# Tweaking Floyd-Warshall Algorithm to detect cycles

Cheers, I am trying to solve the problem of minimum length cycle in a graph, and I came across a solution that suggested that I should tweak the Floyd-Warshall algorithm to solve that. It stated that instead of setting path[i][i] = 0 I should instead set path[i][i] = INFINITY, but I don't exactly understand why that is the case! I find that the main diagonal of the array used by Floyd-Warshall does not change, so how can it help me to see the path of the cycle? I understand that the generated array of the algorithm helps me find the shortest path of a pair. e.g. path[i][j] gives me the shortest path from i to j but, although the intuition stays the same, I see that nothing changes, and I can't take the desired result.

I even tried visualing the process, as seen here. I generated the graph below: but as you can see, although the diagonal is initialized with infinity, it does not get changed, even though the graph clearly has cycles. Can anyone explain what am I missing?

If you let it, running the algorithm will produce, in path[i][i], the length of the shortest path from node i to i. The length of the shortest cycle will be the smallest value on the diagonal. This requires
• updating path[i][i], instead of skipping it, like that visualization does;
• initializing path[i][i] with the actual cost of going from that node to itself in one step: the cost of a self-loop, or if there is no self-loop, infinity.