Stacks and Queues. David Greenstein Monta Vista

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1 Stacks and Queues David Greenstein Monta Vista

2 Stack vs Queue Stacks and queues are used for temporary storage, but in different situations

3 Stacks are Used for handling nested structures: processing directories within directories evaluating expressions within expressions c 2 = a 2 + b 2 2abcos( C )

4 Stacks are Used for handling branching processes: traversing a branching tree structure planning a move in a chess game tracking the sequence of method calls in a Java program (frame)

5 Stack Functions push - adding object to the top of the stack pop - removing object from the top of the stack peek - look at object on top of the stack isempty - true if the stack contains no objects; false otherwise myelements[5] myelements[4] myelements[3] myelements[2] myelements[1] myelements[0] value3 value2 value1 value0 Stack pointer sp

6 Hardware Stack Frame A Java method call creates a stack frame. The frame contains all the necessary information to return (the state ). Pointers SP - Stack Pointer (in all stacks) BP - Base Pointer (in stack frames) Base Stack overflow means large (infinite) number of method calls. Top

7 Properties of a Stack In an efficient implementation, push, pop, and peek methods run in O(1) time. pop and peek are expected to throw a NoSuchElementException if the stack is empty. If we implement the stack using an ArrayList or LinkedList, we do not need to explicitly throw a NoSuchElementException (Ch 20). myelements[5] myelements[4] myelements[3] myelements[2] myelements[1] myelements[0] value3 value2 value1 value0 Stack pointer sp

8 Properties of a Stack A stack of objects holds references to objects. If necessary, a stack can hold multiple references to the same object, but be careful!!! Changing a mutable object on a stack changes them all. It is best to push a copy of the object onto the stack, especially mutable objects, to insure all stack objects retain their original value. myelements[5] myelements[4] myelements[3] myelements[2] myelements[1] myelements[0] value3 value2 value1 value0 Stack pointer sp

9 java.util.stack class Part of the Java Collections Framework (Chapter 20). Stack is a generic class. It works with objects of a specified type (e.g. Stack<String>). Based on the legacy Vector class, similar to ArrayList. Stack methods: push, pop, peek, isempty. Has other methods but do not use them! myelements[5] myelements[4] myelements[3] myelements[2] myelements[1] myelements[0] value3 value2 value1 value0 Stack pointer sp

10 Stack using ArrayList import java.util.arraylist; import java.util.list; public class ArrayStack<E> implements Stack<E> { private List<E> elements; public ArrayStack ( ) { elements = new ArrayList<E>( ); } } public boolean isempty ( ) { return elements.isempty ( ); } public void push (E obj) { elements.add (obj); } public E pop ( ) { return elements.remove (elements.size ( ) - 1); } public E peek ( ) { return elements.get (elements.size ( ) - 1); } Where is the top of an ArrayStack?

11 Queues are Used for Processing events or messages in order of their arrival System tasks Queueing print jobs Entering keystrokes Processing mouse clicks

12 Queue Functions add - adding object to the back of the queue (enqueue) remove - removing object from the front of the queue (dequeue) c peek - look at object at 2 front of the queue isempty - true if the queue contains no objects; false otherwise

13 Properties of a Queue In an efficient implementation, add, remove, and peek methods run in O(1) time. A queue of objects holds references to objects. Like a stack, a queue can hold multiple references to the same object. Like a stack, it is best to add a copy of the object onto the queue, especially mutable objects, to insure all queue objects retain their original value.

14 Ring Buffer An efficient implementation of a queue. Instead of shifting elements to the front, just keep track of front ( read ) and back ( write ) pointers. Actual Implementation Conceptual Model

15 java.util.queue interface boolean isempty () boolean add (E obj) E remove () E peek () A generic interface, part of the Java Collections Framework (Chapter 20) Implemented by other classes like java.util.linkedlist and java.util.priorityqueue

16 SimpleCalc Project Evaluates arithmetic expressions using stacks. Two stacks: operand stack and operator stack * 7 5 * ( ) 7 9 * 4 + Operand Stack Operator Stack Operand Stack Operator Stack

17 Questions?

csci 210: Data Structures Stacks and Queues

csci 210: Data Structures Stacks and Queues 1 Summary Topics Stacks and Queues as abstract data types ( ADT) Implementations arrays linked lists Analysis and comparison Applications: searching with stacks