I originally posted this to the wrong group, but I intended it for this group. I have a boost::multi_index question. My current container has looked like the following for a very long time: typedef std::map<double,int> mtype_t; What I am looking for is a way to index the mtype_t::value_type.first value so that, syntactically, it appears like a vector. For example, the container I need would let me do something like: { container_t c; c.insert( container_t::value_type( keycount_t( 1.414,88) )); c.insert( container_t::value_type( keycount_t( 3.141,77) )); c.insert( container_t::value_type( keycount_t( 23.000,99) )); for(int index=0; index<c.size(); ++index) printf("%d\n",c[index].second); } You can see that I want to be able to index into the container much like one would do with a vector. But I want my indexing to be O(1) and not O(n). Now I still need to be able to iterate the container, and I need to be able to do efficient insertions, and yes, the contents of the container will get bigger and smaller and the values vary quite a lot. A basic sketch of the multi_index container (but without the vector-like index) would be: typedef struct { double key; int count; } keycount_t; struct by_key{}; typedef boost::multi_index_container< keycount_t, boost::multi_index::indexed_by < boost::multi_index::ordered_unique < boost::multi_index::tag<by_key>, boost::multi_index::member< keycount_t,double,&keycount_t::key > > >
container_t;
This gives me the basics. But I cannot think of a way to add a secondary index that would give me the O(1) vector-like indexing. A use-case of the grammar I am after would be something like the following: void useless_function(container_t& c) { for(container_t::iterator i = c.begin(); i!=c.end(); ++i) print("key=%f\tval=%d\n",i->first,i->second); for(index=0; index<c.size(); ++index) print("key=%f\tval=%d\n",c[index].first,c[index].>second); } Here's WHY I need this: I have a stream of floating point values coming in (fixed to 5 significant digits). I need to keep count of how often any one of the floating point values is repeated. Very often I need to respond to requests as to the contents of the container at the Nth position. The slow O(n) method would be: const keycount_t& get_nth_record(int index,const container_t& c) { container_t::const_iterator i; for(i=c.begin(); i!=c.end(); ++i) if(--index==0 ) break; return( *i ); } But again, I need O(1) time, so I need some sort of index added to my container_t. Has anyone done this sort of thing before, and met the O(1) positional index requirement? Many thanks! -zap