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Posted on 1998-10-04
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Last Modified: 2010-05-18
I have a conversion problem for you:
#define MAX 100
int queue[max+1];head,tail
void put(int v)
{
   queue[tail++];
   if (tail >max) tail=0;
}

int get (void)
{
  int t= queue[head++];
  if (head>max) head=0;
  return t;
}

void queueinit (void)
{
  head=0; tail=0;
}

Please convert this to a dynamically allocated Queue, John
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Question by:JTM100498
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Accepted Solution

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scrapdog earned 150 total points
Comment Utility
#include <alloc.h>

struct node
  {
       int value;
       node *next;
  };

node *tail;
node *head;

void put(int v)
{
  node *newnode;
  tail->value = v;
  newnode = (node *) malloc(sizeof(node));
  tail->next = newnode;
  tail = newnode;
  tail->next = NULL;
}

int get (void)
{
  node *newhead;
  int t= head->value;
  newhead = head->next;
  free(head);
  head = newhead;
  return(t);
}

void queueinit (void)
{
  head=(node *) malloc(sizeof(node));
  head->next = NULL;
  tail = head;
}

------------------------------

I left out error checking for the sake of simplicity.

When the queue is initialized, the head pointer and the tail pointer point to the same node.  If the queue is used correctly (i.e. never take something out that is not there), it should not cause any errors.


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Expert Comment

by:scrapdog
Comment Utility
However, if you do want to catch when an error occurs, you could add a statement to the get function:

int get (void)
{
  node *newhead;
  int t= head->value;
  newhead = head->next;
  free(head);
  head = newhead;

  if (newhead==NULL)
       {
             //head has surpassed the tail
             //act accordingly
       }

  return(t);
}

The "if (newnode==NULL)" statement checks if the node following the head pointer is defined.  If it is not, then the head pointer has passed the tail pointer, and you can put any error handling code in here if you want.

One thing I might not have made clear when I posted the answer is that in my example, there is no variable named "queue".  There is simply a head and tail pointer which are transparent to the user.  All you have to do after calling queueinit is call the get and put functions.  

You can also add this function to return the number of nodes currently in the queue:

int getnumnodes(void)
{
  int n=0;
  node *current;
  current=head;
  if (current != NULL)
       {
             n=1;
             while (current->next != NULL) n++;
       }
  return(n);
}




scrapdog

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