Question

typedef struct vertex{
    int num;
    struct vertex *next;
} Vertex;

Vertex *adj[1];

void buildList(){
    Vertex *v=NULL;
    Vertex *t=NULL;

    v = malloc(1*sizeof(*v));
    v->num = 1;
    adj[0] = v;  //a NODE with value 1
    t = v;

    v = malloc(1*sizeof(*v));
    v->num = 1;
    t->next = v; // and ANOTHER NODE but it should be the SAME NODE with the above one
    t = v;


    //v = malloc(1*sizeof(*v));
    //v->num = 1;
    //t->next = adj[0]; // causes infinite loop...
    //t = v;
}

The expected output is a node with value 1 having itself in its adjacency list, an output like 1 -> 1.

Here my problem is it looks like I have two different nodes. When I made a change on one of them the other doesn’t change, acting like an another node.

For instance after building the list if I change the value of the node I should get an output like 3 -> 3. But I get 3 -> 1. The change on the node doesn’t affect the other one. When I try to point adj[0] to t->next however I get an infinite loop…

Answer 1

It’s not entirely clear to me what you want. If you want a Vertex pointing to itself, it’s simply

void buildList(){
    adj[0] = malloc(1*sizeof(*adj[0])); // allocate memory for one Vertex
    if (adj[0] == NULL){
        perror("Allocation of Vertex failed\n");
        exit(EXIT_FAILURE);
    }
    // adj[0] contains the adress of a Vertex
    // set the num of the Vertex
    adj[0]->num = 1;
    // set the next pointer of the Vertex to its address
    adj[0]->next = adj[0];
}

Can you clarify in what way this is not what you want?