C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  vector of lists: anyone know how to do this?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message craig Guest Posted: Sat Jun 17, 2006 9:10 am    Post subject: vector of lists: anyone know how to do this? I've declared the following as a "vector" of "lists" using STL vector< list<Edge*> > I've tried many ways to add objects to this but can't figure out how to do it. Does anyone have any tips, or references to material that might explain it? Any help greatly appreciated. Back to top Default User Guest Posted: Sat Jun 17, 2006 9:10 am    Post subject: Re: vector of lists: anyone know how to do this? craig wrote: Quote: I've declared the following as a "vector" of "lists" using STL vector< list<Edge* I've tried many ways to add objects to this but can't figure out how to do it. Does anyone have any tips, or references to material that might explain it? And what is reason you decided to hide your code from us? How can we figure out what you're doing wrong? Brian Back to top Kirit Sælensminde Guest Posted: Sat Jun 17, 2006 9:10 am    Post subject: Re: vector of lists: anyone know how to do this? craig wrote: Quote: I've declared the following as a "vector" of "lists" using STL vector< list<Edge* I've tried many ways to add objects to this but can't figure out how to do it. Does anyone have any tips, or references to material that might explain it? Any help greatly appreciated. Why not something like: Edge *pEdge = NULL; vector< list< Edge * > > edges( 10 ); edges[ 0 ].push_back( pEdge ); Putting pointers into STL containers is not always a good idea. Make sure you know what you're doing. Prefer something like this: boost::shared_ptr< Edge > pEdge; vector< list< boost::shared_ptr< Edge > > edges( 10 ); edges[ 0 ].push_back( pEdge ); Your choice of smart pointer depends on context though, although boost::shared_ptr is for many uses a good one. std::auto_ptr is not normally a good choice. Of course including the code that doesn't work and telling us what you're thinking about might help us to answer better. K Back to top Kirit Sælensminde Guest Posted: Sun Jun 18, 2006 8:00 am    Post subject: Re: followup (better formating of question) vector of lists craig wrote: Quote: void Graph::addEdge( Vertex &v1, Vertex &v2, int weight ) { ind = "compute index of Vertex in vertex list" Edge *e = new Edge (v1, v2, weight ); list <Edge*> lst; <- how to make dynamic ?? adjLists.push_back (lst); <-how to make dynamic?? adjLists[ind].push_back( e ); } The list is already dynamic, the push_back will add to the end. The vector is slightly different though. I think you missed the suggestion of the resize() somewhere. What you need is (I'm not g'teeing it will compile directly) is something like: vector< list< boost::shared_ptr< Edge > > >::size_type ind = /* Your calc */; if ( adjLists.size() < ind + 1 ) // Remember it is zero indexed adjLists.resize( ind + 1 ); adjLists[ ind ].push_back( boost::shared_ptr< Edge >( new Edge( v1, v2, weight ) ) ); Things to note: * The if statement checks to see that the vector is big enough _before_ you access the index. * The use of the smart pointer. Don't get in the habit of using raw pointers. * The use of the type vector<>::size_type for the index. Your compiler vendor has gone to a lot of trouble making sure you won't have any unexpected overflows etc. when they chose this type. Use it rather than make your own guess about what is right. * The vector is zero indexed so index 1 means it must be at least size 2 K Back to top Roland Pibinger Guest Posted: Sun Jun 18, 2006 9:10 am    Post subject: Re: followup (better formating of question) vector of lists On 17 Jun 2006 20:00:27 <kirit.saelensminde (AT) gmail (DOT) com> wrote: Quote: craig wrote: void Graph::addEdge( Vertex &v1, Vertex &v2, int weight ) { ind = "compute index of Vertex in vertex list" Edge *e = new Edge (v1, v2, weight ); list <Edge*> lst; <- how to make dynamic ?? adjLists.push_back (lst); <-how to make dynamic?? adjLists[ind].push_back( e ); } The list is already dynamic, the push_back will add to the end. The vector is slightly different though. I think you missed the suggestion of the resize() somewhere. What you need is (I'm not g'teeing it will compile directly) is something like: vector< list< boost::shared_ptr< Edge > > >::size_type ind = /* Your calc */; if ( adjLists.size() < ind + 1 ) // Remember it is zero indexed adjLists.resize( ind + 1 ); adjLists[ ind ].push_back( boost::shared_ptr< Edge >( new Edge( v1, v2, weight ) ) ); Why shared_ptrs here? They behave like real pointers but obfuscate the code, create a dependency to Boost, add the overhead of one dynamically allocated counter for each Edge object (e.g. 1000 Edges means 1000 superfluous dynamic allocations), ... The usual way is to let the destructor do the cleanup. Quote: Things to note: * The if statement checks to see that the vector is big enough _before_ you access the index. * The use of the smart pointer. Don't get in the habit of using raw pointers. On the contrary. Get the habit to avoid "smart" pointers!! Quote: * The use of the type vector<>::size_type for the index. Your compiler vendor has gone to a lot of trouble making sure you won't have any unexpected overflows etc. when they chose this type. Use it rather than make your own guess about what is right. * The vector is zero indexed so index 1 means it must be at least size 2 The code may now look like this: void Graph::addEdge( Vertex &v1, Vertex &v2, int weight ) { ind = "compute index of Vertex in vertex list" Edge *e = new Edge (v1, v2, weight ); if (! (ind < adjLists.size())) { adjLists.resize(ind + 1); } // push_back dynamically grows the list adjLists[ind].push_back( e ); } // destructor Graph::~Graph() { for (vector<list<Edge*> >::size_tye ind = 0; ind < adjLists.size(); ++ind) { for (list<Edge*>::iterator it = adjLists[ind].begin(); it != adjLists[ind].end(); ++it) { delete *it; } } } Back to top Kirit Sælensminde Guest Posted: Mon Jun 19, 2006 8:13 am    Post subject: Re: followup (better formating of question) vector of lists Roland Pibinger wrote: Quote: On 17 Jun 2006 20:00:27 <kirit.saelensminde (AT) gmail (DOT) com> wrote: * The use of the smart pointer. Don't get in the habit of using raw pointers. On the contrary. Get the habit to avoid "smart" pointers!! Hmmm. Not sure that is great advice. Obviously there are places where raw pointers are appropriate, but once you have the level of skill to know where they are, then you should know how to use them safely too. Smart pointers are much safer for use by beginners. I suppose that we could argue about this until the cows come home. I am somewhat surprised though. I would have thought recommending smart pointers to be particularly non-contentious. K Back to top Old Wolf Guest Posted: Mon Jun 19, 2006 8:30 am    Post subject: Re: vector of lists: anyone know how to do this? craig wrote: Quote: I've declared the following as a "vector" of "lists" using STL vector< list<Edge* The elements of a vector must be copyable and assignable. If you copy a list<Edge*> you will get another list that has all the same pointers in it. If you then 'delete' the pointers in one list then the other list will have dangling pointers. What is your reason for not using a list of Edges? vector< list<Edge> > Back to top Alf P. Steinbach Guest Posted: Mon Jun 19, 2006 9:10 am    Post subject: Re: followup (better formating of question) vector of lists * Kirit Sælensminde: Quote: Roland Pibinger wrote: On 17 Jun 2006 20:00:27 <kirit.saelensminde (AT) gmail (DOT) com> wrote: * The use of the smart pointer. Don't get in the habit of using raw pointers. On the contrary. Get the habit to avoid "smart" pointers!! Hmmm. Not sure that is great advice. Obviously there are places where raw pointers are appropriate, but once you have the level of skill to know where they are, then you should know how to use them safely too. Smart pointers are much safer for use by beginners. I suppose that we could argue about this until the cows come home. I am somewhat surprised though. I would have thought recommending smart pointers to be particularly non-contentious. If you read this group for a while you'll discover that Roland, for some unknown reason, takes a very dim view of template-based stuff, including smart pointers and the Boost library. Both smart pointers and the Boost library (which contains a number of smart pointer classes) are highly recommended by most experts, for both novices and professionals -- although the professionals generally don't need the recommendation, since most professional C++ programmers are already using smart pointers and Boost. The only reason I can think of why Roland recommends against is that he's stuck with a code base that requires some old compiler that doesn't handle templates well. Happily that isn't a problem for a novice. Nor is it a problem in general, but it's an issue one does not control (except by making sure one isn't made responsible for such old code...). -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? Back to top Guest Posted: Mon Jun 19, 2006 9:10 am    Post subject: Re: vector of lists: anyone know how to do this? Some of the possible reasons are: - Each edge object might take lots of memory and if the graph is big, you might end up using lot of memory - If you have multiple lists each with a different object, if you modify one object, the same object in other list does not get updated. This is because these are two different objects now. It will be hard to track down bugs that can arise because of this. Best solution for a situation like this is to use auto/smart pointers for Edge class. That will allow you to make as many copies of list<Edge*> and only when the last one is deleted is when your object will be really deleted. Atul Old Wolf wrote: Quote: craig wrote: I've declared the following as a "vector" of "lists" using STL vector< list<Edge* The elements of a vector must be copyable and assignable. If you copy a list<Edge*> you will get another list that has all the same pointers in it. If you then 'delete' the pointers in one list then the other list will have dangling pointers. What is your reason for not using a list of Edges? vector< list<Edge Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.