Thread: 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....

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" );
   } 
}