programtoimplementacircularqueueasalinke

Program to implement a circular queue as a linked list

/* Program to implement a circular queue as a linked list. */

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

/* structure containing a data part and link part */
struct node
{
    int data ;
    struct node * link ;
} ;

void addcirq ( struct node **, struct node **, int ) ;
int delcirq ( struct node **, struct node ** ) ;
void cirq_display ( struct node * ) ;

void main( )
{
    struct node *front, *rear ;

    front = rear = NULL ;

    addcirq ( &front, &rear, 10 ) ;
    addcirq ( &front, &rear, 17 ) ;
    addcirq ( &front, &rear, 18 ) ;
    addcirq ( &front, &rear, 5 ) ;
    addcirq ( &front, &rear, 30 ) ;
    addcirq ( &front, &rear, 15 ) ;

    clrscr( ) ;

    printf ( "Before deletion:\n" ) ;
    cirq_display ( front ) ;

    delcirq ( &front, &rear ) ;
    delcirq ( &front, &rear ) ;
    delcirq ( &front, &rear ) ;

    printf ( "\n\nAfter deletion:\n" ) ;
    cirq_display ( front ) ;
}

/* adds a new element at the end of queue */
void addcirq ( struct node **f, struct node **r, int item )
{
    struct node *q ;

    /* create new node */
    q = malloc ( sizeof ( struct node ) ) ;
    q -> data = item ;

    /* if the queue is empty */
    if ( *f == NULL )
        *f = q ;
    else
        ( *r ) -> link = q ;

    *r = q ;
    ( *r ) -> link = *f ;
}

/* removes an element from front of queue */
int delcirq ( struct node **f, struct node **r )
{
    struct node *q ;
    int item ;

    /* if queue is empty */
    if ( *f == NULL )
        printf ( "queue is empty" ) ;
    else
    {
        if ( *f == *r )
        {
            item = ( *f ) -> data ;
            free ( *f ) ;
            *f = NULL ;
            *r = NULL ;
        }
        else
        {
            /* delete the node */
            q = *f ;
            item = q -> data ;
            *f = ( *f ) -> link ;
            ( *r ) -> link = *f ;
            free ( q ) ;
        }
        return ( item ) ;
    }
    return NULL ;
}

/* displays whole of the queue */
void cirq_display ( struct node *f )
{
    struct node *q = f, *p = NULL ;

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

        q = q -> link ;
        p = f ;
    }
}