSD rand() that fostered most of
the paranoia about rand. With it, rand() % 1 generated the sequence 0,
1, 0, 1, 0, 1, ....
That would be bad, as rand() % 1 should only ever produce 0.
Obviously Larry is using a font that doesn't distinguish clearly
enough between '%' and '&'. Yeah, that's it.
I had presumed that ``const int l=2;'' was the line before.
Obfuscatorially yours,
Phil
--
The fact that a believer is happier than a sceptic is no more to the
point than the fact that a drunken man is happier than a sober one.
The happiness of credulity is a cheap and dangerous quality.
-- George Bernard Shaw (1856-1950), Preface to Androcles and the Lion
CBFalconer wrote:
la************@ siemens.com wrote:
>Paul Hsieh <we******@gmail .comwrote:
>>No C compiler's rand() that I have ever seen has, by itself, a
worse effect on random output than that.
Then you've never seen the truely bad BSD rand() that fostered
most of the paranoia about rand. With it, rand() % 1 generated
the sequence 0, 1, 0, 1, 0, 1, ....
FYI the value of "rand() % 1" is identically 0. Except it may be
considerably slower than just writing "0".
Not if your compiler has a good optimizer... ;-)
--
+----------------------------------------------------------------+
| Charles and Francis Richmond richmond at plano dot net |
+----------------------------------------------------------------+
Charles Richmond <fr*****@tx.rr. comwrites:
CBFalconer wrote:
>la************@ siemens.com wrote:
>>Paul Hsieh <we******@gmail .comwrote:
No C compiler's rand() that I have ever seen has, by itself, a
worse effect on random output than that.
Then you've never seen the truely bad BSD rand() that fostered
most of the paranoia about rand. With it, rand() % 1 generated
the sequence 0, 1, 0, 1, 0, 1, ....
FYI the value of "rand() % 1" is identically 0. Except it may be
considerably slower than just writing "0".
Not if your compiler has a good optimizer... ;-)
Maybe. rand() has side effects, so a call to it can't be optimized
away, *unless* the compiler can prove that the side effects don't
affect the program's output.
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Keith Thompson wrote:
Charles Richmond <fr*****@tx.rr. comwrites:
>CBFalconer wrote:
>>la************@ siemens.com wrote:
Paul Hsieh <we******@gmail .comwrote:
No C compiler's rand() that I have ever seen has, by itself, a
worse effect on random output than that.
Then you've never seen the truely bad BSD rand() that fostered
most of the paranoia about rand. With it, rand() % 1 generated
the sequence 0, 1, 0, 1, 0, 1, ....
FYI the value of "rand() % 1" is identically 0. Except it may be
considerabl y slower than just writing "0".
Not if your compiler has a good optimizer... ;-)
Maybe. rand() has side effects, so a call to it can't be optimized
away, *unless* the compiler can prove that the side effects don't
affect the program's output.
srand( rand() % 1 );
--
Eric Sosman es*****@ieee-dot-org.invalid This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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