Floyd's warshall algorithm infinity

Question

I'm working on implementing the floyd warshall algorithm. I apply this algorithm on a graph with different vertices and some of them are not linked. My code isn't getting the right answer.

The final path generated from one vertex to another sometimes includes an edge which doesn't exist. I think my error comes from the fact that I compare infinity to infinity. I currently do that: I assume that a big integer will represent infinity for example 10000. What should I do when I met a situation like that 10000 > 10000 + n? with n < 10000

What is the maximum weight of an edge? Are there edges with negative weights? — kraskevich, Jan 13 '15 at 18:41
And what do you mean by `10000 > 10000 + n`? It is true for any `n < 0`, but how is this related to your question? — kraskevich, Jan 13 '15 at 18:42
The maximum weight is 1000 and there is no edge with negative weights — mel, Jan 13 '15 at 18:43
10000 represent the infinity but in fact we cannot compare to infinity, so is there a special case in floyd warshall algorithm for this kind of thing? — mel, Jan 13 '15 at 18:45
If the infinity is big enough, there is no need handle it specially. — kraskevich, Jan 13 '15 at 18:50
If your algorithm is returning a result that contains an edge that does not exist, then your weight calculations and comparisons are only part of the problem. IIRC, the FW algorithm should not be creating new edges... — twalberg, Jan 13 '15 at 18:53

score 1 · Accepted Answer · answered Jan 13 '15 at 18:45

1

The longest possible valid path can have length = (n - 1) * 1000. Thus, "infinity" must be strictly greater than this value. And there is no need to handle it specially as long as 2 * infinity fits into the type you are using to store distances.

answered Jan 13 '15 at 18:45

kraskevich

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Ok so if my number of vertex is 7 and i met a case like that: 10000 > 10000 + 450; Should I do a similar thing that if I would have met: 300> 100 +50; – mel Jan 13 '15 at 18:51
@mel I do not get your question. How do you implement Floyd-Warshall algorithm? In the implementation I usually use there is no need to handle infinity in a special way. – kraskevich Jan 13 '15 at 19:00

Floyd's warshall algorithm infinity

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