function linked lists help

**revelation437** · 05-08-2003

Code:

#include <iostream>
#include <stdlib.h>
#include <conio.h>


#define NEXT 50
#define PREV 49

using namespace std;

typedef struct node
{
    int a;
    node *next;
    node *prev;
}NODE;


//Function prototypes
void display(node *head);
void addnode(node *current, node *head);

int main()
{
    NODE *current = NULL;
    NODE *head = NULL;


    for (int i = 0; i < 1; i++)
		addnode(current,head);

    display (head);
    system("PAUSE");
    return 0;
}

void addnode(node *current, node *head)
{
	node *newnode = new node;
		cout << "input a value: ";
		cin >> newnode->a;
			if (head == NULL)
				{
					head = newnode;
					current = newnode;
					current->next = NULL;
					current->prev = NULL;
				}// if statement
			else
				{		
					newnode->prev = current;
					current->next = newnode;
					current = newnode;
					current->next = NULL;
				}// else statement
}

void display(node *head)
{
   int ch;
   node *show = head;
   cout << show->a << endl;
   while(1)
   {
        ch =getch();

		if (ch == PREV)
		{
			if (show->prev != NULL)
			{
				show = show->prev;
				cout << show->a << endl;
			}
		}
		if (ch == NEXT)
		{
			if (show->next != NULL)
			{
				show = show->next;
				cout << show->a << endl;
			}
		}
   }
}

ok, i know for a fact that the display function works correctly. and i know the coding for the actual adding of the linked lists also works, but it only works when i keep it in main. i just tried to add it to another function (the adding part) and it keeps erroring out as soon as i try to display. like i said, if i keep the code for adding the linked list in main, everything works perfectly. so i know its a problem with the code to add. i heard you're suppose to use pointers to pointers, but im not sure how or why you use them.
please advise

thanks

**revelation437** · 05-08-2003

figured it out

!

**Sebastiani** · 05-08-2003

Right, to change what head and current actually point to you need to pass their address:

Code:

void addnode(node **current, node **head)
{
	node *newnode = new node;
		cout << "input a value: ";
		cin >> newnode->a;
			if (*head == NULL)
				{
					*head = newnode;
					*current = newnode;
					(*current)->next = NULL;
					(*current)->prev = NULL;
				}// if statement
			else
				{		
					newnode->prev = *current;
					(*current)->next = newnode;
					*current = newnode;
					(*current)->next = NULL;
				}// else statement
}

in main the syntax would be:

Code:

int main()
{
    NODE *current = NULL;
    NODE *head = NULL;


    for (int i = 0; i < 1; i++)
		addnode(&current, &head);

    display (head);
    system("PAUSE");
    return 0;
}

**revelation437** · 05-08-2003

exactly how i did it

i may just be a programmer after all!
hah

**Sebastiani** · 05-08-2003

Food for thought...

Code:

typedef struct node
{
    int data;
    node *next;
    node *prev;
}NODE;




node * newnode(int data)
{
 node * nd = new node;
   if(nd != NULL)
  {
   nd->prev = 0;
   nd->next = 0;
   nd->data = data;
  }
 return nd;
}



node * headnode(node * index)
{
 while(index && index->prev) index = index->prev;
 return index;
}




node * tailnode(node * index)
{
 while(index && index->next) index = index->next;
 return index;
}




bool insertnode(node * insert, node * prev, node * next)
{
 if(insert == NULL) return false;

 // repoint old neghbors

 if(insert->next != NULL) insert->next->prev = insert->prev;
 if(insert->prev != NULL) insert->prev->next = insert->next;

 insert->next = next;
 insert->prev = prev;

 // repoint new neghbors

 if(insert->next != NULL) insert->next->prev = insert;
 if(insert->prev != NULL) insert->prev->next = insert;

 return true;
}




bool pushback(node ** head, int data)
{
 node * nd = newnode(data);

 if(*head == NULL) *head = nd;

 else insertnode(nd, tailnode(*head), 0);

 return (nd != NULL);
}





bool pushfront(node ** head, int data)
{
 node * nd = newnode(data);

 insertnode(nd, 0, *head);

 *head = nd;  // nd is new head

 return (nd != NULL);
}






void displaylist(node * head)
{
  while(head)
 {
  cout << head->data << endl;
  head = head->next;
 }
}




int main()
{
 NODE * head = 0;

 for(int i = 0; i < 10; i++)
  pushback(&head, i);

 for(int i = 0; i < 10; i++)
  pushfront(&head, i);

 displaylist(head);

 cin.get();

 return 0;
}

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