How to do it...

We refer to the same graph that was in the previous recipe; however, this time, there are no edges:

Figure 10.5

Its adjacency matrix representation is as follows:

5,5          1              2              3              4              5
----------------------------------------------------------------------------
1            0              1              1              0              1
2            1              0              0              1              1
3            1              0              0              1              1
4            0              1              1              0              1
5            1              1              1              1              0

The only difference between the programs of the directed and undirected graphs is that, in the latter, the edge is simply repeated. That is, if there is an edge between a and b, then it is considered to be two edges: one from a to b and the other from b to a.

The program for creating an adjacency matrix representation of an undirected graph is as follows:

//adjmatundirect.c

#include <stdio.h>

#define max 10

int main() {
  static int edg[max][max], i, j, v1, v2, numb;
  printf("How many vertices are there? ");
  scanf("%d", & numb);
  printf("We assume that the vertices are numbered from : ");
  for (i = 1; i <= numb; i++) printf("%d ", i);
  printf("\nEnter the edges of the graph. Like 1 4 if there is an \n");
  printf("edge between vertex 1 and 4. Enter 0 0 when over\n");
  for (i = 1; i <= numb * (numb - 1); i++) {
    /* The for loop will run for at most numb*(numb-1) times because, the 
       number of edges are at most numb*(numb-1) where numb is the number 
       of vertices */
    scanf("%d %d", & v1, & v2);
    if (v1 == 0 && v2 == 0) break;
    edg[v1][v2] = 1;
    edg[v2][v1] = 1;
  }
  printf("\nThe adjacency matrix for the graph is \n");
  for (i = 1; i <= numb; i++) printf("\t%d", i);
  printf("\n----------------------------------------------------------\n");
  for (i = 1; i <= numb; i++) {
    printf("%d |\t", i);
    for (j = 1; j <= numb; j++) {
      printf("%d\t", edg[i][j]);
    }
    printf("\n");
  }
  return 0;
}