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Subject: Re: [Boost-users] [BGL] Confusion about iterator_property_map and initialization of OffsetMap
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2011-04-02 15:00:01

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.

-- Jeremiah Willcock

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