Getting info from an array of pointers

[cathym]

Hi.
I have an array of pointers (called g) that point to the dummy header of a linked list.
The linked list struct is as follows:

typedef struct node *node_ptr;
struct node
{
    int to_vertex;
    node_ptr next;
};

I have a separate int array, called e, which holds a number.

What I am trying to do is to use a 'for loop' to travel along g, and at each element, look at the linked list it points to and decrement the number in the e array for each to_vertex in that linked list.
eg, e[0] holds the number 2.
if g[0] has a struct in its linked list with a to_vertex of 2, I want e[0] to become 1.
I keep getting a segmentation fault. Can you please tell me where I am going wrong.

Assuming i is the 'for loop' integer, and current has been declared as:

node_ptr current;

this is the area the seg fault occurs:

current = g[i];
while(current)
{
e[current->to_vertex]--;
current=current->next;
}

It is the line e[current->to_vertex]--; that causes the problem.

Thanks in anticipation of help
Cathy

[Salem]

You need to show how you contructed your linked lists.

Try using a debugger, and checking the value of current and current->to_vertex at the point of the crash.

[cathym]

Thanks for the quick reply
I used some printf statements to see what was in current and current->to_vertex, it seems that it points to the memory address rather than the actual value?

Firstly, I constructed the linked list like this:

for (i=0; i<n_vertices; i++)
{
scanf("%d", &e[i]);		
g[i] = create();		
for(j=0; j<e[i]; j++)	
{
    scanf("%d", &to); 
    insert_at_front(to, g[i]);	
}

where 'to' is a scanned int, and create() is:

node_ptr create(void)
{
	node_ptr list = (node_ptr) malloc(sizeof(struct node));
	list->next = NULL;
	return list;
}

and insert_at_front (with n equal to 'to' and list equal to g[i]) is:

void insert_at_front(int n, node_ptr list)
{
	node_ptr new_node = (node_ptr) malloc(sizeof(struct node));
	new_node->to_vertex = n;
	new_node->next = list->next;
	list->next = new_node;
}

When I run it, I get the following output:

The value of current is: 135536
The value of current->to_vertex is: 137088
Segmentation fault( core dumped)

I really am stuck on this

[Salem]

> g[i] = create();
Your "dummy" node at the start of the list has no value stored in the to_vertex member.

So you get whatever junk was in that memory when you created the first node.

> node_ptr list = (node_ptr) malloc(sizeof(struct node));
There's no need to cast malloc in C, see the FAQ

[cathym]

Thanks again
I changed the create function to initialise a new node with a to_vertex of 0 and that has fixed the problem!.
Thanks so much for pointing me in the right direction.
Sometimes it is one simple small thing that causes a big problem...