[Bug libstdc++/48114] New: binomial_distribution incorrect for p > .5

[aaz at althenia dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48114

           Summary: <random> binomial_distribution incorrect for p > .5
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: a...@althenia.net


Created attachment 23651
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23651
Probability table for binomial_distribution.

The attached program samples binomial_distribution(10, .75) and
calculates the correct probabilities of each outcome. The output shows
that the probabilities are reversed. With p > .5 the distribution should
be biased toward higher values.

        sample  correct
p_0     0.056   0.000
p_1     0.185   0.000
p_2     0.283   0.000
p_3     0.252   0.003
p_4     0.147   0.016
p_5     0.058   0.058
p_6     0.016   0.146
p_7     0.003   0.250
p_8     0.000   0.282
p_9     0.000   0.188
p_10    0.000   0.056

The problem is here, __param.p() is a double.

--- include/c++/bits/random.tcc
+++ include/c++/bits/random.tcc
@@ -1434,7 +1434,7 @@
       {
        result_type __ret;
        const _IntType __t = __param.t();
-       const _IntType __p = __param.p();
+       const double __p = __param.p();
        const double __p12 = __p <= 0.5 ? __p : 1.0 - __p;
        __detail::_Adaptor<_UniformRandomNumberGenerator, double>
          __aurng(__urng);