Inserting into hash table - c

Question

I am trying to insert an integer into a hash table. To do this, I'm creating an array of node*'s and I'm trying to make assignments like listarray[i]->data=5 possible. However, I'm still very confused with pointers and I'm crashing at the line with the comment '//crashes here' and I don't understand why. Was my initialization in main() invalid?
#include <stdio.h>
#include <stdlib.h>
typedef struct node
{
int data;
struct node * next;
} node;
//------------------------------------------------------------------------------
void insert (node **listarray, int size)
{
node *temp;
int value = 11; //just some random value for now, eventually will be scanned in
int index = value % size; // 11 modulo 8 yields 3
printf ("index is %d\n", index); //prints 3 fine
if (listarray[index] == NULL)
{
printf("listarray[%d] is NULL",index); //prints because of loop in main
listarray[index]->data = value; //crashes here
printf("listarray[%d] is now %d",index,listarray[index]->data); //never prints
listarray[index]->next = NULL;
}
else
{
temp->next = listarray[index];
listarray[index] = temp;
listarray[index]->data = value;
}
}//end insert()
//------------------------------------------------------------------------------
int main()
{
int size = 8,i; //set default to 8
node * head=NULL; //head of the list
node **listarray = malloc (sizeof (node*) * size); //declare an array of Node *
//do i need double pointers here?
for (i = 0; i < size; i++) //malloc each array position
{
listarray[i] = malloc (sizeof (node) * size);
listarray[i] = NULL; //satisfies the first condition in insert();
}
insert(*&listarray,size);
}
output:
index is 3
listarray[3] is NULL
(crash)
desired output:
index is 3
listarray[3] is NULL
listarray[3] is now 11

Answer 1

There are various issues here:
If you have a hash table of a certain size, then the hash code must map to a value between 0 and size - 1. Your default size is 8, but your hash code is x % 13, which means that your index might be out of bounds.
Your insert function should also pass the item to insert (unless that's the parameter called size, in which case it is severely misnamed).
if (listarray[index] == NULL) {
listarray[index]->data = value; //crashes here
listarray[index]->next = NULL;
}
It's no wonder that it crashes: When the node is NULL, you cannot dereference it with either * or ->. You should allocate new memory here.
And you shouldn't allocate memory here:
for (i = 0; i < size; i++) //malloc each array position
{
listarray[i] = malloc (sizeof (node) * size);
listarray[i] = NULL; //satisfies the first condition in insert();
}
Allocating memory and then resetting it to NULL is nonsense. NULL is a special value that means that no memory is at the pointed-to location. Just set all nodes to NULL, which means that the hash table starts out without any nodes. Allocate when you need a node at a certain position.
In the else clause, you write:
else
{
temp->next = listarray[index];
listarray[index] = temp;
listarray[index]->data = value;
}
but temp hasn't been allocated, but you dereference it. That's just as bad as dereferencing ´NULL`.
Your hash table also needs a means to handle collisions. It looks as if at every index in the hash table, there is a linked list. That's a good way to deal with it, but you haven't implemented it properly.
You seem to have problems to understand pointers. Perhaps you should start with a simpler data structure like a linked list, just to practice? When you have gotten a firm grasp of that, you can use what you've learned to implement your hash table.