Package: libstdc++6-4.5-dev Version: 4.5-20100202-1 Severity: normal
The random-number distributions in C++0x <random> include a special "one-shot" facility, where the random bounds can be passed to the generator function. This is explicitly intended (judging from committee writings, and source comments in the libstdc++ header files) for use as a rng generator for std::random_shuffle. e.g.: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1933.pdf As best as I can tell, std::random_shuffle wants its random-number generating functor to return results in the range [0,N), where N is the parameter it passes to the functor; i.e., the upper-bound is exclusive (and the lower-bound inclusive). The <random> implementation in libstdc++ also notes this, e.g., in the comment on the associated operator() method in the std::uniform_int_distribution template class: /** * Gets a uniform random number in the range @f$[0, n)@f$. * * This function is aimed at use with std::random_shuffle. */ template<typename _UniformRandomNumberGenerator> result_type operator()(_UniformRandomNumberGenerator& __urng, const param_type& __p); However, in practice, it actually will return the upper-bound as well; it seems to treat it as a maximum, rather than an exclusive bound. For instance, compiling the attached test program (at end of message), "rand-bug.cc", results in the following output: $ g++-4.5 -o rand-bug -std=c++0x rand-bug.cc $ ./rand-bug trying 1000000 random numbers in range [0,25)... returned upper-bound (25) 38394 times (should be 0) [Note that the same issue exists with other ways of invoking using the generator (e.g., a std::uniform_real_distribution<float> with bounds of 0 and 1 will indeed return 1); but it's less clear its a bug in those cases (although, for instance, boost's random implementation never seems to return the upper bound).] Thanks, -Miles Here's the test program: #include <iostream> #include <random> int main () { std::mt19937 rng; typedef std::uniform_int_distribution<unsigned> dist_type; dist_type dist; unsigned num_loops = 1000000; unsigned upper_bound = 25; std::cout << "trying " << num_loops << " random numbers in range [0," << upper_bound << ")..." << std::endl; // // According to the function documentation for this, it should never // return the upper bound: // // * Gets a uniform random number in the range @f$[0, n)@f$. // * // * This function is aimed at use with std::random_shuffle. // unsigned returned_upper_bound_count = 0; for (unsigned i = 0; i < num_loops; i++) { unsigned num = dist (rng, dist_type::param_type (0, upper_bound)); if (num == upper_bound) returned_upper_bound_count++; } std::cout << "returned upper-bound (" << upper_bound << ") " << returned_upper_bound_count << " times (should be 0)" << std::endl; return 0; } -- System Information: Debian Release: squeeze/sid APT prefers unstable APT policy: (500, 'unstable'), (500, 'testing'), (101, 'experimental') Architecture: amd64 (x86_64) Kernel: Linux 2.6.32-trunk-amd64 (SMP w/4 CPU cores) Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8) Shell: /bin/sh linked to /bin/dash Versions of packages libstdc++6-4.5-dev depends on: ii g++-4.5 4.5-20100202-1 The GNU C++ compiler ii gcc-4.5-base 4.5-20100202-1 The GNU Compiler Collection (base ii libc6-dev 2.10.2-5 Embedded GNU C Library: Developmen ii libstdc++6 4.5-20100202-1 The GNU Standard C++ Library v3 libstdc++6-4.5-dev recommends no packages. Versions of packages libstdc++6-4.5-dev suggests: pn libstdc++6-4.5-doc <none> (no description available) -- no debconf information -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
