Stacks Chapter 5. Chapter 5: Stacks2 Chapter Objectives To learn about the stack data type and how to use its four methods: push, pop, peek, and empty.

Stacks

Chapter 5

Chapter 5: Stacks 2

Chapter Objectives

• To learn about the stack data type and how to use its four methods: push, pop, peek, and empty

• To understand how Java implements a stack• To learn how to implement a stack using an underlying

array or a linked list• To see how to use a stack to perform various

applications, including finding palindromes, testing for balanced (properly nested) parentheses, and evaluating arithmetic expressions

Chapter 5: Stacks 3

Stack Abstract Data Type

• A stack can be compared to a Pez dispenser• Only the top item can be accessed• Can only extract one item at a time

• A stack is a data structure with the property that only the top element of the stack is accessible

• The stack’s storage policy is Last-In, First-Out

Chapter 5: Stacks 4

Specification of the Stack Abstract Data Type

• Only the top element of a stack is visible, therefore the number of operations performed by a stack are few

• Need the ability to• Inspect the top element• Retrieve the top element• Push a new element on the stack• Test for an empty stack

Chapter 5: Stacks 5

Specification of the Stack Abstract Data Type (continued)

Chapter 5: Stacks 6

Stack Applications

• Two client programs using stacks• Palindrome finder• Parentheses matcher

• Palindrome: string that reads the same in either direction• Example: “Able was I ere I saw Elba”

Chapter 5: Stacks 7

Stack Applications (continued)

Chapter 5: Stacks 8

Stack Applications (continued)

• When analyzing arithmetic expressions, it is important to determine whether an expression is balanced with respect to parentheses• (a+b*(c/(d-e)))+(d/e)

• Problem is further complicated if braces or brackets are used in conjunction with parenthesis

• Solution is to use stacks!

Chapter 5: Stacks 9

Stack Applications (continued)

Chapter 5: Stacks 10

Stack Applications (continued)

Chapter 5: Stacks 11

Implementing a Stack as an Extension of Vector

• The Java API includes a Stack class as part of the package java.util

• The vector class implements a growable array of objects• Elements of a vector can be accessed using an integer

index and the size can grow or shrink as needed to accommodate the adding and removing of elements

Chapter 5: Stacks 12

Implementing a Stack as an Extension to Vector (continued)

Chapter 5: Stacks 13

Implementing a Stack with a List Component

• Can use either the ArrayList, Vector, or the LinkedList classes as all implement the List interface

• Name of class illustrated in text is ListStack<E>• ListStack is an adapter class as it adapts the methods

available in another class to the interface its clients expect by giving different names to essentially the same operations

Chapter 5: Stacks 14

Implementing a Stack Using an Array

• Need to allocate storage for an array with an initial default capacity when creating a new stack object

• Need to keep track of the top of the stack• No size method

Chapter 5: Stacks 15

Implementing a Stack Using an Array (continued)

Chapter 5: Stacks 16

Implementing a Stack as a Linked Data Structure

• We can implement a stack using a linked list of nodes

Chapter 5: Stacks 17

Comparison of Stack Implementations

• Extending a Vector (as is done by Java) is a poor choice for stack implementation as all Vector methods are accessible

• Easiest implementation would be to use an ArrayList component for storing data

• All insertions and deletions are constant time regardless of the type of implementation discussed• All insertions and deletions occur at one end

Chapter 5: Stacks 18

Additional Stack Applications

• Consider two case studies that relate to evaluating arithmetic expressions• Postfix and infix notation

• Expressions normally written in infix form• Binary operators inserted between their operands

• A computer normally scans an expression string in the order that it is input; easier to evaluate an expression in postfix form

Chapter 5: Stacks 19

Additional Stack Applications (continued)

Chapter 5: Stacks 20

Additional Stack Applications (continued)

• Advantage of postfix form is that there is no need to group subexpressions in parentheses

• No need to consider operator precedence

Chapter 5: Stacks 21

Evaluating Postfix Expressions

Chapter 5: Stacks 22

Evaluating Postfix Expressions (continued)

Chapter 5: Stacks 23

Evaluating Postfix Expressions (continued)

Chapter 5: Stacks 24

Evaluating Postfix Expressions (continued)

Chapter 5: Stacks 25

Converting from Infix to Postfix

Chapter 5: Stacks 26

Additional Stack Applications (continued)

Chapter 5: Stacks 27

Evaluating Postfix Expressions (continued)

Chapter 5: Stacks 28

Evaluating Postfix Expressions (continued)

Chapter 5: Stacks 29

Chapter Review

• A stack is a last-in, first-out (LIFO) data structure• A stack is a simple but powerful data structure; its four

operations include empty, peek, pop, and push• Stacks are useful to process information in the reverse of

the order that it is encountered• Java.util.Stack is implemented as an extension of the

Vector class

Chapter 5: Stacks 30

Chapter Review (continued)

• Three ways to implement a stack: • Using an object of a class that implements the List

interface as a container• Using an array as a container• Using a linked list as a container

• Stacks can be applied in programs for evaluating arithmetic expressions