[Bug libstdc++/67011] division by zero in std::exponential_distribution
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67011 Jonathan Wakely changed: What|Removed |Added Status|UNCONFIRMED |NEW Last reconfirmed||2018-05-14 Ever confirmed|0 |1 --- Comment #4 from Jonathan Wakely --- (In reply to Matthias Kretz from comment #3) > The problem is that (result_type(1) - __aurng()) must never be zero (because > std::log might raise FE_DIVBYZERO for std::log(0)). If __aurng() is a > distribution guaranteed to exclude result_type(1) then this works fine. > AFAIK std::generate_canoncial sometimes generates a 1 (cf. > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63176). PR 63176 is fixed now. > Also, I'm wondering whether the 1 - rng wouldn't reduce the randomness. > Consider: > std::log(numeric_limits::min()) -> -87.3365478515625 > std::log(1.f - nextafter(1.f, 0.f))-> -16.635532379150390625 > IIUC, the effect of log(1 - rng) is that the distance between random values > from std::exponential_distribution is considerable larger than for log(rng). Hmm, let's confirm the bug for that, instead of the division-by-zero that should be fixed.
[Bug libstdc++/67011] division by zero in std::exponential_distribution
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67011 Matthias Kretz kretz at kde dot org changed: What|Removed |Added CC||kretz at kde dot org --- Comment #3 from Matthias Kretz kretz at kde dot org --- The problem is that (result_type(1) - __aurng()) must never be zero (because std::log might raise FE_DIVBYZERO for std::log(0)). If __aurng() is a distribution guaranteed to exclude result_type(1) then this works fine. AFAIK std::generate_canoncialfloat sometimes generates a 1 (cf. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63176). Also, I'm wondering whether the 1 - rng wouldn't reduce the randomness. Consider: std::log(numeric_limitsfloat::min()) - -87.3365478515625 std::log(1.f - nextafter(1.f, 0.f))- -16.635532379150390625 IIUC, the effect of log(1 - rng) is that the distance between random values from std::exponential_distribution is considerable larger than for log(rng).
[Bug libstdc++/67011] division by zero in std::exponential_distribution
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67011 --- Comment #1 from Jonathan Wakely redi at gcc dot gnu.org --- One is for std::exponential_distribution and one is for std::tr1:exponential_distribution. The former is already fixed and the latter is moribund and we don't really care about it any more.
[Bug libstdc++/67011] division by zero in std::exponential_distribution
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67011 --- Comment #2 from Jonathan Wakely redi at gcc dot gnu.org --- Although if you're still seeing a problem with std::exponential_distribution then I guess it's not the same bug. (Please ignore tr1/random.h unless you're actually using std::tr1::exponential_distribution ... and if you are using that, you should consider not using it :-)
