Question

I am trying to implement Distance Vector Routing algorithm, using http://www.cs.bu.edu/fac/byers/courses/791/F99/scribe_notes/cs791-notes-990923.html
(in C++).

Here is what I have done so

far:

i) Read no. of nodes.

ii) Implement the points 1 to 4 in the article as:

     for(i = 0; i < nodes; i++) //nodes is the no. of nodes
     {
         for(j = 0; j < nodes; j++)
         {
             if(distanceVector[i][j] != 0) //distanceVector holds the cost value between every pair of links, 0 if no link exists
             {
                  for(k = 0; k < nodes; k++)
                  {
                       if((distanceVector[i][j] + distanceVector[j][k]) < distanceVector[i][k])
                       {
                             distanceVector[i][k] = distanceVector[i][j] + distanceVector[j][k];
                             via[i][j] = i; // intermediate node, in case no link exists
                             via[j][i] = j;
                       }
                  }
              }
         }
     }

I get the same array / matrix as it is. I have also tried juggling i, j, and k, but of no use.

Am I right in my implementation…???

Answer 1

Two things bother me about your code. First, you are using “0” to represent “no link”. This can get you in trouble. The code basically read as follows: “if there is an intermediary point j, that makes that path from i to k shorter, change the path from i to k to pass via k”. Therefore, using “0” to represent “no link” might make your code choose wrong “via”s. Try instead to use infinity (if you are using floating point) or a really big value (e.g., MAX_INT).

Second, these lines look wrong:

via[i][j] = i; // intermediate node, in case no link exists
via[j][i] = j;

Since you found a path from i to k via j that is shorter, it should be:

via[i][k] = j; // intermediate node, in case no link exists
via[k][i] = j;