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Subject: Re: [boost] [graph]Direct Edge Access
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2013-03-07 13:57:59

On Thu, 7 Mar 2013, Takatoshi Kondo wrote:

> Hello,
> I'm using Boost.Graph version 1.53.0. I'm trying to write some graph
> algorithms and I want to minimize the requirements of the input graph.
> I have a question about Boost.Graph concepts. I wrote a very simple
> code as follows:
> typedef boost::adjacency_list<
> boost::vecS,
> boost::vecS,
> boost::bidirectionalS> G;
> int main() {
> G g;
> boost::add_edge(0, 1, g);
> auto res = boost::edge(0, 1, g); // Why can I call it?
> std::cout << std::boolalpha << res.second << std::endl;
> boost::graph_traits<G>::edge_descriptor e = res.first;
> std::cout << boost::source(e, g) << "," << boost::target(e, g) << std::endl;
> }
> It works, but I don't understand why I can call boost::edge().
> According to the following document, AdjacencyMatrix concept provides
> Direct Edge Access functionality, so I can call edge(u,v,g) :
> The class template adjacency_list is a model of
> VertexAndEdgeListGraph, MutablePropertyGraph, CopyConstructible,
> Assignable, and Serializable.
> So, adjacency_list is not satisfied AdjacencyMatrix concept.

That's right; it (and a few other graph types) provide edge() but do not
satisfy the complexity requirement for it given in the Adjacency Matrix

> Also, I found the following document:
> This document describes that:
> edge()
> The time complexity for this operation is O(E/V) when the OutEdgeList
> type is a Sequence and it is O(log(E/V)) when the OutEdgeList type is
> an AssociativeContainer.
> It seems that edge() is a part of the public interface of
> adjacency_list. But I think that it is not a part of any graph
> concepts.

You are right: the one with that complexity is public but not in a concept

> If I want to write an algorithm that depends on several graph
> concepts, not actual classes such as adjancency_list, I can use
> edge(u,v,g) only if the input graph class satisfies AdjacencyMatrix
> concept.
> Am I understanding correctly?

Yes. You might want to consider using lookup_edge() from
<boost/graph/lookup_edge.hpp>; it will use edge() for graphs that model
Adjacency Matrix and do a manual search of a vertex's out_edges() for
other Incidence Graph models. That will make your algorithms work on all
Incidence Graphs but get the better performance when possible. Note that
lookup_edge() will not take advantage of logarithmic-time edge() functions
available in some graphs that do not model Adjacency Matrix, however; it
either uses a constant-time implementation or falls back to linear search.
The edge() function that you found will itself use linear search in many
cases, however.

-- Jeremiah Willcock

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