Thread: Plz help with placing node in a data structure

This funtion halts the program. I'm trying to organize a list of nodes full of city data. They have to be in ascending order of Zip code. Any help with this would be great. I've racked my brain and realize it's a very simple problem. Thanks.

Scott

Code:

struct SListHeader{
	struct SListNode *psHead;
   struct SListNode *psTail;
   int iNodeCount;
   };

struct SListNode{
	char City[41];
   char State[3];
   char Zip[6];
   double Longitude;
   double Latitude;
   long Population;
   struct SListNode *Next;
   struct SListNode *Prev;
   };

void vAddListNode (struct SListHeader *psList, struct SListNode *psNode)
	{
   struct SListNode *psNext;
   psNext = mallloc (sizeof(struct SListNode));
   psList->iNodeCount+1;
   if (psNode->Zip > psList->psHead->Next->Zip)
   	{
      psNode->Next = psNext;
      psNode->Prev = psNext-Prev;
      psNode->Next->Prev = psNext;
      psNode->Prev->Next = psNext;
   	}
   }

> psList->iNodeCount+1;

What is the meaning of this?

> psNode->Prev = psNext-Prev;

Don't do this with pointers. Or did you mean

psNode->Prev = psNext->Prev;

In that case, psNext->Prev does not point to anything. You should first initialize the psNext node.

Only if

psNode->Zip > psList->psHead->Next->Zip

you add the node, but you don't do that if

psNode->Zip =< psList->psHead->Next->Zip.

Your algorithm looks not complete to me. What you could do, in a kind of pseudo:

Code:

/* Add a note to a list */

new_node = initialise_new_node()

/* Check if node must be put after the tail */
if (new_node->element > list_tail->element)
    add_element_after_tail (list_tail, new_node);
    return;

/* Check if node must be put before the head */
if (new_node->element < list_head->element)
    add_element_before_head (list_head, new_node);
    return;

/* Else the new node must be inserted in the list, search for the
first element which is larger than the element of new node. The 
new node must be inserted before that element. */

list_ptr = find_first_larger_element_position (list, new_node);
insert_new_node (list_ptr, new_node);

Those were two typos. I don't have the program with me...just a printout.

iNodeCount is just an intiger counter. It keeps track of how many nodes have been read in the loop.

Thanks for your help so far. I can't wait to try it out.

Scott

>It keeps track of how many nodes have been read in the loop.

Then this

> psList->iNodeCount+1;

Doesn't do much, there is no assignment in it. It should be:

psList->iNodeCount += 1;

Or without an assignment but with the ++ operator:

psList->iNodeCount++;

As far as I can tell the iNodeCount works. Actually, I set it to zero in another funtion. Borland gets to the

psNode->Next->Prev = psNext;

line before it stops. I'm just trying to figure out the pointer logic mostly I think.

Scott