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Floyd's algorithm source code in C

Updated on September 19, 2011

Algorithm

The Floyd–Warshall algorithm compares all possible paths through the graph between each pair of vertices. It is able to do this with only Θ(|V|3) comparisons in a graph. This is remarkable considering that there may be up to Ω(|V|2) edges in the graph, and every combination of edges is tested. It does so by incrementally improving an estimate on the shortest path between two vertices, until the estimate is optimal.

Consider a graph G with vertices V, each numbered 1 through N. Further consider a function shortestPath(i, j, k) that returns the shortest possible path from i to j using vertices only from the set {1,2,...,k} as intermediate points along the way. Now, given this function, our goal is to find the shortest path from each i to each j using only vertices 1 to k + 1.

There are two candidates for each of these paths: either the true shortest path only uses vertices in the set {1, ..., k}; or there exists some path that goes from i to k + 1, then from k + 1 to j that is better. We know that the best path from i to j that only uses vertices 1 through k is defined by shortestPath(i, j, k), and it is clear that if there were a better path from i to k + 1 to j, then the length of this path would be the concatenation of the shortest path from i to k + 1 (using vertices in {1, ..., k}) and the shortest path from k + 1 to j(also using vertices in {1, ..., k}).

If w(i,j) is the weight of the edge between vertices i and j, we can define shortestPath(i, j, k) in terms of the following recursive formula: the base case is

shortestPath(i,j,0) = w(i,j)

and the recursive case is

This formula is the heart of the Floyd–Warshall algorithm. The algorithm works by first computing shortestPath(i, j, k) for all (i, j) pairs for k = 1, then k = 2, etc. This process continues until k = n, and we have found the shortest path for all (i, j) pairs using any intermediate vertices.

Floyd's Algorithm

/*C PROGRAM TO IMPLEMENT ALL PAIR SHORTEST PATH USING FLOYDS ALGORITHM

INPUT: N VALUE FOR NUMBER OF VERTICES

OUTPUT: ADJACENCY MATRIX
SHORTEST DISTANCE MATRIX
*/

#include<stdio.h>
#include<stdlib.h>

void AdjacencyMatrix(int a[][100],int n){ //To generate adjacency matrix for given nodes

int i,j;
for(i = 0;i < n; i++)
{
for(j = 0;j <= n; j++)
{
a[i][j] = 0;
}

}

for(i = 1; i < n; i++)
{
for(j=0;j<i;j++)
{
a[i][j] = rand()%10;
a[j][i] = 99;
}
}
}


int min(int a,int b){

if(a < b)
return a;
else
return b;
}

void floyds(int a[][100],int n){

int i,j,k;
for(k = 0;k < n ; k++)
{
for(i = 0;i < n; i++)
{
for(j = 0;j < n ; j++)
{
a[i][j] = min (a[i][j], a[i][k] + a[k][j] );
}
}
}

}

int main()
{
int a[100][100],n,i,j;

FILE *fp = fopen("floyds.dot","w"); /* FOR DIGRAPH */

fprintf(fp,"digraph A {\n"); /* WRITE TO FILE */

printf("Enter the vertices of the digraph\n");
scanf("%d",&n);

AdjacencyMatrix(a,n);

printf("\t\tAdjacency Matrix of the graph\n"); /* PRINT ADJACENCY MATRIX */
for(i = 0;i < n; i++)
{
for(j = 0;j < n; j++)
{
printf("\t%d",a[i][j]);
if(a[i][j] != 0)
{
fprintf(fp,"%d -> %d\n",i,j);
}
}
printf("\n");
}

floyds(a,n);

printf("\n Shortest distance matrix\n"); /*PRINT SHORTEST DISTANCE MATRIX*/
for(i = 0;i < n; i++)
{
for(j = 0;j < n; j++)
{
printf("\t%d",a[i][j]);
}
printf("\n");
}

fprintf(fp,"}\n\n");

fclose(fp);

return 0;
}

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    • profile image

      santhosh 7 years ago

      good.... helps in lab

    • profile image

      Rick G 4 years ago

      Concerning floyds(int a[][100],int n).

      What does 'a' and represent and what does each of the two dimensions of a represent?

      What does 'n' represent?

      I have a list of locations, with a list of connections between those locations and the distance associated with those connections. I need to find shortest path for any given two locations - but need to understand how to apply "floyds(int a[][100],int n)" to it.

    • prabhakar gouda profile image
      Author

      prabhakar gouda 4 years ago from Bangalore

      Hi Rick,

      a[][100] is an adjacent matrix, and 'n' is the number of rows/columns in adjacent matrix. In other words 'n' represents the number of node in a graph.

      Ex:

      Consider an adjacency matrix:

      0 1 2

      1 0 2

      9 8 0

      Here n=3

      first rows says distance from 1st node to all other nodes.

      Second rows represents distance from 2nd node to all other nodes.

      In my code i generated random adjacency matrix, if you want to calculate shorted distance for your matrix then please comment random matrix generation part and take manual input.

    • profile image

      namratha 4 years ago

      i wanted code for floyd algorithm using openGL

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