#  Stack.py  Copyright (c) Kari Laitinen

#  http://www.naturalprogramming.com

#  2006-05-22  File created.
#  2022-12-27  Converted to Python 3.


class Stack :

   def __init__( self ) :

      self.stack_memory_space  =  []


   def push( self, item_to_the_stack ) :

      #  We do not need to increase the memory space because
      #  a dynamic list is used to store the values that are
      #  pushed to the stack. The memory space grows automatically
      #  when necessary.

      self.stack_memory_space.append( item_to_the_stack )
 
   def pop( self ) :

      if  len( self.stack_memory_space )  ==  0 :

         print( "\n ERROR: Attempt to pop on empty stack !" )
         item_from_the_stack  =  -1

      else :

         # The standard list method pop() is used to pop the last element
         item_from_the_stack  =  self.stack_memory_space.pop() 

      return  item_from_the_stack



   def stack_is_not_empty( self ) :

      return  len( self.stack_memory_space )  >  0




#  The main program begins.

test_stack  =  Stack()

print( "\n Let's test a stack. Please, type in integers."       \
       "\n I will push those numbers onto a stack, and later"   \
       "\n take them away from the stack. I will stop reading"  \
       "\n numbers when you type in a zero."                    \
       "\n Please, press <Enter> after each number. \n" )


value_to_the_stack  =  -1  # a value other than zero

while  value_to_the_stack  !=  0  :

   print( " Give a number: ", end="" )
   value_to_the_stack  =  int( input() )

   test_stack.push( value_to_the_stack )


print( "\n The numbers popped from the stack are:\n" )

while  test_stack.stack_is_not_empty()  :

   value_popped_from_the_stack  =  test_stack.pop()

   print(  "  %d"  %  value_popped_from_the_stack, end="" )


print( "" )