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What is the purpose of a graph that is "undirected, with cost and reverse cost"? Where is the difference to a graph that is "undirected, with cost" ?

For me it does not really make sense, because for my understanding the reverse cost should just come into play when the graph is directed ... (Figure from http://docs.pgrouting.org/dev/doc/src/developer/sampledata.html#fig2) graph that is undirected, with cost and reverse cost

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The reason why you can set undirected graph with reverse_cost has a simple reason: the function has two arguments, one for directed true/false and one for reverse_cost true/false. But you're right that it only makes sense for a directed graph. In the other case it will be ignored.

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Maybe the documentation at http://docs.pgrouting.org/dev/src/dijkstra/doc/index.html#pgr-dijkstra can shed some light onto the isse. It would seem like - in an undirected graph - one can go in either direction with either cost or reverse_cost:

enter image description here

Not sure which real-world application this is used for though.

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    Not tried this myself, but this answer suggests a couple of practical applications for this and gives an example. – Steven Kay Dec 8 '15 at 23:48

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