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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - std::piecewise_constant_distribution calculates wrong densities"
href="http://llvm.org/bugs/show_bug.cgi?id=22805">22805</a>
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<th>Summary</th>
<td>std::piecewise_constant_distribution calculates wrong densities
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>gnuetzi@gmail.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu, mclow.lists@gmail.com
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<th>Classification</th>
<td>Unclassified
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<pre>It seems that ``std::piecewise_constant_distribution`` computes the densities
wrongly , at least with GCC and its standard library.
According to
<a href="http://www.cplusplus.com/reference/random/piecewise_constant_distribution/">http://www.cplusplus.com/reference/random/piecewise_constant_distribution/</a>:
The densities should be computed as:
![enter image description here][1]
Checking this manually reveals the bug!
This can be seen here: <a href="http://coliru.stacked-crooked.com/a/ca171bf600b5148f">http://coliru.stacked-crooked.com/a/ca171bf600b5148f</a>
The source code related to this is found in
`/usr/include/c++/4.8/bits/random.tcc` (on linux) and the extract of the
initialization function `_M_initialize` called by the constructor shows that
there is something incorrect here:
const double __sum = std::accumulate(_M_den.begin(),
_M_den.end(), 0.0);
__detail::__normalize(_M_den.begin(), _M_den.end(), _M_den.begin(),
__sum); <----- WRONG
// THIS is not the cummulative distribution (since the above
normalization does not give the probability of the intervalls!)
_M_cp.reserve(_M_den.size());
std::partial_sum(_M_den.begin(), _M_den.end(),
std::back_inserter(_M_cp));
// Make sure the last cumulative probability is one.
_M_cp[_M_cp.size() - 1] = 1.0;
// Dividing here by the interval length is WRONG!!!
for (size_t __k = 0; __k < _M_den.size(); ++__k)
_M_den[__k] /= _M_int[__k + 1] - _M_int[__k];
[1]: <a href="http://i.stack.imgur.com/LDthp.gif">http://i.stack.imgur.com/LDthp.gif</a></pre>
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