Program to maintain a doubly linked list

/* Program to maintain a doubly linked list */

#include <stdio.h>
#include <conio.h>
#include <alloc.h>

/* structure representing a node of the doubly linked list */
struct dnode
{
struct dnode *prev ;
int data ;
struct dnode * next ;
} ;

void d_append ( struct dnode **, int ) ;
void d_addatbeg ( struct dnode **, int ) ;
void d_addafter ( struct dnode *, int , int ) ;
void d_display ( struct dnode * ) ;
int d_count ( struct dnode * ) ;
void d_delete ( struct dnode **, int ) ;

void main( )
{
struct dnode *p ;

p = NULL ; /* empty doubly linked list */

d_append ( &p , 11 ) ;
d_append ( &p , 2 ) ;
d_append ( &p , 14 ) ;
d_append ( &p , 17 ) ;
d_append ( &p , 99 ) ;

clrscr( ) ;
d_display ( p ) ;
printf ( "\nNo. of elements in the DLL = %d\n", d_count ( p ) ) ;

d_addatbeg ( &p, 33 ) ;
d_addatbeg ( &p, 55 ) ;

d_display ( p ) ;
printf ( "\nNo. of elements in the DLL = %d\n", d_count ( p ) ) ;

d_addafter ( p, 4, 66 ) ;
d_addafter ( p, 2, 96 ) ;

d_display ( p ) ;
printf ( "\nNo. of elements in the DLL = %d\n", d_count ( p ) ) ;

d_delete ( &p, 55 ) ;
d_delete ( &p, 2 ) ;
d_delete ( &p, 99 ) ;

d_display ( p ) ;
printf ( "\nNo. of elements in the DLL = %d\n", d_count ( p ) ) ;
}

/* adds a new node at the end of the doubly linked list */
void d_append ( struct dnode **s, int num )
{
struct dnode *r, *q = *s ;

/* if the linked list is empty */
if ( *s == NULL )
{
/*create a new node */
*s = malloc ( sizeof ( struct dnode ) ) ;
( *s ) -> prev = NULL ;
( *s ) -> data = num ;
( *s ) -> next = NULL ;
}
else
{
/* traverse the linked list till the last node is reached */
while ( q -> next != NULL )
q = q -> next ;

/* add a new node at the end */
r = malloc ( sizeof ( struct dnode ) ) ;
r -> data = num ;
r -> next = NULL ;
r -> prev = q ;
q -> next = r ;
}
}

/* adds a new node at the begining of the linked list */
void d_addatbeg ( struct dnode **s, int num )
{
struct dnode *q ;

/* create a new node */
q = malloc ( sizeof ( struct dnode ) ) ;

/* assign data and pointer to the new node */
q -> prev = NULL ;
q -> data = num ;
q -> next = *s ;

/* make new node the head node */
( *s ) -> prev = q ;
*s = q ;
}

/* adds a new node after the specified number of nodes */
void d_addafter ( struct dnode *q, int loc, int num )
{
struct dnode *temp ;
int i ;

/* skip to desired portion */
for ( i = 0 ; i < loc ; i++ )
{
q = q -> next ;
/* if end of linked list is encountered */
if ( q == NULL )
{
printf ( "\nThere are less than %d elements", loc );
return ;
}
}

/* insert new node */
q = q -> prev ;
temp = malloc ( sizeof ( struct dnode ) ) ;
temp -> data = num ;
temp -> prev = q ;
temp -> next = q -> next ;
temp -> next -> prev = temp ;
q -> next = temp ;
}

/* displays the contents of the linked list */
void d_display ( struct dnode *q )
{
printf ( "\n" ) ;

/* traverse the entire linked list */
while ( q != NULL )
{
printf ( "%2d\t", q -> data ) ;
q = q -> next ;
}
}

/* counts the number of nodes present in the linked list */
int d_count ( struct dnode * q )
{
int c = 0 ;

/* traverse the entire linked list */
while ( q != NULL )
{
q = q -> next ;
c++ ;
}

return c ;
}

/* deletes the specified node from the doubly linked list */
void d_delete ( struct dnode **s, int num )
{
struct dnode *q = *s ;

/* traverse the entire linked list */
while ( q != NULL )
{
/* if node to be deleted is found */
if ( q -> data == num )
{
/* if node to be deleted is the first node */
if ( q == *s )
{
*s = ( *s ) -> next ;
( *s ) -> prev = NULL ;
}
else
{
/* if node to be deleted is the last node */
if ( q -> next == NULL )
q -> prev -> next = NULL ;
else
/* if node to be deleted is any intermediate node */
{
q -> prev -> next = q -> next ;
q -> next -> prev = q -> prev ;
}
free ( q ) ;
}
return ; /* return back after deletion */
}
q = q -> next ; /* go to next node */
}
printf ( "\n%d not found.", num ) ;
}