chapter 6

Data Structures Using Java 1

Chapter 6

Stacks


Chapter Objectives

• Learn about stacks• Examine various stack operations• Learn how to implement a stack as an array• Learn how to implement a stack as a linked list• Discover stack applications• Learn to use a stack to remove recursion


Stacks

• Definition: – list of homogeneous elements– addition and deletion of elements occurs only at

one end, called the top of the stack

• Last In First Out (LIFO) data structure• Used to implement method calls• Used to convert recursive algorithms

(especially not tail recursive) into nonrecursive algorithms


Various Types of Stacks


LIFO

• Last In First Out (LIFO) data structure– Top element of stack is last element to be added

to stack– Elements added and removed from one end

(top)– Item added last are removed first


Empty Stack


Stack Operations


Basic Operations on a Stack

• initializeStack: Initializes the stack to an empty state

• isEmptyStack: Checks whether the stack is empty. If empty, it returns true; otherwise, it returns false

• isFullStack: Checks whether the stack is full. If full, it returns true; otherwise, it returns false



• push:– Add new element to the top of the stack– The input consists of the stack and the new

element– Prior to this operation, the stack must exist and

must not be full



• top: Returns the top element of the stack. Prior to this operation, the stack must exist and must not be empty

• pop: Removes the top element of the stack. Prior to this operation, the stack must exist and must not be empty


Example of a Stack


Empty Stack


initializeStack

public void initializeStack(){ for(int i = 0; i < stackTop; i++) list[i] = null; stackTop = 0;}//end initializeStack


emptyStack and fullStack

public boolean isEmptyStack(){ return(stackTop == 0);}//end isEmptyStack

public boolean isFullStack()

{

return(stackTop == maxStackSize);

}//end isFullStack


Push


Pushpublic void push(DataElement newItem) throws

StackOverflowException{ if(isFullStack()) throw new StackOverflowException(); list[stackTop] = newItem.getCopy(); //add newItem at the //top of the stack stackTop++; //increment stackTop}//end push


Return Top Element

public DataElement top() throws StackUnderflowException{ if(isEmptyStack()) throw new StackUnderflowException(); DataElement temp = list[stackTop - 1].getCopy(); return temp; }//end top


Pop

public void pop() throws StackUnderflowException{ if(isEmptyStack()) throw new StackUnderflowException(); stackTop--; //decrement stackTop list[stackTop] = null;}//end pop


Pop


copy

private void copy(StackClass otherStack){ list = null; System.gc(); maxStackSize = otherStack.maxStackSize; stackTop = otherStack.stackTop; list = new DataElement[maxStackSize]; //copy otherStack into this stack for(int i = 0; i < stackTop; i++) list[i] = otherStack.list[i].getCopy();}//end copy6


Constructors //constructor with a parameterpublic StackClass(int stackSize) { if(stackSize <= 0) { System.err.println(“The size of the array to implement “ + “the stack must be positive.”); System.err.println(“Creating an array of size 100.”); maxStackSize = 100; } else maxStackSize = stackSize; //set the stack size to //the value specified by //the parameter stackSize stackTop = 0; //set stackTop to 0 list = new DataElement[maxStackSize]; //create the array}//end constructor


Constructors

//default constructor

public StackClass()

{

maxStackSize = 100;

stackTop = 0; //set stackTop to 0

list = new DataElement[maxStackSize]; //create array

}//end default constructor


Copy Constructor and copyStack

public StackClass(StackClass otherStack){ copy(otherStack);}//end copy constructor

public void copyStack(StackClass otherStack){ if(this != otherStack) //avoid self-copy

copy(otherStack);}//end copyStack


Time Complexity of Operations of class stackType


Programming Example: Highest GPA

Input The program reads an input file consisting of each student’s GPA, followed by the student’s name. Sample data is:

3.8 Lisa3.6 John3.9 Susan3.7 Kathy3.4 Jason3.9 David3.4 Jack


Programming Example: Highest GPA (Algorithm)

1. Declare the variables2. Create a DecimalFormat object to output a

decimal number to two decimal places3. Open the input file4. If the input file does not exist, exit the program5. Read the next input line


Highest GPA (Algorithm)6. while (not end of file) {

6.a. Tokenize the input line6.b. Get the next GPA6.c. Get the next name6.d. if (GPA > highestGPA)

{ 6.d.i initialize stack 6.d.ii push(stack, student name) 6.d.iii highestGPA = GPA } 6.e. else if(GPA is equal to highestGPA) push(stack, student name) 6.f Read the next input line

}


Programming Example: Highest GPA (Algorithm)

7. Output the highest GPA.

8. Output the names of the students having the highest GPA.


Programming Example: Highest GPA (Sample Run)

Input File (Ch6_HighestGPAData.txt)3.4 Holt3.2 Bolt2.5 Colt3.4 Tom3.8 Ron3.8 Mickey3.6 Pluto3.5 Donald3.8 Cindy3.7 Dome3.9 Andy3.8 Fox3.9 Minnie2.7 Goofy3.9 Doc3.4 Danny


Programming Example: Highest GPA (Sample Run)

Output

Highest GPA = 3.90

The students holding the highest GPA are:

Doc

Minnie

Andy


Empty and Nonempty Linked Stack

Empty linked stack Nonempty linked stack


Default Constructor

public LinkedStackClass()

{

stackTop = null;

}


initializeStack, isStackEmpty, and isStackFull

public void initializeStack(){ stackTop = null;}//end initializeStack

public boolean isEmptyStack(){ return(stackTop == null);}public boolean isFullStack(){ return false;}


Push

Stack before the push operation

Stack and newNode


Push

Stack after the statement newNode.link = stackTop; executes

Stack after the statement stackTop = newNode; executes


Return Top Element

public DataElement top() throws StackUnderflowException{ if(stackTop == null) throw new StackUnderflowException(); return stackTop.info.getCopy();}//end top


Pop

Stack before the pop operation

Stack after the statement stackTop = stackTop.link; executes

Stack after popping the top element


Application of Stacks:Postfix Expression Calculator

• Prefix/Polish Notation

• Suffix/Postfix/Reverse Polish Notation



Stack after pushing 6


Stack after retrieving the top two elements and popping twice

Stack after pushing the result of op1 + op2, which is 9




Stack after retrieving the top two elements and popping twice

Stack after pushing the result of op1 * op2, which is 18

Stack after popping the element


Postfix Expression Calculator (Main Algorithm)

Get the next expression

while more data to process

{

a. initialize the stack

b. process the expression

c. output the result

d. get the next expression

}


Nonrecursive Algorithm to Print Linked List

current = first; //Line 1while(current != NULL) //Line 2{ stack.push(current); //Line 3 current = current.link; //Line 4}


List After Execution of Statementcurrent = first;


Repeated Execution of:stack.push(current);

current = current.link;


Java class Stack

• Java provides a class to implement a stack in a program

• The name of the Java class defining a stack is Stack

• The class Stack is contained in the package java.util

• Table 6-3 lists the members of the class Stack


Java class Stack


Chapter Summary

• Stack Data Structure• Last In First Out (LIFO)• Stacks Implemented as Arrays• Stacks Implemented as Linked Lists• Postfix Expression Calculator• Nonrecursive Algorithm to Print Linked List• Java class Stack

chapter 6

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