Op 14 mrt. 2012 09:00 schreef "Jim Michaels" <[email protected]> het
volgende:
>
> this is not about rand_s.
> I am using mt19937 from <random>. this is mersenne twister random number
number generator, entirely different, much more random, has a *far longer*
(like 10^604 or 2^604 or something like that) sequence and more complicated.
>
> anyway, my point *wasn't* about the random number generator - you missed
the point - I even put the point of the problem in the Abstract so people
would have a clue.
>
> I was pointing out that the output range of uniform_int is severely
crippled. it stays in the upper end of the data type instead of
spreading/distributing the input evenly across the range of min0 to max0,
which are specified in the arguments of uniform_int's constructor.
>
> that *IS* what uniform_int is supposed to do isn't it? I assume I read
http://en.wikipedia.org/wiki/Uniform_distribution_%28discrete%29
> correctly...
What GCC version? Does the same happen with std::uniform_int_distribution
with -std=c++0x?
Using tr1 now is stupid btw, every decent compiler and library have these
things in the std namespace where they belong.
Ruben
>
>
>> ________________________________
>> From: Kai Tietz <[email protected]>
>> To: [email protected]
>> Sent: Wednesday, March 14, 2012 12:13 AM
>> Subject: Re: [Mingw-w64-public] problem with <random>'s uniform_int?
>>
>> 2012/3/14 Jim Michaels <[email protected]>:
>> > /*
>> > Author: Jim Michaels
>> > Creation Date: 3/13/2012
>> > Current Date: 3/13/2012
>> > Abstract: Shows what I think is a bug in <random>'s uniform_int
>> > distribution where the mt19937 random number generator I am
>> > feeding through it has its numbers concentrated around the
>> > upper portion of the range rather than spread out across the
>> > entire range of the data type. Am I not undertsanding what
>> > uniform_int is supposed to do? or is it not supposed to
>> > produce discrete statistical uniform distributions?
>> > */
>> >
>> >
>> >
>> > #include <tr1/random>
>> > #include <tr1/stdint.h>
>> > #include <iostream>
>> > using namespace std;
>> >
>> > std::tr1::mt19937 gen;//mersenne twister random number generator
>> > #if defined(__MINGW32__)||defined(_MSC_VER)||defined(__BORLANDC__)
>> > std::tr1::uniform_int<> u(0ULL, 0xffffffffffffffffULL);
>> > #else
>> > std::tr1::uniform_int<> u(0Ui64, 0xffffffffffffffffUi64);
>> > #endif
>> >
>> > inline uint64_t random(void) {
>> > return u(gen);
>> > }
>> >
>> > int main(void) {
>> > int x;
>> > bool isFirst=true;
>> > uint64_t mn=0,mx=0,cur=0;
>> > for (x=0; x < 100; x++) {
>> > cout.fill('0');
>> > cout.width(20);
>> > cur=random();
>> > if (isFirst) {
>> > mn=cur;
>> > mx=cur;
>> > isFirst=false;
>> > }
>> > cout<<cur<<endl;
>> > if (cur>mx) mx=cur;
>> > if (cur<mn) mn=cur;
>> > }
>> > cout<<endl<<"min=";
>> > cout.fill('0');
>> > cout.width(20);
>> > cout<<mn<<"\nmax=";
>> > cout.fill('0');
>> > cout.width(20);
>> > cout<<mx<<"\ndifference=";
>> > cout.fill('0');
>> > cout.width(20);
>> > cout<<mx-mn<<endl;
>> > return 0;
>> > }
>> > //problem: this hovers way too long around
>> > /*
>> > sample output:
>> > 18446744072913795932
>> > 18446744073304931054
>> > 18446744073000918905
>> > 18446744073575839711
>> > 18446744073337503749
>> > 18446744072130546618
>> > 18446744071764878885
>> > 18446744071763422559
>> > 18446744073678977040
>> > 18446744073527044839
>> > 18446744073694353124
>> > 18446744073558749017
>> > 18446744073570802426
>> > 18446744072532038929
>> > 18446744073583248563
>> > 18446744073628418359
>> > 18446744073525585066
>> > 18446744072842422873
>> > 18446744072851762780
>> > 18446744073347638446
>> > 18446744072162347015
>> > 18446744072817088873
>> > 18446744073187415213
>> > 18446744073535572907
>> > 18446744071577799048
>> > 18446744072230969164
>> > 18446744072841661626
>> > 18446744073061566917
>> > 18446744073426054765
>> > 18446744072341001254
>> > 18446744072329729627
>> > 18446744072669053378
>> > 18446744072595640013
>> > 18446744072606313980
>> > 18446744072229840436
>> > 18446744072447029159
>> > 18446744072838875481
>> > 18446744073161637570
>> > 18446744073684075993
>> > 1844674407295130874 5
>> > 18446744072944631962
>> > 18446744072398850529
>> > 18446744072694865646
>> > 18446744073495756929
>> > 18446744072264748328
>> > 18446744072702453906
>> > 18446744072829941902
>> > 18446744072190477567
>> > 18446744072824680856
>> > 18446744072461283062
>> > 18446744073369711431
>> > 18446744072655938920
>> > 18446744072882903664
>> > 18446744072445861649
>> > 18446744072333885098
>> > 18446744072228208822
>> > 18446744072467034220
>> > 18446744072180375568
>> > 18446744072738533078
>> > 18446744073536653217
>> > 18446744071878841848
>> > 18446744073180228744
>> > 18446744071928290152
>> > 18446744072885671619
>> > 18446744072641251774
>> > 18446744072941808995
>> > 18446744072940068643
>> > 18446744071587653283
>> > 18446744073452171529
>> > 18446744072417095975
>> > 18446744073240984662
>> > 18446744073534709601
>> > 18446744071909774250
>> > 18446744071764857140
>> > 18446744072686194971
>> > 18446744072892367725
>> > 18446744073025437414
>> > 18446744073660252266
>> > 18446744072911911023
>> > 18446744073405741205
>> > 18 446744073311686108
>> > 18446744073058725738
>> > 18446744073516353565
>> > 18446744072060476037
>> > 18446744072759924511
>> > 18446744073655691123
>> > 18446744072982966111
>> > 18446744072823059978
>> > 18446744071928274454
>> > 18446744071967957672
>> > 18446744071775629235
>> > 18446744072561930879
>> > 18446744073353901113
>> > 18446744072400586818
>> > 18446744072334388088
>> > 18446744072666734544
>> > 18446744072651826116
>> > 18446744071826455169
>> > 18446744071853359699
>> > 18446744071679627487
>> >
>> > min=18446744071577799048
>> > max=18446744073694353124
>> > difference=00000000002116554076
>> > */
>> >
>> >
>> > -------------
>> > Jim Michaels
>>
>> Hello Jim,
>>
>> This random-number generation is based on value-range of rand (gcc
>> doesn't use here rand_s which would provide better random-values).
>> Its range for random-numbers is in range of 0-0x7fff (which your test
>> explicit proves, as C++'s random-implementation expands range by
>> another 16-bits).
>>
>> So the issue isn't in <random> implementation, but in the limited
>> range of random-number generation by msvcrt's rand() function.
>>
>> By looking more detailed into this you can find that MS' rand function
>> is using here a linear congruential scheme. The function is
>> multiplying and adding to a number, which is taken module 2^32, and
>> returning the upper 16 bit modulo 2^15 as "random" number.
>> Because it is multiplying and adding numbers, which are coprime to the
>> modulus, this creates a sort of uniformaly distributed numbers.
>> Base function is a linear function f(x) = ((x * prime-1) + prime-2) %
2^32.
>> The final result is (f(x) >> 16) & 0x7fff;
>> The prime-1 is 214013, and the prime-2 is 2531011.
>>
>> Regards,
>> Kai
>>
>>
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>
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