Question

I’ve been trying to make a tree with directories with pointers in C.

I thought it would be easier to use some kind of struct with pointers, which would resemble a class in Java.

so I would using something like this:

  struct d
  {
      struct d *up /*point to parent*/
      struct d *down /*point to child*/
      struct d *right /*point to right*/
  }

  generate_tree(struct **d)
  {
   /*my code*/
  }

I’m trying to make multiple struct d’s and connect them. So what I’m assuming that I have to do is create and allocate a pointer for the directory then initialize a struct d and have the created pointer to point inside the struct d. My question is how would I create the d and set the *up, *down, and *right?

Thanks for helping

Answer 1

You dynamically allocate a struct d with malloc(). Ie,

struct d *dp = malloc(sizeof(*dp));

Once you have a pointer to an newly-created struct d, you assign to it’s fields just like any other C structure:

dp->up = /* ... */;
dp->down = /* ... */;
dp->right = /* ... */;

The trick, of course, is what you assign to those fields. Well, from their types, struct d *, it has to be a pointer to a struct d. So, either you assign the NULL pointer constant, which indicates there’s nothing there, or you assign the address of some other structd d objects. For example,

struct d *up = malloc(sizeof(*up));
struct d *down = malloc(sizeof(*down));
struct d *right = malloc(sizeof(*right));

d->up = up;
d->down = down;
d->right;

I don’tknow if this struct is intended to be more than a toy example, but it looks like a node in some sort of tree structure. If that’s the case, you’d want to write various helper functions that hide most of this pointer manipulation.

struct d *new_node() { /* allocate a new d,  set all fields to NULL */ }
void insert_node_in_tree(struct d *tree, struct d *node) { ... }

and so on for the relevant operations on whatever it is you’re building.