Hash Tables- Segmentation Fault - c

Question

Not sure why I'm getting a segmentation fault here. I'm trying to create a hash table that would have collisions and the data should be string. So I'm using a char array for the data.
Need to detect collisions at the end.
#define SIZE 20
struct DataItem
{
char data[50];
int key;
};
struct DataItem* hashArray[SIZE];
struct DataItem* dummyItem;
struct DataItem* item;
int hashCode(int key)
{
return key % SIZE;
}
void insert(int key, char data[50])
{
struct DataItem *item = (struct DataItem*) malloc(sizeof(struct DataItem));
strcpy(item->data, data);
item->key = key;
//get the hash
int hashIndex = hashCode(key);
//move in array until an empty or deleted cell
while (hashArray[hashIndex] != NULL && hashArray[hashIndex]->key != -1)
{
//go to next cell
++hashIndex;
//wrap around the table
hashIndex %= SIZE;
}
hashArray[hashIndex] = item;
}
struct DataItem* delete(struct DataItem* item)
{
int key = item->key;
//get the hash
int hashIndex = hashCode(key);
//move in array until an empty
while (hashArray[hashIndex] != NULL)
{
if (hashArray[hashIndex]->key == key)
{
struct DataItem* temp = hashArray[hashIndex];
//assign a dummy item at deleted position
hashArray[hashIndex] = dummyItem;
return temp;
}
//go to next cell
++hashIndex;
//wrap around the table
hashIndex %= SIZE;
}
return NULL;
}
int detect_collisions()
{
int i = 0;
int collision = 0;
for (i = 0; i < SIZE; i++)
{
if (hashArray[i] != NULL)
{
if (hashArray[i]->key == hashArray[i + 1]->key)
;
collision++;
}
}
return collision;
}
void display()
{
int i = 0;
for (i = 0; i < SIZE; i++)
{
if (hashArray[i] != NULL)
{
printf("Phli dafa: %d\n", i);
printf(" (%d,%s)", hashArray[i]->key, hashArray[i]->data);
}
else
printf(" ~~ ");
}
printf("\n");
}
int main()
{
dummyItem = (struct DataItem*) malloc(sizeof(struct DataItem));
strcpy(dummyItem->data, NULL);
dummyItem->key = -1;
insert(1, "check");
display();
delete(item);
}

Answer 1

You have a problem here:
strcpy(dummyItem->data,NULL);
as strcpy will dereference a NULL pointer.
To put in an empty string use:
strcpy(dummyItem->data, "");
Your second problem is:
delete(item);
item is NULL at this point. So here:
struct DataItem* delete(struct DataItem* item)
{
int key = item->key;
^^^^^^
you dereference a NULL pointer.
Maybe your delete function should rather be:
struct DataItem* delete(int key)
{
The third problem is here (there are actually 3 problems here):
for (i = 0; i < SIZE; i++)
{
if (hashArray[i] != NULL)
{
if (hashArray[i]->key == hashArray[i + 1]->key)
^^^^^^^^^^^^^^^^
a) May be NULL
b) Out of range when i == size-1
;
^^^
c) Delete this
collision++;
}
}
My guess is that you want:
for (i = 0; i < SIZE-1; i++) // Notice
{
if (hashArray[i] != NULL && hashArray[i + 1] != NULL) // Notice
{
if (hashArray[i]->key == hashArray[i + 1]->key)
{
collision++;
}
}
}

Answer 2

The problem is
strcpy(dummyItem->data, NULL);
It will try to copy a string pointed by NULL (that is not pointing a string..) to your array. So it invokes Undefined Behavior
What you want (I guess) is
memset(dummyItem->data, 0x00, sizeof(dummyItem->data));
or
dummyItem->data[0] = 0x00;
or
dummyItem->data[0] = '\0';
Take also note that you must always check malloc return value: it can fail returning NULL
dummyItem = malloc(sizeof(struct DataItem));
if ( dummyItem != NULL)
{
// YOUR STUFF
}
You have also a ; just after
if (hashArray[i]->key == hashArray[i + 1]->key)
remove it.
Last thing I can see
delete(item);
is UB because of item is not initialized because the code in insert function uses a local variable, maybe you want change
struct DataItem *item = (struct DataItem*) malloc(sizeof(struct DataItem));
to
item = malloc(sizeof(struct DataItem));
EDIT 1
You are also accessing the array out of bounds with
if (hashArray[i]->key == hashArray[i + 1]->key)
hashArray[i + 1] is out of bounds when i = SIZE-1
Moreover hashArray[i + 1] can be NULL or not initialized.

Answer 3

If you compile your code with debugging turned on ( ie -g option with gcc ) and then run it in a debugger it will tell you what lines the seg faults are happen on. There are two places that are causing a seg fault that I can see - the strcpy being passed a NULL and also the call to delete(item) where item is still NULL. Presumably because you think you're using the one defined in insert rather than the globally defined one.