singly linked list to doubly linked list

I was just wondering how to change this singly linked list code into a doubly linked list. I know i have to use:

struct Node{
char d;
Node *next;
Node *prev;
}

i just want to be able to move forward and backwards in the list.

i am confused.... :confused:

and by book doesn't cover it, it just basically says doubly linked list are useful to move forward and backwards

Code:

---

#include <stdio.h>
#include <stdlib.h>


struct listNode {           
  char data;
  struct listNode *nextPtr;
};

typedef struct listNode ListNode;
typedef ListNode *ListNodePtr;


void insert( ListNodePtr *sPtr, char value );
char delete( ListNodePtr *sPtr, char value );
int isEmpty( ListNodePtr sPtr );
void printList( ListNodePtr currentPtr );
void instructions( void );

int main()
{
  ListNodePtr startPtr = NULL;
  int choice;
  char item; 

  instructions();
  printf( "? " );
  scanf( "%d", &choice );

 
  while ( choice != 3 ) {

      switch ( choice ) {

        case 1:
            printf( "Enter a character: " );
            scanf( "\n%c", &item );
            insert( &startPtr, item );
            printList( startPtr );
            break;

        case 2:

           
            if ( !isEmpty( startPtr ) ) {
              printf( "Enter character to be deleted: " );
              scanf( "\n%c", &item );

             
              if ( delete( &startPtr, item ) ) {
                  printf( "%c deleted.\n", item );
                  printList( startPtr );
              }
              else {
                  printf( "%c not found.\n\n", item );
              }

            }
            else {
              printf( "List is empty.\n\n" );
            }

            break;

        default:
            printf( "Invalid choice.\n\n" );
            instructions();
            break;
     
      }

      printf( "? " );
      scanf( "%d", &choice );
  }

  printf( "End of run.\n" );
 
  return 0;

}


void instructions( void )
{
  printf( "Enter your choice:\n"
      "  1 to insert an element into the list.\n"
      "  2 to delete an element from the list.\n"
      "  3 to end.\n" );
}


void insert( ListNodePtr *sPtr, char value )
{
  ListNodePtr newPtr;     
  ListNodePtr previousPtr;
  ListNodePtr currentPtr; 

  newPtr = malloc( sizeof( ListNode ) );

  if ( newPtr != NULL ) {
      newPtr->data = value;
      newPtr->nextPtr = NULL;

      previousPtr = NULL;
      currentPtr = *sPtr;

     
      while ( currentPtr != NULL && value > currentPtr->data ) {
        previousPtr = currentPtr;         
        currentPtr = currentPtr->nextPtr; 
      }
     
      if ( previousPtr == NULL ) {
        newPtr->nextPtr = *sPtr;
        *sPtr = newPtr;
      }
      else {
        previousPtr->nextPtr = newPtr;
        newPtr->nextPtr = currentPtr;
      }

  }
  else {
      printf( "%c not inserted. No memory available.\n", value );
  }

}


char delete( ListNodePtr *sPtr, char value )
{
  ListNodePtr previousPtr;
  ListNodePtr currentPtr; 
  ListNodePtr tempPtr;   

 
  if ( value == ( *sPtr )->data ) {
      tempPtr = *sPtr;
      *sPtr = ( *sPtr )->nextPtr;
      free( tempPtr );
      return value;
  }
  else {
      previousPtr = *sPtr;
      currentPtr = ( *sPtr )->nextPtr;

     
      while ( currentPtr != NULL && currentPtr->data != value ) {
        previousPtr = currentPtr;       
        currentPtr = currentPtr->nextPtr;
      }

     
      if ( currentPtr != NULL ) {
        tempPtr = currentPtr;
        previousPtr->nextPtr = currentPtr->nextPtr;
        free( tempPtr );
        return value;
      }
   
  }

  return '\0';

}


int isEmpty( ListNodePtr sPtr )
{
  return sPtr == NULL;

}


void printList( ListNodePtr currentPtr )
{

 
  if ( currentPtr == NULL ) {
      printf( "List is empty.\n\n" );
  }
  else {
      printf( "The list is:\n" );

   
      while ( currentPtr != NULL ) {
        printf( "%c --> ", currentPtr->data );
        currentPtr = currentPtr->nextPtr; 
      }

      printf( "NULL\n\n" );
  }
}

---

I wrote some simple functions to manipulate doubly linked list a while ago and I'll share it with you. Note that I've done that a while ago so it could be some possible bugs. If you find it please let me know.
Anyway I hope these can help you to learn more

Go Here and scroll down to the Link List link .

xeddiex

Do you know how to walk through a single linked list? If so, what's the problem?

Code:

---

struct snode *s;
struct dnode *d;

for ( s = slist; s; s = s->next )
{
    d->data = s->data;
    d->next = s->next;
    d->prev = dlist;
    dlist = d;
}

---

Quzah.