I am creating a doubly linked list and printing a backwards function. Everything is working great in my code, but when the function is printed in reverse, I need it to say NULL at the end, as it does going forward. When I put puts("NULL") (line 193) it splits up the list and puts NULL after each one. And if I remove that from my code, it works fine, just without NULL printed at the end. I just need NULL to print at the end and I am unsure of how to do this. Thank you!!!
#include <stdio.h>
#include <stdlib.h>
struct listNode {
char data;
struct listNode *nextPtr;
struct listNode *prevPtr;
};
typedef struct listNode ListNode;
typedef ListNode *ListNodePtr;
// prototypes
void insert( ListNodePtr *sPtr, char value );
char delete( ListNodePtr *sPtr, char value );
int isEmpty( ListNodePtr sPtr );
void printList( ListNodePtr currentPtr );
void printBackwards( ListNodePtr startPtr );
void instructions( void );
int main( void )
{
ListNodePtr startPtr = NULL; // initially there are no nodes
int choice; // user's choice
char item; // char entered by user
instructions(); // display the menu
printf( "? " );
scanf( "%d", &choice );
// loop while user does not choose 3
while ( choice !=3 ) {
switch ( choice ) {
case 1:
printf( "Enter a character: " );
scanf( "\n%c", &item );
insert( &startPtr, item ); // insert item in list
printList( startPtr );
printBackwards( startPtr );
break;
case 2:
// if list is not empty
if ( !isEmpty( startPtr ) ) {
printf( "Enter character to be deleted: " );
scanf( "\n%c", &item );
// if character is found, remove it
if ( delete( &startPtr, item ) ) { // remove item
printf( "%c deleted.\n", item );
printList( startPtr );
printBackwards( startPtr );
} // end if
else {
printf( "%c not found.\n\n", item );
} // end else
} // end if
else {
printf( "List is empty.\n\n" );
} // end else
break;
default:
printf( "Invalid choice.\n\n" );
instructions();
break;
} // end switch
printf( "\n\n? " );
scanf( "%d", &choice );
} // end while
return 0; // indicates successful termination
} // end main
// display program instructions to user
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");
} // end function instructions
// Insert a new value into the list in sorted order
void insert( ListNodePtr *sPtr, char value )
{
ListNodePtr newPtr; // pointer to new node
ListNodePtr previousPtr; // pointer to previous node in list
ListNodePtr currentPtr; // pointer to current node in list
newPtr = malloc( sizeof( ListNode ) ); // create node
if ( newPtr != NULL ) { // is space available
newPtr->data = value; // place value in node
newPtr->nextPtr = NULL; // node does not link to another node
previousPtr = NULL;
currentPtr = *sPtr;
// loop to find the correct location in the list
while ( currentPtr != NULL && value > currentPtr->data ) {
previousPtr = currentPtr; // walk to ...
currentPtr = currentPtr->nextPtr; // ... next node
} // end while
// insert new node at beginning of list
if ( previousPtr == NULL ) {
newPtr->nextPtr = *sPtr;
*sPtr = newPtr;
} // end if
else { // insert new node between previousPtr and currentPtr
previousPtr->nextPtr = newPtr;
newPtr->nextPtr = currentPtr;
} // end else
} // end if
else {
printf( "%c not inserted. No memory available.\n", value );
} // end else
} // end function insert
// Delete a list element
char delete( ListNodePtr *sPtr, char value )
{
ListNodePtr previousPtr; // pointer to previous node in list
ListNodePtr currentPtr; // pointer to current node in list
ListNodePtr tempPtr; // temporary node pointer
// delete first node
if ( value == ( *sPtr )->data ) {
tempPtr = *sPtr; // hold onto node being removed
*sPtr = ( *sPtr )->nextPtr; // de-thread the node
free( tempPtr ); // free the de-threaded node
return value;
} // end if
else {
previousPtr = *sPtr;
currentPtr = ( *sPtr )->nextPtr;
// loop to find the correct location in the list
while ( currentPtr != NULL && currentPtr->data != value ) {
previousPtr = currentPtr; // walk to ...
currentPtr = currentPtr->nextPtr; // ... next node
} // end while
// delete node at currentPtr
if ( currentPtr != NULL ) {
tempPtr = currentPtr;
previousPtr->nextPtr = currentPtr->nextPtr;
free( tempPtr );
return value;
} // end if
} // end else
return '\0';
} // end function delete
// Return 1 if the list is empty, 0 otherwise
int isEmpty( ListNodePtr sPtr )
{
return sPtr == NULL;
} // end function isEmpty
// Print the list
void printList( ListNodePtr currentPtr )
{
// if list is empty
if ( currentPtr == NULL ) {
printf( "List is empty.\n\n" );
} // end if
else {
printf( "\nThe list is:\n" );
// while not the end of the list
while ( currentPtr != NULL ) {
printf( "%c --> ", currentPtr->data );
currentPtr = currentPtr->nextPtr;
} // end while
printf( "NULL\n\n" );
} // end else
} // end function printList
void printBackwards(ListNodePtr currentPtr)
{
// if list is empty
if(currentPtr==NULL) {
puts("List is empty.\n");
return;
} // end if
else if ( currentPtr->nextPtr != NULL ) {
printBackwards(currentPtr->nextPtr);
} // end else if
else
puts("\nThe list in reverse is:");
printf("%c --> ", currentPtr->data);
puts("NULL");
} // end function printBackwards
