Bellman–Ford Algorithm for Shortest Paths

Bellman-Ford algorithm is used to find minimum distance from the source vertex to any other vertex. The main difference between this algorithm with Dijkstra’s the algorithm is, in Dijkstra’s algorithm we cannot handle the negative weight, but here we can handle it easily.

Bellman-Ford algorithm finds the distance in a bottom-up manner. At first, it finds those distances which have only one edge in the path. After that increase the path length to find all possible solutions.

Input and Output

Input:
The cost matrix of the graph:
0  6  ∞ 7  ∞
∞  0  5 8 -4
∞ -2  0 ∞  ∞
∞  ∞ -3 0  9
2  ∞  7 ∞  0

Output:
Source Vertex: 2
Vert:   0   1   2   3   4
Dist:  -4  -2   0   3  -6
Pred:   4   2  -1   0   1
The graph has no negative edge cycle

Algorithm

bellmanFord(dist, pred, source)

Input − Distance list, the predecessor list, and the source vertex.
Output − True, when a negative cycle is found.

Begin
   iCount := 1
   maxEdge := n * (n - 1) / 2     //n is number of vertices

   for all vertices v of the graph, do
      dist[v] := ∞
      pred[v] := Ï•
   done

   dist[source] := 0
   eCount := number of edges present in the graph
   create edge list named edgeList

   while iCount < n, do
      for i := 0 to eCount, do
         if dist[edgeList[i].v] > dist[edgeList[i].u] + (cost[u,v] for edge i) dist[edgeList[i].v] > dist[edgeList[i].u] + (cost[u,v] for edge i) pred[edgeList[i].v] := edgeList[i].u
      done
   done

   iCount := iCount + 1
   for all vertices i in the graph, do
      if dist[edgeList[i].v] > dist[edgeList[i].u] + (cost[u,v] for edge i),
         then return true
   done

   return false
End

Example

#include<iostream>
#include<iomanip>
#define V 5
#define INF 999
using namespace std;
//Cost matrix of the graph (directed) vertex 5

int costMat[V][V] = {
   {0, 6, INF, 7, INF},
   {INF, 0, 5, 8, -4},
   {INF, -2, 0, INF, INF},
   {INF, INF, -3, 0, 9},
   {2, INF, 7, INF, 0}
};

typedef struct {
   int u, v, cost;
}edge;

int isDiagraph() {

   //check the graph is directed graph or not

   int i, j;
   for(i = 0; i<V; i++) {
      for(j = 0; j<V; j++) {
         if(costMat[i][j] != costMat[j][i]) {
            return 1;      //graph is directed
         }
      }
   }
   return 0;//graph is undirected
}

int makeEdgeList(edge *eList) {
   //create edgelist from the edges of graph
   int count = -1;
   if(isDiagraph()) {
      for(int i = 0; i<V; i++) {
         for(int j = 0; j<V; j++) {
            if(costMat[i][j] != 0 && costMat[i][j] != INF) {
               count++;         //edge find when graph is directed
               eList[count].u = i; eList[count].v = j;
               eList[count].cost = costMat[i][j];
            }
         }
      }
   }else {
      for(int i = 0; i<V; i++) {
         for(int j = 0; j<i; j++) {
            if(costMat[i][j] != INF) {
               count++;         //edge find when graph is undirected
               eList[count].u = i; eList[count].v = j;
               eList[count].cost = costMat[i][j];
            }
         }
      }
   }
   return count+1;
}

int bellmanFord(int *dist, int *pred,int src) {
   int icount = 1, ecount, max = V*(V-1)/2;
   edge edgeList[max];

   for(int i = 0; i<V; i++) {
      dist[i] = INF;      //initialize with infinity
      pred[i] = -1;      //no predecessor found.
   }

   dist[src] = 0;//for starting vertex, distance is 0

   ecount = makeEdgeList(edgeList);       //edgeList formation

   while(icount < V) {       //number of iteration is (Vertex - 1)
      for(int i = 0; i<ecount; i++) {
         if(dist[edgeList[i].v] > dist[edgeList[i].u] + costMat[edgeList[i].u][edgeList[i].v]) {      //relax edge and set predecessor
            dist[edgeList[i].v] = dist[edgeList[i].u] + costMat[edgeList[i].u][edgeList[i].v];
            pred[edgeList[i].v] = edgeList[i].u;
         }
      }
      icount++;
   }

   //test for negative cycle
   for(int i = 0; i<ecount; i++) {
      if(dist[edgeList[i].v] > dist[edgeList[i].u] + costMat[edgeList[i].u][edgeList[i].v]) {
         return 1;    //indicates the graph has negative cycle
      }
   }

   return 0;     //no negative cycle
}

void display(int *dist, int *pred) {
   cout << "Vert: ";
   for(int i = 0; i<V; i++)
      cout <<setw(3) << i << " ";
   cout << endl;
   cout << "Dist: ";

   for(int i = 0; i<V; i++)
      cout << setw(3) << dist[i] << " ";
   cout << endl;
   cout << "Pred: ";

   for(int i = 0; i<V; i++)
      cout << setw(3) << pred[i] << " ";
   cout << endl;
}

int main() {
   int dist[V], pred[V], source, report;
   source = 2;
   report = bellmanFord(dist, pred, source);
   cout << "Source Vertex: " << source<<endl;
   display(dist, pred);

   if(report)
      cout << "The graph has a negative edge cycle" << endl;
   else
      cout << "The graph has no negative edge cycle" << endl;
}

Output

Source Vertex: 2
Vert:   0   1   2   3   4
Dist:  -4  -2   0   3  -6
Pred:   4   2  -1   0   1
The graph has no negative edge cycle