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Why the command "graphallshortestpaths" gives me Inf value for a weighted indirect graph that I know it doesn't have disconnections?

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I am trying to get the shortest path of all the nodes of a network for a weighted non-negative sparse matrix. When I use "graphallshortestpath", I get some "Inf" values that indicates there is no path between 2 nodes. I tried the same graph with a dense matrix and used Dijkstra's algorithm and I had no "Inf" values at all. So, I don't know why "graphallshortestpath" built in function -that is much more faster because works with sparse matrix- is giving me "Inf" values.
Any ideas, please.
Thanks,
  1 Comment
Christine Tobler
Christine Tobler on 16 Sep 2016
Could you post an example of the graph you are using? I tried an example and I don't see this happening for that case:
>> M = sparse([1 3], [2 4], 1, 4, 4);
>> graphallshortestpaths(M)
ans =
0 1 Inf Inf
Inf 0 Inf Inf
Inf Inf 0 1
Inf Inf Inf 0
>> graphshortestpath(M, 1,'Method','Dijkstra')
ans =
0 1 Inf Inf

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Accepted Answer

Christine Tobler
Christine Tobler on 16 Sep 2016
I'm not sure what function you are using to compute Dijkstra with a dense matrix - graphshortestpath for a dense matrix returns an error for me.
I would assume that the problem is that when a graph is represented as a sparse matrix, the assumption is that all zeros stand for edges that do not exist. But when you use a dense matrix, the algorithm might be interpreting every zero entry as an edge with length zero - so every node would be reachable from every other node.

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