Re: [Boost-users] Iterater for subset extraction?

-----Original Message----- From: Stephan Diederich

Hi Thom,

2006/11/8, Thom DeCarlo :

...

I have a couple of n00b questions. I have constructed a BGL graph that uses the edge_name_t property to road names. I want to change the weights of some of the roads based on input from the user. Is there a way to set up an iterator to step through edges that have names which match elements of a set? (e.g., "A,B,B1,L,M")

In a related question, I need to extract a vertex whose vertex_index_t value matches a specific value. The vertices were not entered in any specific order, so the "vertex(1, g)" function returns the first vertex to have been entered into the graph, but not necessarily the one whose index value is "1".

If I don't get you wrong, there's no direct way in the BGL to do that. The properties you attach to a vertex or an edge are normally accessed through the edge or vertex descriptor respectively. What you want to do is the other way around. Get from a property to the edge desciptor(s) that match that property or in the case of vertex_index to the vertex descriptor. Maybe you can use Bimap (http://tinyurl.com/y95qng), where you can get keys (edge/vertex-descriptors) from values together with the concept of exterior property maps (http://tinyurl.com/yhju58) and Constructing an Exterior Property Map.

Reading further in the Bimap docs, I think I found what you want: http://tinyurl.com/yxzpme

HTH, Stephan

Well, once I figure out how to use it, I've found the Filter Iterator (http://www.boost.org/libs/iterator/doc/filter_iterator.html) which may solve the first part of my problem. For the second part, I'm not sure exactly what you mean. Let me describe the problem a bit more. Each vertex has an internal property that I've created: // define a struct for the vertex coordinate properties typedef struct vCoord { double Lon; //degrees double Lat; //degrees double Alt; //meters above WGS84 ellipsoid } Coord; The user will provide a specific coordinate for his starting and ending points and a preferred route of street names. I need to traverse the BGL graph to find the vertex nearest the user's coordinate that is also on the route. (Initially, I thought I was going to give each vertex a name and the user would provide that name, hence the search for a specific vertex index value. But, it is more likely that the user will know his position than the neighborhood of road names.) I think the filter iterator will also help with this problem. At least it will let me narrow down the number of vertices that need to be compared when searching for the closest one. Does this sound reasonable? Thanks, Thom