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example with Matrix iterator1, iterator2
by DrJekyll 23 Sep '05

23 Sep '05

dear group members I read through the Ublas document but could not see any example with the iterator1 and iterator2 of matrix. Could someone kindly send me some example and explanation please? thanks XS ________________________________________________________________ Sent via the WebMail system at empirics.net

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[BBv2] getting started problem with headers
by Tino S. 23 Sep '05

23 Sep '05

Hi boosters, i can't build my first stand alone project with boost.build. I did'nt found any solutions in the doc. :( Here the facts: envirement: - winxp with vc7.1 and cygwin with gcc3.4 - bjam version 03.01.11 (which comes with boost 1.33) - set the SystemVar BOOST_ROOT=D:\Temp\boost_1_33_0 (under win) In my home directory are these files: // FILE: site-config.jam # Get Boost location from build system path-constant BOOST_ROOT : [ modules.peek : BOOST_ROOT ] ; // FILE: user-config.jam import toolset : using ; using msvc ; and here is my simple project dir + my_prj // FILE: hello.cpp #include <boost/array.hpp> #include <iostream> int main() { boost::array<int,4> a = { { 1, 2, 3 } }; std::cout << "Hello!\n"; std::cout << "a[1]:" << a[1] << '\n'; std::cin.get(); return 1; } // FILE: Jamfile # nothing works, :( .. could not find the boost header # nothing works.. exe hello : hello.cpp ; #exe hello1 : hello.cpp : <include>boost ; #exe hello2 : hello.cpp : <include>BOOST_ROOT ; #exe hello3 : hello.cpp : <include>$(BOOST_ROOT) ; // FILE: project-root.jam path-constant MY_DIR : . ; using lex ; using bison ; using testing ; What's wrong? Why could'nt found the boost header files? For beginners it would wounderfull to add samples to the doc which shows the integration of Boost (include and libs). Thanks in advance Tino Schumacher ___________________________________________________________ Gesendet von Yahoo! Mail - Jetzt mit 1GB Speicher kostenlos - Hier anmelden: http://mail.yahoo.de

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Re: [Boost-users] [BGL] detect cycles in a directed graph
by Andrea Olgiati 23 Sep '05

23 Sep '05

Giulio, Here's an idea. You say you want to list all the vertices that make up any one of the (potentially very numerours) cycles that originate on a vertex X. If _any_ cycle will do, how about the shortest? You can use Dijkstra. From http://www.boost.org/libs/graph/doc/dijkstra_shortest_paths.html: "Also you can record the shortest paths tree in a predecessor map: for each vertex u in V, p[u] will be the predecessor of u in the shortest paths tree (unless p[u] = u, in which case u is either the source or a vertex unreachable from the source)." I don't think Dijkstra will work on a path that looks like V->X->Y->U->V (ie, a cycle). But, to get around this, you do know that U->V is an edge, so you can compute the shortest path between V and U, and know that there's an edge between U and V. HTH, Andrea -- Andrea Olgiati - Elixent - Castlemead, Lwr Castle St., Bristol BS1 3AG, UK andrea.olgiati(a)elixent.com ++44 (0)117 9175612 > -----Original Message----- > From: boost-users-bounces(a)lists.boost.org > [mailto:boost-users-bounces@lists.boost.org] On Behalf Of > Giulio Veronesi > Sent: 22 September 2005 11:05 > Cc: boost-users(a)lists.boost.org > Subject: Re: [Boost-users] [BGL] detect cycles in a directed graph > > Thanks Vladimir. > > I'm not sure that I've followed well your description. Here > is my test implementation... but it doesn't work. > > Please, could you help me to discover the problem? > > Thanks in advance, > Giulio > > > typedef adjacency_list<vecS, vecS, directedS, > no_property, no_property> MyGraph; > > typedef graph_traits<MyGraph>::vertex_descriptor Vertex; > > > struct CycleDetector : public dfs_visitor<> { > CycleDetector(std::vector<Vertex> p, > std::vector<Vertex>& _cycle) : > m_predecessor(p), cycle(_cycle) { } > > template <class Edge, class Graph> > void back_edge(Edge e, Graph& g) > { > Vertex t = target(e, g); > Vertex c = source(e, g); > > cycle.push_back(c); > do { > c = m_predecessor[c]; > cycle.push_back(c); > } while(c != t); > } > > protected: > std::vector<Vertex>& cycle; > std::vector<Vertex> m_predecessor; > }; > > int main(int,char*[]) > { > > typedef std::pair<int,int> Edge; > std::vector<Edge> edges; > edges.push_back(Edge(0,1)); > edges.push_back(Edge(1,2)); > edges.push_back(Edge(2,3)); > edges.push_back(Edge(3,1)); > edges.push_back(Edge(3,4)); > edges.push_back(Edge(4,0)); > > MyGraph g(edges.begin(), edges.end(), 5); > > std::vector<Vertex> cycle; > std::vector<Vertex> p(num_vertices(g)); > CycleDetector vis(p, cycle); > depth_first_search(g, visitor(vis)); > > for (int i=0; i<cycle.size(); i++) > std::cout << cycle[i] << ", "; > std::cout << std::endl; > > return 0; > } > > > Vladimir Prus ha scritto: > >>something like this? > >> > >>template <class Edge, class Graph> > >>void back_edge(Edge e, Graph& g) { > >> clinks.push_back(TLink(source(e, g), target(e, g))); } > >> > >>where: > >> > >>typedef std::pair<int,int> TLink; > >>vector<TLink> clinks; > > > > > > Something more like: > > > > struct vis > > { > > vis(vector_property_map<int> predecessor) > > : m_predecessor(predecessor) {} > > > > template<......> > > void back_edge(Edge e, Graph& g) > > { > > vertex_descriptor t = target(e, g); > > vertex_description c = source(e, g); > > vector<vertex_descriptor> cycle; > > cycle.push_back(c); > > do { > > c = m_predecessor[c]; > > cycle.push_back(c); > > } while(c != t); > > } > > }; > > > > On construction, you need to pass the same > vector_property_map both to > > the visitor and the depth_first_search algorithm. > > > > Yes, this is untested code, sorry :-( > > > > - Volodya > > > > > > _______________________________________________ > Boost-users mailing list > Boost-users(a)lists.boost.org > http://lists.boost.org/mailman/listinfo.cgi/boost-users > >

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Re: [Boost-users] A question about boost::lambda::if_ behavior
by tom gee 23 Sep '05

23 Sep '05

Thanks to Simon for the elaborate explanation. Thanks to Bjorn again for the url, getting me to the right place. Yes, I should have gone through the docs. (*^�n^*) > Message: 1 > Date: Thu, 22 Sep 2005 20:44:08 +1200 > From: Simon Buchan <simon(a)hand-multimedia.co.nz> > Subject: Re: [Boost-users] A question about boost::lambda::if_ > behavior > To: boost-users(a)lists.boost.org > Message-ID: <dgtqts$e0j$1(a)sea.gmane.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > Bjorn.Karlsson(a)readsoft.com wrote: > > Hello Tom, > > > > -----Original Message----- > > From: tom gee [mailto:rockonedge@gmail.com] > > Subject: [Boost-users] A question about boost::lambda::if_ behavior > > > > > >>Can anyone please explain this code: > > > > [snip] > > > > The problem is that the expression std::cout << constant("Bingo:<") is > > evaluated immediately. Why? Because it's not a lambda expression. You have > > two options that will help you turn your if-branch into a lambda expression; > > either use boost::lambda::var or boost::lambda::constant. > > > > Using boost::lambda::var: > > > > std::for_each(setFinalVal.begin(),setFinalVal.end(), > > (std::cout << _1 << " ", if_(_1 < 24.000001 && _1 > 23.999999) > > [var(std::cout) << "Bingo:<" << _1 << ">"])); > > > > Using boost::lambda::constant: > > > > std::for_each(setFinalVal.begin(),setFinalVal.end(), > > (std::cout << _1 << " ", if_(_1 < 24.000001 && _1 > 23.999999) > > [std::cout << constant("Bingo:<") << _1 << ">"])); > > > > > > Cheers, > > Bjorn Karlsson > constant should be better (lower overhead, conceptually closer), > but no biggy. I beleive this is the No.1 FAQ, is it not? > (PS: why does not the lambda docs say in the function call overload > section to use BLL's bind? It took me AGES to figure out how to do > that!) > > Message: 7 > Date: Thu, 22 Sep 2005 22:37:28 +1200 > From: Simon Buchan <simon(a)hand-multimedia.co.nz> > Subject: Re: [Boost-users] A question about boost::lambda::if_ > behavior > To: boost-users(a)lists.boost.org > Message-ID: <dgu1ic$2g2$1(a)sea.gmane.org> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > tom gee wrote: > > Hello Bjorn, > > > > Thanks for such a quick help. I works now. > > > > Can you please further explain the usage of var and constant . > > in the expression > > if_(_1 < 24.000001 && _1 > 23.999999)[ cout <<"Bingo:<"<<_1<<">"]) > > I understand "Bingo:<" is evaluated immediately, why not ">", which is not a > > lambda expression either, is not > > evaluated along. > > > Refresher course: an expression like: > cout << foo << bar << endl; > is read (aka parsed) by the compiler to be shorthand for the following: > (((cout << foo) << bar) << endl); > which, incidently is just shorthand for: > operator<<(operator<<(operator<<(cout, foo), bar), endl); > Thus, '<<' is known as a 'left assosiative' (I think :D) operator. > Refresher over. > Basicly, the 'cout << "Bingo:<"' sub-expression has no 'lambda stuff' in > it, and therefore calls the _normal_ operator<<, which immediately > prints "Bingo:<". This operator<< returns a reference to cout, so the > next '<<' sees: > cout << _1 > Which (obviously) is an expression containing a lambda (sub-)expression. > The guys who made this library were nice enough to provide overloads of > '<<' (along with every other possible operator) that return a 'lambda > function object', you just have to know that it is a valid lambda > expression that _will_ evalute as "cout << _the_first_argument_" when > called. The same happens with the next <<, it sees: > _lambda_operator_left_shift_ << ">" > , where the left argument is the previous lambda function object made > from the "cout << _1" sub-expression. Now there is _another_ overload of > << that creates _another_ lambda function object, that _will_ evalute as > if it was: > cout << _the_first_argument_ << ">" > So, basicly, anytime there is a sub-expression that doesn't have a > lambda object in it, it is treated as a normal expression, but if it > does, it creates a function object that will do what it looks like it > would do. What 'constant' and 'var' do is make a (lambda?) function > object that just returns it's argument, as, respectively, a constant > copy, or a reference to the original (I think, again, use boost::ref to > be sure) > > Forgive me if this question seems stupid, but I am new to Lambda, fascinated > > by its power, but convoluted by its mechanism yet. > > > > Thanks. > > > [snip] > > Don't worry about it, but don't forget to read the documentation! > One more thing to keep in mind, the overload for '()', the call > operator, for a lambda function object immeadiatly evaluates it's > contained expression, ie: > (_1 + 3)(4) == (4 + 3); > to get around this if you want to delay a call to that object, use: > boost::lambda::bind(_1 + 3, 4); > which returns a lambda function object that when called, calls the first > argument with the rest of bind's arguments as it's arguments, so: > bind(_1 + 3, 4)() == (_1 + 3)(4) == (4 + 3) > Which is VERY usefull if you want to, say call functions in a STL container: > > int i, j; > std::vector<void(*)(int, int)> some_funcs; > ... > std::for_each(some_funcs.begin(), some_funcs.end(), bind(*_1, i, j)); > > Although, Boost.Signals is probably a better bet if you do stuff like > that a bit. >

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why not boost::bind::_1 ?
by ClassyK 22 Sep '05

22 Sep '05

when include "boost\bind.hpp", why _1 is in global space rather than boost::bind, while in lamba, there is boost::lamba::_1

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Re: [Boost-users] [BGL] detect cycles in a directed graph
by Vladimir Prus 22 Sep '05

22 Sep '05

On Thursday 22 September 2005 10:50, Giulio Veronesi wrote: [replying to list, no need to keep this off-list] > Vladimir Prus ha scritto: > > Giulio Veronesi wrote: > >>assistance in the following problem: "get all the edges of a cycle". > > > > Of which cycle? > > > > If you want to get at least one cycle in a graph that has one, then you > > need to create a visitor that implements "back_edge" method and use > > predecessor map to traverse the cycle in reverse order. > > Yes! Only one cycle. Please, could you give me a pratical example (just > the pseudo code)? I've very little experience with BGL. Do you intend > something like this? > > template <class Edge, class Graph> > void back_edge(Edge e, Graph& g) { > clinks.push_back(TLink(source(e, g), target(e, g))); > } > > where: > > typedef std::pair<int,int> TLink; > vector<TLink> clinks; Something more like: struct vis { vis(vector_property_map<int> predecessor) : m_predecessor(predecessor) {} template<......> void back_edge(Edge e, Graph& g) { vertex_descriptor t = target(e, g); vertex_description c = source(e, g); vector<vertex_descriptor> cycle; cycle.push_back(c); do { c = m_predecessor[c]; cycle.push_back(c); } while(c != t); } }; On construction, you need to pass the same vector_property_map both to the visitor and the depth_first_search algorithm. Yes, this is untested code, sorry :-( - Volodya

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[bind] use with shared_ptr
by Neal Coombes 22 Sep '05

22 Sep '05

Bind is very convenient for use with shared_ptr: struct C { void func() {} }; shared_ptr<C> c(new C); bind(&C::func, c)(); // calls c->func() However I've found a snag when trying to do function composition: class A { shared_ptr<C> func() { return shared_ptr<C>(new C); } }; A a; bind(&C::func, bind(&A::func, a)); // doesn't work, instead must: bind(&C::func, bind(&get_pointer<B>, bind(&A::func, a))); The fact that I can work around this is obviously a good thing. But the work around is significantly less clear, especially when attempting to convince a development team that boost and bind are Good Things(tm) and they should be adopted. Is it horribly difficult to make the simpler composition work (i.e. was it tried and impossible or simply overlooked)? Thanks, Neal

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Re: [Boost-users] A question about boost::lambda::if_ behavior
by Bjorn.Karlsson＠readsoft.com 22 Sep '05

22 Sep '05

From: tom gee [mailto:rockonedge@gmail.com] > Can you please further explain the usage of var and constant . > in the expression if_(_1 < 24.000001 && _1 > 23.999999)[ > cout <<"Bingo:<"<<_1<<">"]) > I understand "Bingo:<" is evaluated immediately, why not ">", > which is not a lambda expression either, is not evaluated along. Actually it is a lambda expression! The reason is the preceding streaming of _1, which implicitly marks the expression as a lambda expression. So, std::cout << "Bingo:<" is not a lambda expression, and is thus evaluated immediately. The last part of the expression is a lambda expression, _1 << ">" (or rather std::cout << _1 << ">"), but it won't be invoked. See http://tinyurl.com/7jq3v for details on how to use constant and var. > Forgive me if this question seems stupid, but I am new to Lambda, > fascinated by its power, but convoluted by its mechanism yet. The question isn't stupid, and you're more than welcome to keep asking. And I agree that Boost.Lambda's power is fascinating. Bjorn Karlsson

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Re: [Boost-users] A question about boost::lambda::if_ behavior
by tom gee 22 Sep '05

22 Sep '05

Hello Bjorn, Thanks for such a quick help. I works now. Can you please further explain the usage of var and constant . in the expression if_(_1 < 24.000001 && _1 > 23.999999)[ cout <<"Bingo:<"<<_1<<">"]) I understand "Bingo:<" is evaluated immediately, why not ">", which is not a lambda expression either, is not evaluated along. Forgive me if this question seems stupid, but I am new to Lambda, fascinated by its power, but convoluted by its mechanism yet. Thanks. -----Original Message----- From: tom gee [mailto:rockonedge@gmail.com] Subject: [Boost-users] A question about boost::lambda::if_ behavior > Can anyone please explain this code: [snip] The problem is that the expression std::cout << constant("Bingo:<") is evaluated immediately. Why? Because it's not a lambda expression. You have two options that will help you turn your if-branch into a lambda expression; either use boost::lambda::var or boost::lambda::constant. Using boost::lambda::var: std::for_each(setFinalVal.begin(),setFinalVal.end(), (std::cout << _1 << " ", if_(_1 < 24.000001 && _1 > 23.999999) [var(std::cout) << "Bingo:<" << _1 << ">"])); Using boost::lambda::constant: std::for_each(setFinalVal.begin(),setFinalVal.end(), (std::cout << _1 << " ", if_(_1 < 24.000001 && _1 > 23.999999) [std::cout << constant("Bingo:<") << _1 << ">"])); Cheers, Bjorn Karlsson

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Re: [Boost-users] algorithms and member functions
by Foster Gareth 22 Sep '05

22 Sep '05

> What's wrong with: > std::for_each > ( as.begin > , as.end() > , bool some_function_keyword(A& a) > { return a.wee() & SOME_FLAG; > } > ); > I think the previous poster (whos comments where appreciated) implied that while some languages (he mentioned Lisp and Perl I think, and I mentioned Java) could/do support this, C++ would struggle to. It would make things hairy for the compiler because a function definition is not preceeded by any keyword. I think you can also do things a bit like this in java ... Interesting perhaps that there is a keyword involved in this case, would this still bog down compilers so much as to outweight the advantages of the sytax style (assuming there are any, I'm happy to be argued out of thiking this kind of coding is neat). button_click_event.set_handler ( new class : public button_click_event_handler { void on_click() { std::cout << "anon_class::on_click() : printing wee" << std::endl; } } ); > I don't know what a good function keyword name would be. > Obviously: fn, function, lambda are in common use everywhere > (and fn is too short for a C++ keyword! :D) > Reuse 'inline'? I suppose inline would make sense, but it might be a bit of a hack to have it used in two senses. I wonder if you can do this stuff in Delphi, they have a function keyword. Thesaurus :) " function noun 1. The proper activity of a person or thing: job, purpose, role, task. See do/not do. 2. A large or important social gathering: affair, celebration, festivity, fete, gala, occasion, party, soiree. Informal do. Slang bash. See group, work/play. verb 1. To react in a specified way: act, behave, operate, perform, work. See action/inaction. 2. To perform a function effectively: go, operate, run, take, work. See thrive/fail/exist. 3. To perform the duties of another: act, officiate, serve. See do/not do, substitute. " Cheers for the responses anyway, Gaz

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