Stacks

2 Outline and Reading The Stack ADT (§2.1.1) Array-based implementation (§2.1.1) Growable array-based stack (§1.5) Java.util.Stack class Java.util.Vector Applications for Stack

Stacks3 Abstract Data Types (ADTs) An abstract data type (ADT) is an abstraction of a data structure An ADT specifies: Data stored Operations on the data Error conditions associated with operations Example: ADT modeling a simple stock trading system The data stored are buy/sell orders The operations supported are  order buy(stock, shares, price)  order sell(stock, shares, price)  void cancel(order) Error conditions:  Buy/sell a nonexistent stock  Cancel a nonexistent order

Stacks4 The Stack ADT The Stack ADT stores arbitrary objects Stack follows Last In First Out (LIFO) discipline. That is, insertions and deletions follow the last-in first-out scheme Think of a spring-loaded plate dispenser Main stack operations: push(object): inserts an element object pop(): removes and returns the last inserted element Auxiliary stack operations: object top(): returns the last inserted element without removing it integer size(): returns the number of elements stored boolean isEmpty(): indicates whether no elements are stored

Stacks5 Handling Exceptions/Error Conditions Attempting the execution of an operation of ADT may sometimes cause an error condition, called an exception Exceptions are said to be “thrown” by an operation that cannot be executed In the Stack ADT, operations pop and top cannot be performed if the stack is empty Attempting the execution of pop or top on an empty stack throws an EmptyStackException We will look into Java exception handling in the next week.

Stacks6 Applications of Stacks Page-visited history in a Web browser Undo sequence in a text editor Every process in computer system has a user stack to maintain its context. The subroutine/method call structure is maintained in a LIFO basis. Auxiliary data structure for algorithms

Stacks7 Array-based Stack A simple way of implementing the Stack ADT uses an array We add elements from left to right A variable keeps track of the index of the top element S 012 t … Algorithm size() return t + 1 Algorithm pop() if isEmpty() then throw EmptyStackException else t  t  1 return S[t + 1]

Stacks8 Array-based Stack (cont.) The array storing the stack elements may become full A push operation will then throw a FullStackException Limitation of the array- based implementation Not intrinsic to the Stack ADT S 012 t … Algorithm push(o) if t = S.length  1 then throw FullStackException else t  t + 1 S[t]  o

Stacks9 Performance and Limitations Performance Let n be the number of elements in the stack The space used is O(n) Each operation runs in time O(1) Limitations The maximum size of the stack must be defined a priori and cannot be changed Trying to push a new element into a full stack causes an implementation-specific exception

Stacks10 Growable Array-based Stack In a push operation, when the array is full, instead of throwing an exception, we can replace the array with a larger one How large should the new array be? incremental strategy: increase the size by a constant c; O(n) for push operations doubling strategy: double the size; O(1) for push operation Algorithm push(o) if t = S.length  1 then A  new array of size … for i  0 to t do A[i]  S[i] S  A t  t + 1 S[t]  o

Stacks11 Stack Interface in Java Java interface corresponding to our Stack ADT Requires the definition of class EmptyStackException Different from the built-in Java class java.util.Stack We will look at exceptions later. public interface StackInterface { public int size(); public boolean empty(); public Object top() throws EmptyStackException; public void push(Object o); public Object pop() throws EmptyStackException; }

Stacks12 Array-based Stack in Java public class ArrayStack implements StackInterface { private Object[] _items; private int top = -1; private int capacity = 20; public ArrayStack () { _items = new Object[capacity]; }

Stacks13 Push and other methods public int size() { return top +1;} public boolean isEmpty() { return (top < 0);} public void push(Object obj) { if (top == _items.length -1) { // ASSERT: stack capacity full Object[] tmp = new Object[2*length]; for (int i= 0; i< _items.length; i++) tmp [i] = _items[i]; _items = tmp; top++; capacity = capacity*2; _items[top] = obj; }

Stacks14 Pop and top method public Object top() { if (empty() ) { System.out.println(“empty”); return null; } else return _items[top]); } public Object pop() { if (empty() ) { System.out.println(“empty”); return null; } else { Object elem = _items[top]); top = top –1; } return elem; }

Stacks15 Stack Class Hierarchy java.util.Vector Java.util.Stack Vector Stack AbstractList List, Cloneable, Serializable extends implements extends

Stacks16 Stack Class Diagram Stack //constructor Stack() //methods Object push(Object item) Object pop() Object peek() int search(Object o) //predicate method boolean empty()

Stacks17 java.util.Vector Vector Vector() // constructor boolean addElement(Object obj); // add to the end Object elementAt(int index); //return element at index, accessor int size(); boolean isEmpty() Object remove (int index); // remove element at index: mutator Object lastElement(); // accessor String toString(); Object[] toArray(); //other methods

Stacks18 Java.util.Stack Lets see how easy it is to reuse java.util.Vector to implement a Stack