Queue of linked list C programming

Question

I have a problem with my program. I have created a queue of linked lists, and when I clear my queue with my delQueue function, my queue disappears, and I can't push anything in anymore.

How can I fix this? My push function works fine unless I delete everything from my queue.

Here is my code:

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

int count = 0;

struct Node
{
    int Data;
    struct Node* next;
}*rear, *front;

void delQueue()
{

    struct Node *var=rear;
    while(var!=NULL)
    {
        struct Node* buf=var->next;
        free(var);
        count = count + 1;

    }

}

void push(int value)
{
    struct Node *temp;
    temp=(struct Node *)malloc(sizeof(struct Node));
    temp->Data=value;
    if (front == NULL)
    {
        front=temp;
        front->next=NULL;
        rear=front;
    }
    else
    {
        front->next=temp;
        front=temp;
        front->next=NULL;
    }
}

void display()
{
    struct Node *var=rear;
    if(var!=NULL)
    {
        printf("\nElements in queue are:  ");
        while(var!=NULL)
        {
            printf("\t%d",var->Data);
            var=var->next;
        }
    printf("\n");
    } 
    else
    printf("\nQueue is Empty\n");
}

when you push you are updating front. Should not rear be updated when you are pushing? Queue is first in first out...so when you push, rear pointer should be updated. — Bill
– Bill, Commented May 4, 2013 at 2:16
Thank you so much everyone you have been very helpful. I really appreciate all the help that I have recieved here and I have surely learned a lot about queues. than you again. God bless u all — user2133160
– user2133160, Commented May 4, 2013 at 2:43
you should accept the answer which you think best answered your question ;) — Bill
– Bill, Commented May 4, 2013 at 2:51

perreal · Accepted Answer · 2013-05-04 02:31:11Z

0

void delQueue()
{
    while(rear != NULL) {
        struct Node* var=rear->next;
        free(rear);
        count = count + 1;
        rear = var;         /* update rear */
    }
    front = NULL; /* clear the front */
}

edited May 4, 2013 at 2:31

answered May 4, 2013 at 2:09

perreal

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3 Comments

user2133160 Over a year ago

My queue now exists but I can't add anything it won't push anything in it but the queue is there and empty.

perreal Over a year ago

See the update, maybe it is because you are not resetting the front.

Bill Over a year ago

Since it is a queue, I prefer deleting from front, as given in my answer. Otherwise you loose the meaning of a queue, FIFO

andy · Accepted Answer · 2013-05-04 02:07:14Z

0

You are looking at "var" after you freed it (when you go around the loop again). Did you mean to assign "var = buf" in the loop in delQueue()?

Also, don't forget to check the return from malloc() for NULL in your push() routine. Even if this is just a small learning program, you should learn to always check...

answered May 4, 2013 at 2:07

andy

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Comments

Bill · Accepted Answer · 2013-05-04 02:39:11Z

0

 int delQueue()
    {
   int count = 0;
    while ( front != NULL )
    {
    struct Node * temp = front;
    front = front -> next;
    free (temp);
    count++;
    }
    rear= NULL;

    return count;
    }

Since it is a queue, I would prefer to delete elements from the front, rather than rear.

edited May 4, 2013 at 2:39

answered May 4, 2013 at 2:19

Bill

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1 Comment

user2133160 Over a year ago

it works perfect but I can't seem to keed the count of how many items I delete. Where will I put that??

Siddique · Accepted Answer · 2013-05-04 05:23:09Z

0

you have to add the following line at the end of delQueue()

rear = front = NULL;

answered May 4, 2013 at 5:23

Siddique

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