Re: [boost] BGL: dijkstra questions

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2002-01-15 15:46:28

On Tue, 15 Jan 2002, David Abrahams wrote:
david.> I know this will seem like a silly question to some; so much the better, the
david.> answer should be easy:
david.>
david.> I don't understand the reason for the decrease-key operation. I mean, I
david.> understand why it's needed the way that the algorithm is formulated, but
david.> AFAIK a formulation which stores the current weight of any path in the queue
david.> is far, far simpler and does the job:
david.>
david.> DIJKSTRA(G, s, w)
david.>   for each vertex u in V
david.>     d[u] := infinity
david.>     p[u] := u
david.>   end for
david.>   INSERT(Q, (s,s,0))
david.>   while (Q != Ø)
david.>     u,t,x := EXTRACT-MIN(Q)

should the above line instead be

t,u,x := EXTRACT-MIN(Q)

david.>     if u not in S
david.>       d[u] = x         // optional! d not needed for algorithm
david.>       p[u] = t         // also optional
david.>       S := S U { u }
david.>       for each vertex v in Adj[u]
david.>         if v not in S // optimization - first path to v is always shortest
david.>           INSERT(Q, (u, v, x + w(u,v))) // put the path in the queue
david.>       end for
david.>   end while
david.>
david.>
david.> Am I missing something, like the loss of some important event points?

At first glance, your algorithm makes sense to me. I like it. Let's do up
and implementation and play around with it. Similarly for BFS.

Cheers,
Jeremy

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 Jeremy Siek                          http://php.indiana.edu/~jsiek/
 Ph.D. Student, Indiana Univ. B'ton   email: jsiek_at_[hidden]
 C++ Booster (http://www.boost.org)   office phone: [phone]
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