Page 142, Exercise 2

The queue starts at the low end of the memory and grows upward. Only a rear pointer is needed because when an item is deleted the queue is shifted downward. The queue is full when the rear and top pointers meet.

The queue operations are:

Adding to the queue 
void queue_full()
{/* code to check for a full queue is in the 
   addq() function */
   fprintf(stderr, "The queue is Full\n");
}

void addq(element item)
{/* add an item to the global queue
     rear points to the current end of the queue */
  if (rear >= top-1)
     queue_full();
  else
     memory[++rear] = item;
}
Deleting from the queue 
void queue_empty()
{/* check for empty queue is in the deleteq() function */
 fprintf(stderr, "The queue is empty\n");
}

element deleteq()
{/* remove element at the front of the queue */
     int i;
	 element item = memory[0];
     if (!rear)
	     queue_empty();
     else { /* shift downward */
	    for (i = 1; i <= rear; i++)
		    memory[i-1] = memory[i];
		rear--;
	    return item;
	}
}

The stack starts at the high end of memory and grows downward. It is also full when the rear and top pointers meet. The operations are:

Adding to the stack 
void add(element item)
{/* add an item to the global stack
     top (also global) is the current top of the stack,
     MAX_SIZE is the maximum size */

   if (top <= rear+1)
     StackFull();
   else
      memory[--top] = item;
}
Deleting from the Stack 
void StackEmpty()
{
  printf("The stack is empty.  No item deleted \n");
}

element delete()
{/* remove top element from the stack and put it in item */
  if (top == MAX_SIZE)
    StackEmpty();
  else
    return memory[top++];
}