STACKS

1
STACKS

• READ CHAPTER 7
• (P. 315 - 388)

2
COP2532 STACKS

• Introduction
• stacks are a great way to understand the concept
  of providing a discipline over a data structure
• only certain operations can have access to
  features of the stack
• insertion and deletion from stack occurs from
  only one place
• how does it work?
• 1) values are added to or taken from the top of
  the stack
• 2) adding an item is called pushing it onto the
  stack
• 3) removing an item is called popping it off
  stack

3
Introduction to Stacks (cont.)

• as mentioned before, a stack is how the system
  handles function calls in your program
• last function called is first on stack
• compiler keeps track of all variables for each
  call and uses stack to know where to return
• as function ends, function is popped off stack
  and control moves to the next function on the
  stack
• this procedure is called LIFO
• should only use a stack if this type of structure
  is called for
• if items need to be removed from other places
  other than the last entered, another structure
  should be used

4
Conceptual Design of a Stack

• When a stack is created, it starts off empty (no
  items)
• There is a position pointer that tells you where
  the current top of the stack is
• To add a value to the stack, you push it on the
  stack
• Remove values using pop
• Need to ensure that the following two things are
  not done
• 1) Attempting to push an item when the stack is
  full (called stack overflow)
• 2) Attempting to pop an item when the stack is
  empty (called stack underflow)

5
STACK DESIGN (cont)

• so, if we design the Stack as an object
• Stack Object
• Knows
• Its items
• Top position of stack (position pointer)
• Can Do
• Push an item on stack
• Pop an item from stack
• Tell how many items are currently on stack
• Tell if stack is empty
• Tell if stack is full
• Tell you which item is currently sitting on top
  (without removing it)

6
Stack Implementation

• typedef int Item
• typedef Item ItemPtr
• class Stack
• public
• static const int DEFAULTSIZE 30
• Stack()
• Stack(const int StackSize)
• Stack()
• Item Top()
• Item Pop()
• void Push(const Item anItem)
• int Size() const
• bool IsEmpty() const
• bool IsFull() const
• private
• ItemPtr MyStack
• int TopofStack_

7
The Member Functions

• Stack Stack()
• MyStack new ItemDEFAULTSIZE
• TopofStack -1
• Stack Stack(const int StackSize)
• if ((StackSize lt 1) (StackSize gt 30))
• StackSize DEFAULTSIZE
• MyStack new ItemStackSize
• TopofStack -1

8
MORE MEMBER FUNCTIONS

• The Deconstructor - simply deletes the stack
  from the heap
• Stack Stack()
• delete MyStack
• The IsEmpty Function - will return true if the
  stack is empty
• bool Stack IsEmpty() const
• return (TopofStack -1)
• The IsFull Function - will return true if the
  stack is Full
• bool Stack IsFull() const
• return((TopofStack1) gt DEFAULTSIZE)

9
MORE MEMBER FUNCTIONS

• The Size Function - will return the number of
  items currently on stack
• int Stack Size() const
• return(TopofStack 1)
• The Push Function - takes an item as input and
  adds it to the stack
• void Stack Push(const Item anItem)
• if (IsFull())
• cout ltlt "Cannot add - stack is
  full" ltlt endl
• else
• MyStackTopofStack
  anItem

10
MORE MEMBER FUNCTIONS

• The Pop Function - returns the top item off the
  stack and resets TopOfStack
• Item Stack Pop()
• if (IsEmpty()) cout ltlt "Cannot pop -
  stack empty" ltlt endl
• else
• return(MyStackTopofStack--)
• return -999
• The Top Function - gives back the top item on
  stack but does not remove it
• Item Stack Top() const
• if (IsEmpty())
• cout ltlt "No items on stack " ltlt endl
• else
• return(MyStackTopofStack
• return -999

11
AN EXAMPLE OF USING THIS STACK - REVERSING DIGITS

• void main()
• Stack TheStack
• char InputChar '0' //dummy digit to force
  loop to run
• cout ltlt "Enter digits to display in reverse
  "
• while (isdigit(InputChar))
• cin.get(InputChar)
• if (isdigit(InputChar))
• TheStack.Push(InputChar)
• char OutputChar
• while (!TheStack.IsEmpty())
• OutputChar TheStack.Pop()
• cout ltlt OutputChar ltlt " "
• cout ltlt endl

12
QUESTIONS?

• Read section on LINKED STACKS
• (P. 353 - 366)