July 2009 - Boost-users - lists.boost.org

[multi_array] Assertion when resizing empty array
by John Reid 22 Jul '09

22 Jul '09

Resizing an empty array no longer seems to work. My code worked fine with boost 1.38, now it causes an assertion with the latest boost trunk from svn. The code is: #include <boost/multi_array.hpp> int main( int argc, char * argv[] ) { boost::multi_array< double, 2 > a; a.resize( boost::extents[2][3] ); return 0; } The assertion is: multi_array_resize: /home/john/Dev/ThirdParty/boost/boost-trunk/boost/multi_array/base.hpp:446: ArrayRef boost::detail::multi_array::multi_array_impl_base<T, NumDims>::generate_array_view(boost::type<Reference>, const boost::detail::multi_array::index_gen<NumDims, NDims>&, const typename boost::detail::multi_array::associated_types<T, mpl_::size_t<NumDims> >::size_type*, const typename boost::detail::multi_array::associated_types<T, mpl_::size_t<NumDims> >::index*, const typename boost::detail::multi_array::associated_types<T, mpl_::size_t<NumDims> >::index*, TPtr) const [with ArrayRef = boost::detail::multi_array::multi_array_view<double, 2ul>, int NDims = 2, TPtr = double*, T = double, long unsigned int NumDims = 2ul]: Assertion `index_bases[n] <= start && start < index_bases[n]+index(extents[n])' failed. Aborted My compiler is: john@john-dell:~/Dev/MyProjects/Bio/hmm$ gcc -v Using built-in specs. Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.3.3-5ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3 --program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.3.3 (Ubuntu 4.3.3-5ubuntu4) I updated from svn today. I think the code creates a view on the old data in order to retain it. The assertion happens because this view is empty. Regards, John.

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Spirit qi recommendations ?
by Avi Bahra 22 Jul '09

22 Jul '09

In order to reduce compile times I have decided to rewrite the parser/grammar from classic to spirit qi (spirit 2.1). My understanding is that faster compiles come from easier modularisation available under spirit qi. (As shown in mini_c example, in the boost tree) However I need this to be portable to as many platforms as possible. In my case minimum set is Linux,HP-UX, AIX. So my fall-back is boost 1.38. So how much has boost spirit qi changed between 1.38 and present ? How are defects reported against spirit qi, integrated back to past releases, or is there any recommended patch type mechanism, that supports redistribution of my local boost tree. Or is simply better to use spirit independently from boost? Are there any issues/guidelines that people have found in transitioning from classic to Qi. In my case I made heavy use of closures. I have not seen closures in any of spirit qi examples, Assuming there is equivalent functionality to closures ? I would interested in any recommendations before starting on this project. Best regards, Ta, Avi

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weak_ptr and thread safety
by Pavel Shevaev 22 Jul '09

22 Jul '09

Guys, could you please clarify whether the following usage of weak_ptr is thread safe: //thread A: shared_ptr<Foo> p = wp.lock(); //thread B: p.reset(); I believe it's not thread safe as it looks very similar to the example 3 from the shared_ptr documentation: // thread A p = p3; // reads p3, writes p // thread B p3.reset(); // writes p3; undefined, simultaneous read/write Am I right in my assumption? And if yes, could you please give any recommendations for the following use case. I have a networking thread which owns a set of net session objects stored as shared_ptrs. I also have another thread where some entities have weak_ptrs members pointing to net session objects from networking thread. From time to time net sessions are erased and I think when at the very this moment entity object from another thread tries to lock() the net session and access it seg. fault happens. Thanks in advance. -- Best regards, Pavel

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Re: [Boost-users] [Fusion] Understanding Fusion
by Cromwell Enage 22 Jul '09

22 Jul '09

--- On Sat, 7/18/09, Robert Jones wrote: > Hi All > > I'm very much a Fusion newbie, just getting may head around it, so a > really basic question! > > From the intro page, when we write > > #include <boost/fusion/sequence.hpp> > #include <boost/fusion/include/sequence.hpp> > > vector<int, char, std::string> stuff > for_each(stuff, print_xml()); > > > am I correct in believing that there is no loop involved in the > for_each, and that this resolves, at compile time, to a linear > list of function calls? Yes. IIRC, it resolves to the equivalent of: print_xml()(int()); print_xml()(char()); print_xml()(std::string()); HTH, Cromwell D. Enage

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Creating a fixed size managed_mapped_file
by Eli Zakashansky 22 Jul '09

22 Jul '09

Hello, I would like to create a managed_mapped_file which should contain a certain number of elements[n] of a certain size [k], therefore the required size which I would like to use is n*k (size of element * number of elements). But when the file is created [new bip::managed_mapped_file (bip::create_only, fileName, n*k);] besides the elements which I would like to placed in this file, a segment manager<http://www.boost.org/doc/libs/1_39_0/doc/html/interprocess/architecture.htm…> is also placed there and occupies an uncertain space [http://www.boost.org/doc/libs/1_39_0/doc/html/interprocess/architecture.htm…] I would like to know how much memory (size of file) shall I require in the C-tor so that my file will have enough space for n elements of size k, and be as compact as possible. Thanks in advance, Eli.

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Invoking 'brent_find_minima' for a member function of a class?
by s seelenluft 22 Jul '09

22 Jul '09

Hi, I found this example: http://net.pku.edu.cn/~webg/src/paradise/thirdparty/installed/include/boost… works fine but I have trouble figuring out how to adapt this if the function to minimize is a member function of a class (and invoked in another member function). Any example of how to call brent_find_minima (or similar functions) from a member function and minimizing a member function would be very welcome. I have to the advice to wrap the function to be minimized into a static wrapper function but could not find an example how to do this from within a member function (I have it working when calling from main). Thanks. Markus

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Iterator to wrap around ends of the container. Does it exist?
by PB 22 Jul '09

22 Jul '09

Hi, Just wondered if there was anything in Boost so that when an iterator is incremented, if it's at the last element it goes to the begin() of the container. And for decrement if at the beginning it would refer to one before end(). The sequence would have to be finite, so just selecting a different logical begin and end really. I'm looking to solve this using some kind of new iterator (iterator adaptor) rather than a change of container. If there anything in Boost for this? Thanks, Pete

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Variance calcs in the stats library and in Excel
by Archie14 22 Jul '09

22 Jul '09

In the docs (file:///C:/Libraries/boost/boost_1_39_0/doc/html/accumulators/user_s_guide.htm l#accumulators.user_s_guide.the_statistical_accumulators_library.variance) there is an example of calculating variance of values 1,2,3,4,5 accumulator_set<int, stats<tag::variance(lazy)> > acc1; acc1(1); acc1(2); acc1(3); acc1(4); acc1(5); BOOST_CHECK_EQUAL(5u, count(acc1)); BOOST_CHECK_CLOSE(3., mean(acc1), 1e-5); BOOST_CHECK_CLOSE(11., moment<2>(acc1), 1e-5); BOOST_CHECK_CLOSE(2., variance(acc1), 1e-5); When I am doing the same in Excel the mean is indeed 3, which matches the example, but variance is 2.5 and not 2. I am using AVERAGE and VAR formulas in Excel for mean and variance respectively Why there is a mismatch between variance values??

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Re: [Boost-users] where is the pdf documentation?
by Tan, Tom (Shanghai) 22 Jul '09

22 Jul '09

http://boost.cowic.de/rc/pdf/ > ??? I see mention of pdf documentation, but couldn't find it online. > Can someone give the link to it?

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[math] finding c such that w1/c+...+wn/c < math::tools::max_value<T>()
by er 21 Jul '09

21 Jul '09

Hi All, A little while back the following code was suggested to find c such that (x/c)+(y/c) <= C = math::tools::max_value<T>() template<class T>// static T scale_to_max( const T& x, const T& y ) { double max = (std::numeric_limits<T>::max)(); if(x > max - y) { double c = x/max + y/max; while(!boost::math::isinf(x/c + y/c)) { c = boost::math::float_prior(c); } while(boost::math::isinf(x/c + y/c)) { c = boost::math::float_next(c); } return(c); } else { return(static_cast<T>(1)); } } I'm now trying to generalize this i.e. find c such that w1/c+...+wn/c <= C: 1) split w1,..,wn into maximal size, sum less than C, subgroups. 2) apply scale_to_max iteratively over adjacent groups. This does not always suceed (see example at the bottom). Any suggestion would be appreciated. *.hpp template<typename T> class scale_sum_to_max{ public: typedef void result_type; typedef T argument_type; scale_sum_to_max(T cum_sum):cum_sum_(cum_sum),c_(static_cast<T>(1)){} scale_sum_to_max():cum_sum_(static_cast<T>(0)),c_(static_cast<T>(1)){} void operator()(T x){ if( math::isinf(scaled_sum()+(x/c_)) ){ T s = scale_to_max( scaled_sum(), x/c_ ); cum_sum_ /= s; c_ *= s; BOOST_ASSERT(!math::isinf(cum_sum_+x/c_)); } cum_sum_ += x/c_; } T scaling_factor()const{ return c_; } T scaled_sum()const{ return cum_sum_; } private: T cum_sum_; T c_; }; template<typename T> class finite_sums{ typedef std::vector<T> cont_t; public: typedef void result_type; typedef T argument_type; finite_sums() { coll_.push_back(static_cast<T>(0)); } void operator()(T x){ if(math::isinf(sum()+x)){ coll_.push_back(x); }else{ coll_.back() += x; } } const cont_t& collected()const{ return coll_; } T scaling_factor()const{ return for_each( begin(coll_), end(coll_), scale_sum_to_max<T>() ).scaling_factor(); } private: T sum()const{ return coll_.back(); } cont_t coll_; }; }} *.cpp typedef double value_t; typedef std::vector<value_t> vec_t; const unsigned n = 1e5; //n = 1e3 OK value_t w = math::tools::max_value<value_t>(); w /= 2; vec_t vec_w(n,w); value_t scaling_factor = for_each( begin(vec_w), end(vec_w), math_limit::finite_sums<value_t>() ).scaling_factor(); //assertion failed: (!math::isinf(cum_sum_+x/c_))

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