https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88849
Zhang Boyang <zhangboyang.id at gmail dot com> changed:
What |Removed |Added
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CC| |zhangboyang.id at gmail dot com
--- Comment #1 from Zhang Boyang <zhangboyang.id at gmail dot com> ---
Hi,
I think I encountered the same bug. I did 10000000000 tests of B(N=100,P=0.15)
and I observed the "weighted average" was about 15.004, which is about 0.004
larger than the "expected value" (N*P=15).
This behavior is not observed if the naive method (i.e. run
std::bernoulli_distribution(P) for N times) is used. Thus I think this
indicates there is a bug in the implementation of std::binomial_distribution.
Zhang Boyang
My test program is:
#include <cstdio>
#include <random>
#define N 100
#define P 0.15
#define OUTER_LOOP 10000
#define INNER_LOOP 1000000
int main()
{
long long result[N+1] = {};
#if 1
std::mt19937 gen(20240201);
#else
std::mt19937_64 gen(20240201);
#endif
for (int i = 1; i <= OUTER_LOOP; i++) {
#if 1
std::binomial_distribution<int> d(N, P);
for (int j = 1; j <= INNER_LOOP; j++) {
result[d(gen)]++;
}
#else
std::bernoulli_distribution d(P);
for (int j = 1; j <= INNER_LOOP; j++) {
int cur = 0;
for (int k = 1; k <= N; k++) {
cur += d(gen);
}
result[cur]++;
}
#endif
long long cnt = 0, sum = 0;
for (int j = 0; j <= N; j++) {
cnt += result[j];
sum += j * result[j];
}
printf("%d/%d\n", i, OUTER_LOOP);
for (int j = 0; j <= N; j++) {
printf("%5d %20lld %.20f\n", j, result[j], (double)result[j]/cnt);
}
printf("%d/%d\n", i, OUTER_LOOP);
printf(" cnt = %lld\n", cnt);
printf(" sum = %lld\n", sum);
printf(" avg = %.20f\n", (double)sum/cnt);
printf(" N*P = %.20f\n", N*P);
printf("avg/N = %.20f\n", (double)sum/cnt/N);
printf(" P = %.20f\n", P);
}
return 0;
}
The result is:
cnt = 10000000000
sum = 150039073251
avg = 15.00390732510000013633
N*P = 15.00000000000000000000
avg/N = 0.15003907325099999359
P = 0.14999999999999999445
The histogram is:
0 930 0.00000009300000000000
1 15401 0.00000154010000000000
2 135116 0.00001351160000000000
3 779486 0.00007794859999999999
4 3329355 0.00033293549999999998
5 11293727 0.00112937270000000004
6 31552023 0.00315520229999999985
7 74764087 0.00747640870000000016
8 153388141 0.01533881410000000056
9 276729436 0.02767294359999999953
10 444347696 0.04443476960000000003
11 640022574 0.06400225739999999830
12 834915008 0.08349150079999999308
13 996435535 0.09964355349999999556
14 1094566902 0.10945669019999999805
15 1112943209 0.11129432089999999334
16 1043376263 0.10433762629999999982
17 909811402 0.09098114020000000468
18 740331472 0.07403314719999999627
19 563856817 0.05638568170000000240
20 402991697 0.04029916969999999821
21 270919911 0.02709199110000000063
22 171678195 0.01716781950000000057
23 102734220 0.01027342200000000080
24 58150147 0.00581501470000000010
25 31216233 0.00312162330000000012
26 15892051 0.00158920510000000004
27 7686873 0.00076868729999999999
28 3531666 0.00035316660000000001
29 1548848 0.00015488479999999999
30 646371 0.00006463710000000000
31 257351 0.00002573510000000000
32 98083 0.00000980830000000000
33 35443 0.00000354430000000000
34 12256 0.00000122560000000000
35 4202 0.00000042020000000000
36 1320 0.00000013200000000000
37 396 0.00000003960000000000
38 118 0.00000001180000000000
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Compiler information (archlinux gcc-13.2.1-4):
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++
--enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/ --with-build-config=bootstrap-lto
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-libstdcxx-backtrace
--enable-link-serialization=1 --enable-linker-build-id --enable-lto
--enable-multilib --enable-plugin --enable-shared --enable-threads=posix
--disable-libssp --disable-libstdcxx-pch --disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.1 20230801 (GCC)
