C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  sorting maps according to their value_type          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message francesco Guest Posted: Fri Feb 10, 2006 2:06 pm    Post subject: sorting maps according to their value_type I'm designing an algorithm with maps that need the elements to be sorted by value instead of by key. Does anyone have a suggestion on how to do that? #include <map> using namespace std; int main() { // how can I specify a SortingCriterion based on the value so that // the months are ordered from the one with largest number of day // to the one with smallest? map<const char*, int /*, SortingCriterion*/ > months; months["february"] = 28; months["march"] = 31; months["april"] = 30; } Thanks & regards Francesco [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Matjaz Depolli Guest Posted: Fri Feb 10, 2006 3:06 pm    Post subject: Re: sorting maps according to their value_type Maps are always sorted by key. So in your case you only need to switch value and key types. Or maybe I'm missing something? map<int, const char*> months; months[28] = "february"; months[31] = "march"; months[30] = "april"; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ralf Fassel Guest Posted: Fri Feb 10, 2006 5:06 pm    Post subject: Re: sorting maps according to their value_type * "Matjaz Depolli" <matjaz.depolli (AT) gmail (DOT) com> | map<int, const char*> months; Probably better use a multimap if he wants to run his program in summer, too... R' [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ralf Fassel Guest Posted: Sat Feb 11, 2006 6:06 pm    Post subject: Re: sorting maps according to their value_type * "francesco" <fd.calabrese (AT) gmail (DOT) com> | map<const char*, int /*, SortingCriterion*/ > months; Also, using a char* as map key will not do what you expect it to do. Try the following: #include <iostream> #include <ostream> #include <map> #include <cstring> int main() { std::map<const char*, int> months; months["february"] = 28; std::cout << "february has " << months["february"] << " days\n"; std::cout << "february has " << months[strdup("february")] << " days\n"; } => february has 28 days february has 0 days Use a std::string instead as the key. R' [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Vladimir Marko Guest Posted: Sat Feb 18, 2006 1:06 am    Post subject: Re: sorting maps according to their value_type francesco wrote: Quote: I'm designing an algorithm with maps that need the elements to be sorted by value instead of by key. Does anyone have a suggestion on how to do that? Have a look at multi_index in Boost: http://www.boost.org/libs/multi_index/doc/index.html Cheers, Vladimir Marko [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Seungbeom Kim Guest Posted: Sat Feb 18, 2006 5:06 am    Post subject: Re: sorting maps according to their value_type francesco wrote: Quote: I'm designing an algorithm with maps that need the elements to be sorted by value instead of by key. Does anyone have a suggestion on how to do that? Sets and maps are always maintained in the order of the keys so that looking up (by keys) can be efficient. You don't have to sort them in an arbitrary order nor are you allowed to. For a serious task like this, it's probably Boost.MultiIndex that you need. However, for a simple and const data structure like the example, I would just have a vector that has the same information but with a different ordering. struct by_second { template <typename V> bool operator()(const V& a, const V& b) const { return a.second < b.second; } }; map<string, int> months; months["jan"] = 31; months["feb"] = 28; months["mar"] = 31; months["apr"] = 30; // ... // vector<map<string, int>::value_type> cannot be sorted vector<pair<string, int> > months_by_days(months.begin(), months.end()); sort(months_by_days.begin(), months_by_days.end(), by_second()); -- Seungbeom Kim [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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