C linked list can't add Element at beginning

Question

I'm fairly new to C and we got an excersie where we need to programm a linked list with predetermined functions and parameters. Now I got a problem to add an new elemnt to the beginning of the list, everything else works, here the syntax.

int main(){

    Vector3D foo;
    foo.x = 521;
    foo.y = 2;
    foo.z = 3;

    VectorList * head;
    head = create_VL(NULL, &foo);

    insertElementBack(head, &foo);
    foo.x = 456;
    insertElementBack(head, &foo);
    foo.x = 2;
    insertElementFront(head, &foo);
    print_list(head);
    printf("%d\n", size(head));

}

void insertElementFront(VectorList* l, Vector3D* v){

    VectorList *previous, *new_VL;

    previous = &l;

    new_VL = NULL;
    new_VL = malloc(sizeof(VectorList));

    new_VL -> value = *v;

    new_VL -> next = previous;

    l = new_VL;
}

VectorList *create_VL(VectorList* l, Vector3D* v) {

    VectorList* new_VL = (VectorList*)malloc(sizeof(VectorList));

    if(new_VL == NULL)
    {
        printf("Error creating a new node.\n");
        exit(0);
    }
    new_VL->value = *v;
    new_VL->next = l;

    return new_VL;
}

void insertElementBack(VectorList* l, Vector3D* v){

    VectorList *vl = l;

    while( vl -> next != NULL){
        vl = vl -> next;
    }

    VectorList *new_List = create_VL(NULL, v);
    vl -> next  = new_List;

}

The name and the parameters aren't allowed to be changed, I could solve this problem with a double pointer as parameter but it's not allowed. Could someone please give me a hint, I tried a lot of things but nothing worked.

Best

Martin

Answer 1

You really should have included the definition of VectorList instead of making us guess at its structure, but your problem is probably this:

previous = &l;

l is a pointer to a VectorList . By doing &l you're taking the address of the pointer, not the VectorList . So when you do this

new_VL -> next = previous;

your next pointer now points to that pointer, not the VectorList . In fact, aren't you getting a warning on that? Don't ignore warnings.

Your second and bigger problem is head doesn't get updated to reflect the new head, and without being able to use double pointers your only way of solving that is to return the new head node as a return value from insertElementFront .

Answer 2

In void insertElementFront(VectorList* l, Vector3D* v){ l is a local variable just holding the address of head. Changing the address locally does not affect head variable.

You can make head a global variable and approach the following way

void insertElementFront(VectorList* l, Vector3D* v){

    VectorList *new_VL;

    new_VL = NULL;
    new_VL = malloc(sizeof(VectorList));

    new_VL -> value = *v;

    new_VL -> next = l;

    head = new_VL
}

or You can return the head pointer

VectorList* insertElementFront(VectorList* l, Vector3D* v){

    VectorList *new_VL;

    new_VL = NULL;
    new_VL = malloc(sizeof(VectorList));

    new_VL -> value = *v;

    new_VL -> next = l;

    return new_VL
}

and

head = insertElementBack(head, &foo);

For more info look https://www.geeksforgeeks.org/how-to-write-functions-that-modify-the-head-pointer-of-a-linked-list/