Question

Im toying with creating my own hash table data structure, but I have hit an unexpected problem, which I can’t solve and haven’t found a satisfying solution for it yet.

See, I have this linked list struct

struct Link
{
   int v;
   struct Link* next;
}

and then inside the hash table structure I want keep track of an array linked lists, like this:

struct Link** entries;

What I run into is, that for this to work, I first have to initialize the array like this:

entries = malloc(sizeof(struct Link*) * N);

for (i = 0; i < N; i++)
   entries[i] = malloc(sizeof(struct Link));

What I want is not having to do the for loop that initializes the structs, because that’s not how linked lists work, I want to leave entries[x] blank until it actually gets assigned a value.

If I don’t do the for loop this happens:

if (entries[x] != NULL)  /* true, the array is initialized */
  entries[x]->v = value; /* SEGFAULT, there is no struct initialized */

That if statement ‘should’ return false if I haven’t assigned a Link struct to it yet, but it doesn’t.

One way of solving this problem is to just initialize all the first links of the linked list with that for loop and then checking for the value, but that’s not what I want.

So does anyone know a way to solve this the way I want it to?

Answer 1

You can use calloc rather than malloc while allocating entries like:

entries = calloc(sizeof(struct Link*),  N);