Page 142, Exercise 1

The first stack starts at the low end of the memory and grows upward. The second stack starts at the high end of memory and grows downward. The memory is full when the stack pointers collide. The declarations are:

Stack declarations 
#define MAX_SIZE 6
typedef struct 	{
	  int key; } element;

element memory[MAX_SIZE];     /* global queue declaration */
int top [2];
   top[0] = -1;
  top[1] = MAX_SIZE;


CALLS: printf("1. Insert stack 0, 2.Delete stack 0, 3. Insert Stack 1, 4. Delete Stack 1, 0. Quit: ");
  scanf("%d",&choice);
  while (choice > 0) {
    switch  (choice) {
      case 1: printf("Insert in stack 0: ");
	      scanf("%d",&item.key);
	      add(0, item);
	      break;
      case 2: item = delete(0);
	      if (top[0] >= 0 )
		     printf ("%d was deleted from the stack 0.\n\n", item.key);
	      break;
      case 3: printf("Enter the number to insert: ");
	      scanf("%d",&item.key);
	      add(1, item);
	      break;
      case 4: item = delete(1);
	      if (top[1] < MAX_SIZE)
		       printf("%d was deleted from the stack 1 \n\n",item.key);
	      break;
     }
Adding to a stack 
void StackFull()
{
   printf("The stack is full.  No item added \n");
}

void add(int topNo, 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[0]+1 >= top[1])
     StackFull();
   else {
      if (!topNo) 
	     memory[++top[0]] = item;
	  else
          memory[--top[1]] = item;
   }
}
Deleting from a stack 
void StackEmpty()
{
  printf("The stack is empty.  No item deleted \n");
}
element delete(int topNo)
{/* remove top element from the stack and put it in item */
  if (!topNo) {
      if (top[0] < 0)
          StackEmpty();
	  return memory[top[0]--];
 }
  else { /*second stack */
    if (top[1] == MAX_SIZE)
	   StackEmpty();
    return memory[top[1]++];
 }
}