Re: BGL and reversing / moving edges

Thanks for the suggestion to use edge indices rather than descriptors -- I'll give that a shot. And thanks for helping me out!

-Josh --- In Boost-Users@y..., Jeremy Siek wrote:

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Hi Josh,

On Thu, 25 Jul 2002, joshrose wrote: joshro> joshro> However, I'm not sure this is quite what I'm looking for, since in joshro> addition to not invalidating other iterators / descriptors (those joshro> other than the one I'm trying to reverse), it's critical

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joshro> not invalidate the edge descriptor for the edge I'm trying to joshro> reverse, since it's still going to be in the graph, and

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joshro> maps will refer to it. What I think I really need is a way to joshro> move the edge within the graph (or at least remove the edge and joshro> re-add it, in a way that guarantees that the re-added edge has the joshro> same edge_descriptor as the original edge).

Are you using internal property maps for the edges? Is so you, when you removed, and then add an edge, you could first read out all the

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info, and then write it into the new edge.

If you are using an external map for edges... how are you doing the lookup? You aren't using the edge_descriptor itself as the key are you? That is bad... instead you could add an integer ID internal

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edges, and then use that to do lookups.

joshro> Tracing through the implementation a bit of add_edge, I have an joshro> idea on how to do this, but it involves a fair bit of hacking and joshro> is not terribly clean (and probably doesn't work!): you could joshro> directly go into the edge_desc_impl class (really it's superclass joshro> edge_base) [from detail/edge.hpp] and swap the m_source and joshro> m_target members directly, then remove the edge from the

(Apologize for being a bit confused about the property map library, but hopefully this posting doesn't smack too much of ignorance) One quick followup, though -- if I do follow the approach of an external property map keyed by edge id, won't I wind up having to do a potentially expensive lookup on the edge id every time I need to access properties keyed by edges? The reason I was going to originally use edge descriptors is that they're immediately available during the graph traversal at constant additional cost -- the algorithm involves a modified version of Dijkstra's shortest paths algo extended to multigraphs. (If I understand correctly, the standard Dijk algo in BGL doesn't handle mutigraphs, since it records predecessors as vertices, rather than edges.) By adding a lookup, I'm adding (at least) a log (E) factor to all operations that involve accessing edge properties, in particular to looking up edge weights -- in fact, the standard BGL Dijk algo seems to get around this precisely by using an weight_map keyed by edge_descriptors! (again, if I understand correctly). Since this lookup happens deep in the traversal, the potential effect on running time is probably quite noticable, esp. since for my application (statistical machine translation), each subgraph in the bipartite graph can have on the order of 500,000 nodes, and the graph can contain an order of magnitude larger number of edges. I think your suggestion about using the edge id and an extra level of indirection will help me achieve a correct solution for now, and I'll go ahead in that direction. Depending on the performance of the algo (this only needs to be run once in the beginning of a pipeline to bootstap a machine learning process), I'll look into ways to remove the extra lookup cost by using edge_descriptors as keys. Just a suggestion, though, that it would help at least one person out to have some sort of a move_edge function that could move an edge within a graph while maintaining edge properties and the descriptor (grin). If I do wind up writing something along these lines, perhaps I'll try to figure out the whole submission process for BGL. Thanks again. --- In Boost-Users@y..., "joshrose" wrote: that I property property property for source

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joshro> vertex edge list, create a StoredEdge instance and insert it into joshro> the target edge list. But I'm not sure how to link this new joshro> stored edge with the old edge_descriptor.

I suppose I could add the following function:

reverse_edge(edge_descriptor e, graph& g)

which would do what you are describing. However, if the property copying solution described above works for you, I'd rather not add the reverse_edge function.

Cheers, Jeremy

---------------------------------------------------------------------- Jeremy Siek http://php.indiana.edu/~jsiek/ Ph.D. Student, Indiana Univ. B'ton email: jsiek@o... C++ Booster (http://www.boost.org) office phone: (812) 855-3608 ----------------------------------------------------------------------