Boost C++ Libraries: Ticket #8491: boost::random::discrete_distribution ignores the first element

attachment set

astralstorm@… — Thu, 25 Apr 2013 07:22:07 GMT

attachment → randoms.cpp

discrete_distribution testcase

status changed; resolution set

Steven Watanabe — Wed, 29 May 2013 19:54:14 GMT

status new → closed
resolution → invalid

Negative weights are not allowed. I have no idea what behavior you expect from this.

"... w_k, commonly known as the weights, shall be non-negative, non-NaN, and non-infinity." (C++11 26.5.8.6.1 [rand.dist.samp.discrete])

status changed; resolution deleted

anonymous — Wed, 29 May 2013 21:38:04 GMT

status closed → reopened
resolution invalid

The weights are not negative in this testcase - the probs array is actually printed out. Please run the testcase. It offsets them so that they become non-negative.

For negative weights, such a normalization should be done within boost anyway I think, but then the special value would have to be -infty instead of 0 to give probability 0, which is an API change.

status changed; resolution set

Steven Watanabe — Tue, 11 Mar 2014 18:30:12 GMT

status reopened → closed
resolution → invalid

You're using invalid iterators in the printing loop. distr.probabilities() returns by value.

After I fix that, I get:

Weights: 126 215 260 265 277 340 415
Probabilities: 0.0663857 0.113277 0.136986 0.139621 0.145943 0.179136 0.218651

AstralStorm — Tue, 11 Mar 2014 18:56:25 GMT

Thanks, nice catch. It is a bit counterintuitive in terms of API though, I can think of no real use for the by-value semantics here.

Steven Watanabe — Tue, 11 Mar 2014 19:22:34 GMT

by-value is what the standard specifies. In terms of the Boost implementation, it has to be by-value, because it's computed on the fly. The weights aren't actually stored.