How to do it...

Perform the following steps to create an adjacency matrix representation of a graph:

1. Ask the user for the number of vertices in the graph.
2. Define a square matrix that is equal to the number of vertices.
3. Initialize all the elements of the matrix to 0.
4. Ask the user to enter the edges. For each edge (i,j) entered by the user, replace 0 at the mat[i][j] index location.
5. Once all the edges are entered, display all of the elements of the adjacency matrix.

The code for creating an adjacency matrix representation of a graph is as follows:

//adjmatdirect.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;
  }
  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;
}

Now, let's go behind the scenes to understand the code better.