Floyd-Warshall with negative loop in C

Question

im working on graph searching with Floyd-Warshalls algorithm and don´t understand how to alter it to prevent negative loops.

When i enter:

From  Cost   To
0      -1     1
1      -2     0

I get cost matrix:

   0   1
0 -3  -4
1 -5 -10

and then it starts looping till it crashes because its still adding negative edges and further decreases the cost.

void FloydWarshall(int ***distanceMatrix, int maxVertices)
{
int from, to, via;

for(from = 0; from < maxVertices; from++)
{
 for(to = 0; to < maxVertices; to++)
 {
      for(via = 0; via < maxVertices; via++)
       {
         //Fix the negative looping
         //EDIT FIX:
         if((*distanceMatrix)[via][via]<0)
         {fprintf(stderr, "Contains negative cycle!\n"); continue;}
         //END EDIT
         //Searches for the cheapest way from all-to-all nodes, if new path
         // is cheaper than the previous one, its updated in matrix
         (*distanceMatrix)[from][to] = min((*distanceMatrix)[from][to],
         ((*distanceMatrix)[from][via]+(*distanceMatrix)[via][to]));
       }
 }
}
}

Where min is:

int min(int a,int b)
{return (a<b)? a:b;}

and my distanceMatrix has INF wherever there is no cost.

I came across older topic where the altered algorythm was like this:

for(int i = 0; i < n; i++)
for(int j = 0; j < n; j++)    // Go through all possible sources and targets

    for(int k = 0; d[i][j] != -INF && k < n; k++)
        if( d[i][k] != INF && // Is there any path from i to k?
            d[k][j] != INF && // Is there any path from k to j?
            d[k][k] < 0)      // Is k part of a negative loop?

            d[i][j] = -INF;   // If all the above are true
                              // then the path from i to k is undefined

However, even when i use this fix instead of my function, it still loops and further decreases cost. Is this fix correct? If not, how should i rewrite it? Thanks.

Answer 1

From wikipedia

Hence, to detect negative cycles using the Floyd–Warshall algorithm, one can inspect the diagonal of the path matrix, and the presence of a negative number indicates that the graph contains at least one negative cycle.[2] Obviously, in an undirected graph a negative edge creates a negative cycle (i.e., a closed walk) involving its incident vertices.

So if d[k][k] is ever less than 0 you should just exit and say there is a negative cycle.