1. COMPUTER 2430 Object Oriented Programming and Data Structures I

2. public class Stack {
private Object[] items;
private int top;
public void push ( Object obj )
. . .
public Object pop()
public boolean isEmpty()
public boolean isFull() }
Method peek?

3. User Method peek Given a Stack with the methods isEmpty isFull push
pop
How can a user have a look at the element before calling pop?
Can NOT access private data!
User peek method

4. // User method outside Stack class
public Object peek ( Stack s )
Stack myStack = new Stack( 1000 );
...
Object x = peek( myStack );
if (x == null)
. . .
else if (x instanceof FixedPoint)
else if (x instanceof Golfer)
else

5. public Object peek ( Stack s ){
if ( s.isEmpty() )
return null;
// Don’t change the stack
Object obj = s.pop();
s.push( obj );
// peek the stack
return obj; }

6. Passing Object Reference
public Object peek ( Stack s )
// Pass by value or pass by reference?
Java has only passing by value
No passing reference
Each class variable is a reference
The value of s is the address of a Stack object

7. Passing Object Reference
Stack myStack; myStack = new Stack( 1000 ); ... Object x = peek( myStack ); // public Object peek ( Stack s ) // passing by value
myStack
Activation record
return point s

8. Passing Object Reference
Object x = peek( myStack );
Can the function change the object referenced by myStack?
Yes! myStack stores the reference to the stack object.
All objects are passed by reference in Java!
Can the function modify myStack to reference a different stack?
No! Passing by value!
myStack
Activation record
return point s

9. User cannot access private data.
Peek by a User
public Object peek ( Stack s ) {
if ( s.isEmpty() )
return null;
Object x = s.pop();
s.push( x );
return x; }
It works.
Has to pop and push!
User cannot access private data.

10. User versus Implementer
Implementer can provide a peek method
More efficient
Implementer has access to all private data
Upgrade

11. public class Stack {
private Object[] items;
private int top = 0;
...
public Object peek()
if ( top == 0 )
return null;
return items[top - 1];
// Do not modify the stack (top).
// return items[top--];
// top is modified!
// top = top – 1; }

12. What can you do with a Stack?
As User
Call the public methods!
As Implementer
Get count of items
Search
Insert item at any position
Remove item from any position …

13. Binary Arithmetic Operations
Conversion programs
Infix  Prefix, Postfix
Prefix  Infix, Postfix
Postfix  Infix, Prefix
Evaluation programs
Using operator stack
Using operand stack
Using both
Easiest program
Evaluate postfix expressions
Operand stack

14. Evaluating RPN Using Stack
4 / * *
-17
Process tokens in order
If the token is an operand
Push it
If the token is an operator
Pop two operands
Perform the operation
Push the result
Stop if invalid at any time

15. Evaluating RPN Stack: LIFO Just what we need here! 6 4 7 5 10 7 5 -3 5
/ * 9 * +
/ * 9 * +
/ * 9 * +
/ * 9 * +
/ * 9 * +
8 3 / * 9 * +
/ * 9 * +
* 9 * +
9 * +
* + +
-49 6 4 7 5 10 7 5 -3 5 3 8 -3 5 2 -3 5 -6 5 9 -6 5
-54 5
-49
4 + 6
7 - 10
8 / 3
-3 * 2
-6 * 9
5 + (-54)
Stack: LIFO
Just what we need here!

16. Initialize an operand stack, s Set valid  true While valid and more tokens to process read token if token is operand s.push(token) else if token is operator if s empty then valid  false else op2  s.pop() if s empty then valid  false else op1  s.pop() if divide by 0 then valid  false else s.push( op1 <operator> op2 ) else valid  false If s empty then valid  false Else Answer  s.pop() if s not empty then valid  false

17. Invalid Expression
5 +
Second pop fails – check for empty before calling pop +
First pop fails – check for empty before before calling pop /
Divide by 0
3 4
Stack not empty at the end – check for empty at the end
3 4 B
Bad token
Empty expression
Popping answer fails – check for empty at the end

18. Invalid Expression
What about an expression consisting of a single operand? 243 It is valid! An expression can be put on the right hand side of an assignment. X = 243; // valid! X = ; // Invalid!

19. Prog 3
Prog 2

20. Monday Class
Lab 206