1. Stacks
Data Structures and Problem Solving with C++: Walls and Mirrors, Carrano and Henry, © 2013

2. Introduction to the Stack ADT
Stack: a LIFO (last in, first out) data structure
Examples:
plates in a cafeteria serving area
return addresses for function calls

3. Specifications for the ADT Stack
A stack of cafeteria plates
Data Structures and Problem Solving with C++: Walls and Mirrors, Carrano and Henry, © 2013

4. Abstract Data Type: Stack
A finite number of objects
Not necessarily distinct
Having the same data type
Ordered by when they were added
Stack uses LIFO principle
Last In First Out
We have identified the following operations:
See whether stack is empty.
Add a new item to stack.
Remove from the stack item added most recently.
Get item that was added to stack most recently.
Data Structures and Problem Solving with C++: Walls and Mirrors, Carrano and Henry, © 2013

5. Stack Abstract Data Type
Functions:
push: add a value at the top of the stack
pop: remove a value from the top of the stack
peek: show the top of the stack, but don’t remove it.
isEmpty: true if the stack currently contains no elements

6. Stack Basics
Stack is usually implemented as a list, with additions and removals taking place at one end of the list
The active end of the list implementing the stack is the top of the stack
Stack types:
Static – fixed size, often implemented using an array
Dynamic – size varies as needed, often implemented using a linked list

7. An Array Based Implementation
FIGURE 7-1 Using an array to store a stack’s entries: (a) a preliminary sketch; (b) implementation details
Data Structures and Problem Solving with C++: Walls and Mirrors, Carrano and Henry, © 2013

8. Static Stack Implementation
Uses an array of a fixed size
Bottom of stack is at index 0. A variable called top tracks the current top of the stack
const int STACK_SIZE = 3;
char s[STACK_SIZE];
int top = 0;
top is where the next item will be added

9. Array Implementation Example
This stack has max capacity 3, initially top = 0 and stack is empty. K E G K E E
push('E');
push('K');
push('G');
top is 1
top is 2
top is 3

10. Stack Operations Example
After three pops, top is 0 and the stack is empty E K E
pop();
(remove G)
pop();
(remove K)
pop();
(remove E)

11. Stack Operations Example
Push(3) top
Push(4)
Peek()
Pop()

12. Class Implementation { private: char *s; int capacity, top; public:
class STACK {
private:
char *s;
int capacity, top;
public:
void push(char x);
char pop();
char peek();
bool isEmpty();
STACK(int stackSize);
STACK();
~STACK() };
int StackIsEmpty = 1009;
int StackFullException = 1010;
See IntStack.h, IntStack.cpp, pr18-01.cpp

13. Array Implementation
char s[STACK_SIZE]; int top=0; To check if stack is empty: bool isEmpty() { if (top == 0) return true; else return false; }

14. Array Implementation
To add an item to the stack void push(char x) { if (top == STACK_SIZE()) {throw(StackFullException);} // throw an exception s[top] = x; top++; }

15. Array Implementation
To remove an item from the stack char pop() { if (isEmpty()) {throw(StackIsEmptyException);}// throw an exception top--; return s[top]; }

16. Array Implementation
To look at the top item in the stack char peek() { if (isEmpty()) {throw(StackIsEmptyException);}// throw an exception return s[top]; }

17. Simple Uses of a Stack Reverse a list of characters Postfix calculator
Deck of cards!!
Program structure
…. Lots more
Data Structures and Problem Solving with C++: Walls and Mirrors, Carrano and Henry, © 2013

18. Using Stacks with Algebraic Expressions
Evaluating postfix expressions
The effect of a postfix calculator on a stack when evaluating the expression 2 * (3 + 4)
Data Structures and Problem Solving with C++: Walls and Mirrors, Carrano and Henry, © 2013

19. End
Chapter 6
Data Structures and Problem Solving with C++: Walls and Mirrors, Carrano and Henry, © 2013