How do i copy a struct from an array of structs?

Question

I realize it gets kind of long with all the printf, but I didn't really know how to present my issue better.

I am having problems initializing a struct in c properly. I have tried debugging with printf to show my issue. Essentially I'm creating a list of structs, using the following function:

Vertex *Vertex_new(int n){
    Vertex *vert = malloc(sizeof(Vertex));
    if(!vert){return NULL;}

    vert->id = n;
    vert->outNeighbours = malloc(sizeof(LinkedList));
    vert->outNeighbours = LinkedList_new();
    vert->inNeighbours = malloc(sizeof(LinkedList));
    vert->inNeighbours = LinkedList_new();

    return vert;
}

struct Vertex {
    int id; // a number in [0; numVertices[
    LinkedList *outNeighbours; // A linked list of vertices.
    LinkedList *inNeighbours; // A linked list of vertices
};

And that is used for the creation of the following struct, with function right after. The function after, called Graph_new(), I have an issue with saving the Vertex, properly inside the list. I don't know which part I'm doing wrong, but I'm assuming it's something to do with how I'm initializing the list, and as such made a bunch of print statements to try and understand the problem:

struct Graph {
    int numVertices;
    int numEdges;
    Vertex *vertices; // An array of numVertices vertices
};



Graph *Graph_new(int n){
    Graph *grf = malloc(sizeof(Graph));
    if(!grf){return NULL;}

    grf->numEdges = 0;
    grf->numVertices = n;

    Vertex list[n];
    grf->vertices = malloc(sizeof(Vertex*)*n);

    for(int i = 0; i < n;i++){
        list[i] = *Vertex_new(i);
    }
    grf->vertices = list;

    // My attempt at debugging, `grf` is simply returned after these print statements:
    printf("Inside graph_new for list:\n");
    printf("%d ",list[0].id);
    printf("%d ",list[1].id);
    printf("%d ",list[2].id);
    printf("%d ",list[3].id);
    printf("%d \n",list[4].id);
    // Det her virker herinde, men ikke uden for
    // er der et problem i hvordan jeg gemmer listen?
    printf("Inside graph_new for grf->vertices:\n");
    printf("%d ",grf->vertices[0].id);
    printf("%d ",grf->vertices[1].id);
    printf("%d ",grf->vertices[2].id);
    printf("%d ",grf->vertices[3].id);
    printf("%d \n",grf->vertices[4].id);

    printf("%d ",grf->vertices[0].id);
    printf("%d ",grf->vertices[1].id);
    printf("%d ",grf->vertices[2].id);
    printf("%d ",grf->vertices[3].id);
    printf("%d \n",grf->vertices[4].id);



    return grf;
}

In another function, I call graph_new(n). n is here read from a file, n in this case is 5, which I have tested, to make sure it reads it correctly, so it's a guaranteed 5. I tested two way of printing, in order to understand what is happening. The first:

Graph *newG = Graph_new(n);


// outside of graph_new:
printf("outside of graph_new, newG->vertices:\n");
printf("%d ",newG->vertices[0].id);
printf("%d ",newG->vertices[1].id);
printf("%d ",newG->vertices[2].id);
printf("%d ",newG->vertices[3].id);
printf("%d \n",newG->vertices[4].id);
printf("%d ",newG->vertices[0].id);
printf("%d ",newG->vertices[1].id);
printf("%d ",newG->vertices[2].id);
printf("%d ",newG->vertices[3].id);
printf("%d \n",newG->vertices[4].id);

Gives the output:

Inside graph_new for list:
0 1 2 3 4 
Inside graph_new for grf->vertices:
0 1 2 3 4 
0 1 2 3 4 
outside of graph_new, newG->vertices:
0 -285208794 -603392624 -603392624 0 
0 -285208794 -603392624 -603392624 0

So at first I assumed it was because the list was not saved properly, or the vertices was not saved properly inside the list.

But if I then print this outside of Graph_new:

Vertex vert1 = newG->vertices[0];
Vertex vert2 = newG->vertices[1];
Vertex vert3 = newG->vertices[2];
Vertex vert4 = newG->vertices[3];
Vertex vert5 = newG->vertices[4];
printf("works outside:\n");
printf("%d ", vert1.id);
printf("%d ", vert2.id);
printf("%d ", vert3.id);
printf("%d ", vert4.id);
printf("%d \n", vert5.id);  

// Saving the vertices back in the array works
newG->vertices[0] = vert1;
newG->vertices[1] = vert2;
newG->vertices[2] = vert3;
newG->vertices[3] = vert4;
newG->vertices[4] = vert5;

Vertex vert11 = newG->vertices[0];
Vertex vert22 = newG->vertices[1];
Vertex vert33 = newG->vertices[2];
Vertex vert44 = newG->vertices[3];
Vertex vert55 = newG->vertices[4];  
printf("Works again\n");
printf("%d ", vert11.id);
printf("%d ", vert22.id);
printf("%d ", vert33.id);
printf("%d ", vert44.id);
printf("%d \n", vert55.id);
// ------------------------------

printf("Doesn't work: \n");
Vertex vvert1 = newG->vertices[0];
Vertex vvert2 = newG->vertices[1];
Vertex vvert3 = newG->vertices[2];
Vertex vvert4 = newG->vertices[3];
Vertex vvert5 = newG->vertices[4];
printf("%d ", vvert1.id);
printf("%d ", vvert2.id);
printf("%d ", vvert3.id);
printf("%d ", vvert4.id);
printf("%d \n", vvert5.id);

I get the following output:

Inside graph_new for list:
0 1 2 3 4 
Inside graph_new for grf->vertices:
0 1 2 3 4 
0 1 2 3 4 
works outside:
0 1 2 3 4 
Works again
0 1 2 3 4 
Doesn't work: 
0 -2035597530 519880720 -2031896736 -1398417392

If anyone is has the time to understand it and give a helping hand, I'd REALLY appreciate it. I might not make it in time for the exam, but at least I'd know what I did wrong.

Answer 1

Graph *Graph_new(int n){
    …

    Vertex list[n];
    grf->vertices = malloc(sizeof(Vertex*)*n);

    for(int i = 0; i < n;i++){
        list[i] = *Vertex_new(i);
    }
    grf->vertices = list;

    …

    return grf;
}

list array is local to Graph_new function and will be destroyed once control exits the function, thus you cannot take reference of list out of Graph_new it is undefined behavior.

Also *Vertex_new(i); has memory leaks, as you are loosing reference to the allocation.