Re: [Boost-users] [BGL] Confusion about iterator_property_map and initialization of OffsetMap

Subject: Re: [Boost-users] [BGL] Confusion about iterator_property_map and initialization of OffsetMap
From: Cedric Laczny (cedric.laczny_at_[hidden])
Date: 2011-04-03 03:05:22

On Saturday, 2. April 2011 21:00:01 Jeremiah Willcock wrote:
> On Sat, 2 Apr 2011, Cedric Laczny wrote:
> > On Friday, 1. April 2011 20:25:54 Jeremiah Willcock wrote:
> >> On Fri, 1 Apr 2011, Cedric Laczny wrote:
> >>> Hi,
> >>> 
> >>> I have modified the astar-cities.cpp example to use an external
> >>> rank_map based on iterator_property_map.
> >>> 
> >>> The following should give you an idea:
> >>>  vector<cost> v_vec(num_vertices(g), 0.0);
> >>>  
> >>>  typedef property_map< mygraph_t, vertex_index_t>::type VertexIndexMap;
> >>>  VertexIndexMap v_index = get(vertex_index, g); // Initialization of
> >>>  interest // Create the external property map
> >>>  typedef iterator_property_map< std::vector< cost >::iterator,
> >>>  VertexIndexMap
> >>>  
> >>>> CostMap;
> >>>> 
> >>>  CostMap c_map(v_vec.begin(), v_index);
> >>> 
> >>> and the function-call of astar_search is as follows:
> >>>    astar_search
> >>>    
> >>>      (g, start,
> >>>      
> >>>       distance_heuristic<mygraph_t, cost, location*, const char**>
> >>>       
> >>>        (locations, goal, name),
> >>>       
> >>>       predecessor_map(&p[0]).distance_map(&d[0]).
> >>>       visitor(a_star_vis).
> >>>       rank_map(c_map)); // Here we use the "global" cost map
> >>> 
> >>> Basically, the code is the same as in the example, with only the
> >>> addition of the explicit rank_map.
> >>> 
> >>> Now I am fairly confused that the function call seems to behave the
> >>> same if I have "get(vertex_index, g)" and if I don't have it.
> >>> This comes unexpected to me as I would think that it needs OffsetMap
> >>> (v_index) to correctly find the index in the random access container
> >>> (v_vec). But when I simply declare an object it is not yet initialized
> >>> and will not contain any meaningfull offsets (not using
> >>> "get(vertex_index, g)").
> >>> So why does it work, when the map is not initialized? Or what am I
> >>> missing here please?
> >> 
> >> The default index map is just a typed_identity_property_map, and that
> >> class doen't have any parameters to its constructor.  However, you
> >> should not rely on those properties, since algorithms in BGL have
> >> broken because their authors assumed the vertex index map was the
> >> identity then users passed in graphs (such as adjacency_list with listS
> >> as the vertex container) with non-identity maps.
> > 
> > My vertex-list is vecS, therefore the get(vertex_index, g) should work.
> > But this does not confuse me. It's the fact, that it does not seem to
> > matter if the map is actually correctly initialized or not. Simply using
> > the iterator_property_map on an IndexMap() seems to work, which is not
> > what I would expect as the mapping from a vertex to an offset is hence
> > missing. So I am unfortunately still confused.
> 
> The default for the index map when you create an iterator_property_map is
> the identity (since the property map doesn't know what graph you're using
> it with), not the graph's vertex index map.  Thus, it doesn't matter
> whether the graph has an index map or not, leaving that argument off of
> the iterator_property_map's template argument list and constructor will
> give you the identity.

I am really sorry, but I'm still not sure if I get it right.
So, when I declare the iterator_property_map as follows:

typedef iterator_property_map< std::vector< cost >::iterator, VertexIndexMap 
>> CostMap;
CostMap c_map(v_vec.begin(), v_index);

and v_index is _not_ initialized, then what will be the size and the content 
of the CostMap? And what will be the size and content of v_index?
Will it be the size of v_vec? And by identity, do you mean that a vertex is 
basically just an integer and thus each vertex will be mapped to its according 
number, so therefore identity? And because this is the default behavior, it is 
equal to using get(vertex_index, g) as this basically is also just an 
identity-map?
So basically, the default-initialization of VertexIndexMap will result in the 
identity, aswell as for the CostMap, therefore it doesn't matter whether you 
specify vertex-index, as long as it is the identity?

Thank you and appologies for my repeated asking.

> 
> -- Jeremiah Willcock
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> Boost-users_at_[hidden]
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Best,

Cedric