Linked list cstring insertion sort - c++

Question

This is my code for an insertion-sort, sorting via the cstring member of the node. The current code only inserts before the head. The code encapsulated in comments is the sorted insertion I am trying to make work. I'm thinking I have to use a predecessor and successor pointer, but maybe it's the comparison that is confusing me. Any help would be appreciated. Thanks!
#include <iostream>
#include "groclist.h"
void insert_item(Grocery_Item_Ptr &head, int quantity, const char name[])
{
bool exists = false;
bool done = false;
Grocery_Item_Ptr temp = NULL;
Grocery_Item_Ptr current = NULL;
Grocery_Item_Ptr pred = NULL;
Grocery_Item_Ptr succ = NULL;
if (head == NULL) {
head = new Grocery_Item;
head->quantity = quantity;
strncpy(head->name, name, MAX_ITEM_NAME_LEN);
head->next = NULL;
return;
}
else {
current = head;
while (current != NULL) {
if (strncmp(current->name, name, MAX_ITEM_NAME_LEN) == 0) {
current->quantity += quantity;
exists = true;
}
current = current->next;
}
if (exists) {
current = NULL;
return;
}
else {
current = head;
}
if (!exists) {
temp = new Grocery_Item;
temp->quantity = quantity;
strncpy(temp->name, name, MAX_ITEM_NAME_LEN);
/*
while (!done || current != NULL) {
if (strncmp(current->name, name, MAX_ITEM_NAME_LEN) < 0) {
pred = current;
succ = current->next;
current->next = temp;
temp->next = succ;
done = true;
}
if (!done) {
current = current->next;
}
}
*/
temp->next = head;
head = temp;
temp = NULL;
}
}
return;
}

Answer 1

One thing you are missing is keeping a reference to the predecessor while searching. This is necessary in order to keep the chain intact.
Here is a draft that should work (currently untested!):
while (!done || current != NULL)
{
//If we found the place to insert
if (strncmp(current->name, name, MAX_ITEM_NAME_LEN) < 0)
{
//If the place to insert was at head
if(pred == NULL)
{
//The new node becomes the head
head = temp;
}
else
{
//Set the previous nodes next to point at this node.
pred->next = temp;
}
//Always set the node to be inserted's next
//pointing to the node we should be inserted before
temp->next = current;
done = true;
}
if (!done)
{
//No match, keep looking but keep an updated pred pointer
pred = current;
current = current->next;
}
}

Answer 2

It's just pseudocode, but maybe it helps:
if(head == NULl)
{
//make newnode as head
}
if(head.name == new name)
{
//update quantity
}
if(head.name <new name)
{
//insert newnode before head
//make newnode as head
}
if (new name > head.name)
{
current = head;
succ = current.next;
while (succ && new name <succ.name)
{
curent = succ;
succ = succ.next
}
if(succ = NULL)
current->next = newnode
else
if new name = succ->name
update quantity
else
curent->next = newnode
newnode->next = succ;
}

Answer 3

I appreciate your input guys, if only for having me think about it in different ways. However my result is quite different, and I must truly thank my whiteboard.
if (strncmp(name, head->name, MAX_ITEM_NAME_LEN) < 0) {
temp->next = head;
head = temp;
temp = NULL;
}
else {
pred = head;
current = head->next;
do {
if (strncmp(name, current->name, MAX_ITEM_NAME_LEN) < 0) {
pred->next = temp;
temp->next = current;
done = true;
}
else if (current->next == NULL) {
current->next = temp;
done = true;
}
else {
pred = current;
current = current->next;
}
if (done) {
pred = NULL;
current = NULL;
}
} while (!done && current != NULL);
}