July 2003 - Boost-users - lists.preview.boost.org

[BGL] dijkstra_shortest_paths and the single-pair problem
by Richard Jepps 04 Jul '03

04 Jul '03

I would like to use the dijkstra_shortest_paths algorithm to find the shortest path from one specified vertex to another. The standard Boost implementation will find this, but it will continue until it has found the shortest paths to all nodes in the graph (i.e. solved the single source problem). The BGL book (Section 5.1, pp76) says: "It turns out that there are no algorithms for solving the single-pair problem that are asymptotically faster than the algorithms that solve the single-source problem". I think this is because the single-source algorithms all have the property that it is possible to determine whether the shortest path to any given node has been found during the execution of the algorithm, therefore you can use the same algorithm to solve both types of problem provided that the termination criterion is appropriate to the problem. Consider this problem: "find the shortest path from a source vertex u to a nearby destination vertex v in an infinitely large directed weighted graph G in finite time." The single-source problem can only be solved in infinite time, but the single-pair problem can be solved relatively quickly (because the two vertices are near each other). I think that the termination criterion is quite straightforward using a DijkstraVisitor - it is when vis.finish_vertex(u, g) is called and u == destination_node. Unfortunately I can't see how to stop the algorithm in a reasonable way. An unreasonable solution is to provide a custom graph and turn off all its edges when the termination criterion is met. This is unreasonable because the responsibility for termination belongs to the algorithm, not the data structure - I shouldn't have to modify the data structure to terminate the algorithm. It's also unreasonable because I have to customise both the graph and the algorithm to do something very simple. Is there a better way? Thanks Richard

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function object with lambda
by Matt Hurd 04 Jul '03

04 Jul '03

Not sure how to force a for_each with a lambda to invoke the random function object successively. In the for_each version below get_long is called just once. Not sure how to use var, std_function, bind or something else to overcome this. I'm sure the answer is straight forward... anyone? Regards, Matt. #include <iostream> #include <boost/random.hpp> #include <vector> #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> typedef boost::mt19937 base_generator_type; base_generator_type generator( 100123u ); boost::uniform_int<base_generator_type,long> gen_long(generator,-100,100); using namespace std; using namespace boost::lambda; int main(int argc, char *argv[]) { vector<long> v(10); for_each(v.begin(),v.end(), _1 = gen_long() ); for_each(v.begin(), v.end(), cout << _1 << ' '); cout << "\n"; for( vector<>::iterator i = v.begin(); i != v.end(); ++i ) { *i = gen_long(); } for_each(v.begin(), v.end(), cout << _1 << ' '); cout << "\n"; system("PAUSE"); return 0; }

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Re: [Boost-Users] [BGL] dijkstra_shortest_paths and the single-pair problem
by JOAQUIN LOPEZ MU?Z 03 Jul '03

03 Jul '03

Throw an exception? Joaquín M López Muñoz Telefónica, Investigación y Desarrollo ----- Mensaje Original ----- De: Richard Jepps <yg-boost-users(a)m.gmane.org> Fecha: Martes, Julio 1, 2003 3:00 pm Asunto: [Boost-Users] [BGL] dijkstra_shortest_paths and the single-pair problem > I would like to use the dijkstra_shortest_paths algorithm to find the > shortest path from one specified vertex to another. > > The standard Boost implementation will find this, but it will > continue until > it has found the shortest paths to all nodes in the graph (i.e. > solved the > single source problem). > > The BGL book (Section 5.1, pp76) says: "It turns out that there > are no > algorithms for solving the single-pair problem that are asymptotically > faster than the algorithms that solve the single-source problem". > I think this is because the single-source algorithms all have the > propertythat it is possible to determine whether the shortest path > to any given node > has been found during the execution of the algorithm, therefore > you can use > the same algorithm to solve both types of problem provided that the > termination criterion is appropriate to the problem. > > Consider this problem: "find the shortest path from a source > vertex u to a > nearby destination vertex v in an infinitely large directed > weighted graph G > in finite time." > The single-source problem can only be solved in infinite time, but the > single-pair problem can be solved relatively quickly (because the two > vertices are near each other). > > I think that the termination criterion is quite straightforward > using a > DijkstraVisitor - it is when vis.finish_vertex(u, g) is called and > u == > destination_node. Unfortunately I can't see how to stop the > algorithm in a > reasonable way. > > An unreasonable solution is to provide a custom graph and turn off > all its > edges when the termination criterion is met. > This is unreasonable because the responsibility for termination > belongs to > the algorithm, not the data structure - I shouldn't have to modify > the data > structure to terminate the algorithm. > It's also unreasonable because I have to customise both the graph > and the > algorithm to do something very simple. > > Is there a better way? > > Thanks > > Richard > > > > > > > > ------------------------ Yahoo! Groups Sponsor -------------------- > -~--> > Get A Free Psychic Reading! Your Online Answer To Life's Important > Questions.http://us.click.yahoo.com/Lj3uPC/Me7FAA/ySSFAA/EbFolB/TM > ------------------------------------------------------------------- > --~-> > > Info: <" target="l">http://www.boost.org> > Wiki: <" target="l">http://www.crystalclearsoftware.com/cgi- > bin/boost_wiki/wiki.pl>Unsubscribe: <')" >mailto:boost-users- > unsubscribe(a)yahoogroups.com> > > Your use of Yahoo! Groups is subject to > http://docs.yahoo.com/info/terms/ > > >

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boost::numeric::ublas::map_array support for the complete std::map interface?
by Markus Werle 03 Jul '03

03 Jul '03

Hi! While trying to test boost::numeric::ublas::map_array as replacement for a std::map in my own code, I found that some memfuns like lower_bound and key_comp are missing. I also found that these can be added quite easily, since e.g. lower_bound exists as external function and only needs to be called in an appropriate way. Though the functions mentioned may not be useful in ublas apps, they would help improve the reusability of map_array. Before I start stalking through the code: Are there any plans to add these features already? Or is there somebody who knows the ublas code better than me and can help me avoid breaking existsing code? best regards, Markus -- Build your own Expression Template Library with Daixtrose! Visit http://daixtrose.sourceforge.net/

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RE: [Boost-Users] Re: [BGL] dijkstra_shortest_paths and the singl e-pair problem
by Jim.Hyslop 03 Jul '03

03 Jul '03

Mark Sizer wrote: > Eeek! Exceptions as flow control are, well, not exceptions. > > I have no solution to offer (I don't even fully understand the > question), but I hope someone has a better answer. At this time, the only solution is to throw an exception. This whole issue was discussed in September, including the morality of using exceptions for flow control. Please see the thread entitled "[BGL] Best way to abort an algorithm from within a visitor?": http://groups.yahoo.com/group/Boost-Users/message/1819 -- Jim

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regular expression control
by Enrico 03 Jul '03

03 Jul '03

Hi all, I'm using regex with a list of regular expressions written by my team. They start from a set of rules and write down the expression (s). What I'm requested to do now is a function that given a regular expression write down all the combinations available, with some limitations of course: if for example an expression has an infinity of combination this could not be achieved. Since all the expression have a limited number of combinations in this case a warning will suffice. I searched for such a function in the regex code but didn't find it. Any idea? Thanks, Enrico

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Boost.Thread and fork
by Pavol Droba 02 Jul '03

02 Jul '03

Hi, I have problem with boost.threads on linux ( under gcc ). I have a multithreaded aplication using Boost.Thread. It works just fine. However, I want to make a daemon out of it. So I tried to fork() at the start of the program and run all the relevant code ( including all threading ) in the child process. But when I do so, first call to create_thread hangs the process. I'm quite new to linux threading, so it is possible, that the problem is not related to Boost.Threads. Any hints? Thanks in advance. Pavol

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another newbie question on bjam
by Karsten Weinert 01 Jul '03

01 Jul '03

Hi, still trying to set up bjam, I am stumbling on the following problem: I want to use jamfile.jam instead of jamfile (because my editor uses extensions to define actions on files). But when I run bjam on it, with bjam -f jamfile.jam -sTOOLS=mingw it complains about an unknown rule exe. What can I do? Here is the jamfile.jam: exe hello : hello.cpp : <include>c:$(SLASH)dev-cpp$(SLASH)include$(SLASH)c++ <include>c:$(SLASH)dev-cpp$(SLASH)include$(SLASH)c++$(SLASH)mingw32 <include>c:$(SLASH)dev-cpp$(SLASH)include$(SLASH)c++$(SLASH)backward ; Furthermore, can you please advise me how to simplify this jamfile? In my opinion, the include-statements belong to mingw-tools.jam, but how can I specify several include directories there? Any comments appreciated, Karsten

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