Nonrecursive list methods

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Nonrecursive list methods

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Traversing a list. Traversing means 'visiting' each node in order. You can traverse recursively, or with a loop, but never both ... – PowerPoint PPT presentation

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Title: Nonrecursive list methods

1
Non-recursive list methods

There's a different way to display all the nodes
in a list like this
class ListNode
int myNum
ListNode nextNumber
ListNode(int nNum, ListNode next)
myNum nNum nextNumber next
ListNode head new ListNode(6,
new ListNode(4,
new ListNode(2,null)))

2
Non-recursive list methods

Instead of recursion, we can use a loop
class ListNode
int myNum
ListNode nextNumber
ListNode(int nNum, ListNode next)
myNum nNum nextNumber next
ListNode head new ListNode(6,
new ListNode(4,
new ListNode(2,null)))
ListNode temp head //IMPORTANT!
while(temp ! null)
System.out.println(temp.myNum)
temp temp.nextNumber

3
Non-recursive list methods

Here it is again with a for loop
class ListNode
int myNum
ListNode nextNumber
ListNode(int nNum, ListNode next)
myNum nNum nextNumber next
ListNode head new ListNode(6,
new ListNode(4,
new ListNode(2,null)))
for(ListNode temp head
temp ! null
temp temp.nextNumber)
System.out.println(temp.myNum)

4
Non-recursive list methods

Notice both loops have the same parts
ListNode temp head
while(temp ! null)
System.out.println(temp.myNum)
temp temp.nextNumber
for(ListNode temp head
temp ! null
temp temp.nextNumber)
System.out.println(temp.myNum)

5
Traversing a list

Traversing means "visiting" each node in order
You can traverse recursively, or with a loop, but
never both
Don't mess up the head, it will turn the list to
garbage
ListNode temp head //IMPORTANT!
Make a copy of the head instead
while(temp ! null)
System.out.println(temp.myNum)
temp temp.nextNumber

6
Problem write a non-recursive method that returns
the sum of all the nodes

int findSum(ListNode head)
ListNode temp head
int nSum 0
while(temp ! null)
nSum temp.myNum
temp temp.nextNumber
return nSum

7
Inserting list nodes

We can add nodes at the front, at the end or
somewhere in the middle.
Lets start with the easiest example adding a
node to the front of the list
class ListNode
int myNum
ListNode nextNumber
ListNode(int nNum, ListNode next)
myNum nNum nextNumber next

8
Inserting a node at the front

Let's add a node with the value 4 to the front of
this list
ListNode head
head new ListNode(6,
new ListNode(2,
new ListNode(3, null)))

9
Inserting a node at the front
Before
head
6
2
3
After
4
head
6
2
3
3
10
Inserting a node at the front

You can see that head will no longer point to 6
It will point to a new ListNode with value 4
The new ListNode's nextNumber will point to what
used to be the head

Before
head
6
2
3
After
4
head
6
2
3
11
Inserting a node at the front

adding a node with the value 4 to the front of
this list
ListNode head
head new ListNode(6,
new ListNode(2,
new ListNode(3, null)))
head new ListNode(4,head)
That's all there is to it!

12
Inserting a node at the end

This time, let's add a node with the value 4 to
the end of this list
ListNode head
head new ListNode(6,
new ListNode(2,
new ListNode(3, null)))

13
Inserting a node at the end
Before
head
6
2
3
After
6
head
2
3
3
4
14
Inserting a node at the end

You can see that head is unchanged
The next number of the next number of the next
number of head will point to a new ListNode

Before
head
6
2
3
After
6
head
2
3
4
15
Inserting a node at the end

ListNode head
head new ListNode(6,
new ListNode(2,
new ListNode(3, null)))
head.nextNumber.nextNumber.nextNumber
new ListNode(4,null)
This works, but it's not exactly ideal

Before
head
6
2
3
After
6
head
2
3
4
16
Inserting a node at the end

A better idea is to traverse the list until we
get to the last nextNumber, the one that is null
Then "hook" that last nextNumber up to a new
ListNode
ListNode head
head new ListNode(6,
new ListNode(2,
new ListNode(3, null)))
ListNode temp head
while(temp.nextNumber ! null)
temp temp.nextNumber
temp.nextNumber new ListNode(4,null)

17
What is the output of this program?

ListNode head
head new ListNode(-2,null)
head new ListNode(-3,head)
head.nextNumber
new ListNode(7,head.nextNumber)
head.nextNumber.nextNumber.nextNumber
new ListNode(5,null)
displayNodes(head)

18
Inserting a node in the middle of the list

Now, let's add a node with the value 4 before the
node with value 3
ListNode head
head new ListNode(6,
new ListNode(2,
new ListNode(3, null)))

19
Inserting a node in the middle of the list
Before
head
6
2
3
After
6
head
2
4
3
3
20
Inserting a node in the middle of the list

Again, you can see that head is unchanged
The next number of the next number of head will
point to a new ListNode
The nextNumber of the new ListNode will point to 3

head
Before
6
2
3
After
6
head
2
4
3
21
Inserting a node in the middle of the list

head.nextNumber.nextNumber
new ListNode(4,
head.nextNumber.nextNumber)
This works, but it's not ideal

head
Before
6
2
3
After
6
head
2
4
3
22
Inserting a node in the middle of the list

ListNode head
head new ListNode(6,
new ListNode(2,
new ListNode(3,null)))
//traverse the list until we find the
//nextNumber that points to 3
ListNode temp head
while(temp.nextNumber ! null
temp.nextNumber.myNum !3)
temp temp.nextNumber
temp.nextNumber new ListNode(4,
temp.nextNumber)

23
Deleting a node

Deleting a node means "rerouting" a pointer past
the node to be deleted.
Let's say we want to delete the node with value 2

head
Before
6
2
3
After
6
head
2
3
24
Deleting a node

head.nextNumber head.nextNumber.nextNumber
This works, but it's better to traverse the list

head
Before
6
2
3
After
6
head
2
3
25
Deleting a node

ListNode temp head
while(temp.nextNumber ! null
temp.nextNumber.myNum ! 2)
temp temp.nextNumber
temp.nextNumber temp.nextNumber.nextNumber

head
Before
6
2
3
After
6
head
2
3
26
Deleting a node at the front

Deleting a node at the front is a special casewe
need to change head
Luckily, it's pretty simple
head head.nextNumber

head
Before
6
2
3
After
6
head
2
3
27
Writing a Linked List class

A Linked List class needs
A head variable
A constructor
An insert method
A delete method
A method that searches the list for a particular
value

28
The head variable

class List
private ListNode head

29
The constructor

class List
private ListNode head
public List()
???

30
The constructor

class List
private ListNode head
public List()
head null

31
A method that inserts a new node at the front of
the list

class List
private ListNode head
public ListNode()
head null
public void insertAtFront(int nNum)
???

32
A method that inserts a new node at the front of
the list

class List
private ListNode head
public ListNode()
head null
public void insertAtFront(int nNum)
head new ListNode(nNum,head)

33
A method that displays all the nodes in the list

class List
private ListNode head
public ListNode()
head null
public void insertAtFront(int nNum)
head new ListNode(nNum,head)
void displayNodes (ListNode node)
if (node null)
System.out.println("End")
else
System.out.println(node.myNum)
displayNodes(node.nextNumber)

34
Using the List class what's wrong with this code?

List theList new List()
theList.insertAtFront(1)
theList.insertAtFront(2)
theList.insertAtFront(3)
theList.displayNodes(head)

35
head is a private member of the List class. We
don't have access to head outside of the List
class

List theList new List()
theList.insertAtFront(1)
theList.insertAtFront(2)
theList.insertAtFront(3)
theList.displayNodes(head)

36
A "helper" method that "gets the display started"

class List
private ListNode head
public ListNode()
head null
public void insertAtFront(int nNum)
head new ListNode(nNum,head)
void display()displayNodes(head)
void displayNodes (ListNode node)
if (node null)
System.out.println("End")
else

37
Using the List class Now it works correctly

List theList new List()
theList.insertAtFront(1)
theList.insertAtFront(2)
theList.insertAtFront(3)
theList.display()
Note recursive member methods often use helpers
to "get them started"

38
Writing a "remove" method

When you write a "remove" method, you search the
list for a particular value to remove
There are three possibilities
The list is empty
The node to remove is the first node in the list
You need to traverse the list to see if the node
is in the list

39
Writing a "remove" method

class List
private ListNode head
//lots more Java
public void remove(int nNum)
??

40
Writing a "remove" method

public void remove(int nNum)
//check if list is empty

41
Writing a "remove" method

public void remove(int nNum)
//check if list is empty
if(head null)
??

42
Writing a "remove" method

public void remove(int nNum)
//check if list is empty
if(head null)
??
//remove first node?

43
Writing a "remove" method

public void remove(int nNum)
//check if list is empty
if(head null)
??
//remove first node?
else if(head.myNum nNum)
??

44
Writing a "remove" method

public void remove(int nNum)
//check if list is empty
if(head null)
??
//remove first node?
else if(head.myNum nNum)
??
else
//traverse the list

45
Applications

Assignments 13.5 and 14 are applications
They have a main method
//A simple application
public class SimpleApp
public static void main(String args)
System.out.println("hello")

46
file input and output

Applets have security restrictions they are not
allowed access to your hard drive
If we want a program to read data from a file (or
save data to a file), we need to write an
application
The standard file input and output classes in
java are Ugly (and are not on the AP)
A simpler alternative is the EasyReader class

47
A "simple" standard Java file input application

import java.io.
public class FileReaderTest
public static void main(String args)
File inFilenew File("inFile.txt")
try
FileReader frnew FileReader(inFile)
BufferedReader in new BufferedReader(fr)
String sLine
do
sLinein.readLine()//read one line
if(sLine!null)
System.out.println(sLine)
while(sLine!null)
in.close()
catch(IOException ioe)

48
The same program using the EasyReader class

public class EasyReaderDemo
public static void main(String args)
String sFileName "inFile.txt"
EasyReader inFile new EasyReader(sFileNam
e)
if (inFile.bad())
System.err.println("Can't open "
sFileName)
System.exit(1)
String sLine
do
sLine inFile.readLine())
if(sLine ! null)
System.out.println(sLine)
while (sLine ! null)
inFile.close()

49
The same program using the EasyReader class

public class EasyReaderDemo
public static void main(String args)
//Setup a new EasyReader
String sFileName "inFile.txt"
EasyReader inFile
new EasyReader(sFileName)
if (inFile.bad())
System.err.println("Can't open "
sFileName)
System.exit(1)
String sLine
do
sLine inFile.readLine())
if(sLine ! null)
System.out.println(sLine)

50
The same program using the EasyReader class

public class EasyReaderDemo
public static void main(String args)
String sFileName "inFile.txt"
EasyReader inFile new EasyReader(sFileNam
e)
//Check to see if the file exists
if (inFile.bad())
System.err.println
("Can't open " sFileName)
System.exit(1)
String sLine
do
sLine inFile.readLine())
if(sLine ! null)
System.out.println(sLine)

51
The same program using the EasyReader class

public class EasyReaderDemo
public static void main(String args)
String sFileName "inFile.txt"
EasyReader inFile new EasyReader(sFileNam
e)
//Check to see if the file exists
if (inFile.bad())
System.err.println
("Can't open " sFileName)
System.exit(1)
//Read data one line at a time
String sLine
do
sLine inFile.readLine())
if(sLine ! null)

52
The same program using the EasyReader class

public class EasyReaderDemo
public static void main(String args)
String sFileName "inFile.txt"
EasyReader inFile new EasyReader(sFileNam
e)
//Check to see if the file exists
if (inFile.bad())
System.err.println
("Can't open " sFileName)
System.exit(1)
//Read data one line at a time
String sLine
do
sLine inFile.readLine())
if(sLine ! null)

53
Head recursion vs. Tail recursion

If the last line of a method is a recursive call,
the method is said to be tail recursive
void displayNodes (ListNode node)
if (node null)
System.out.println("End")
else
System.out.println(node.myNum)
displayNodes(node.nextNumber)

54
Head recursion vs. Tail recursion

If the last line of a method is a recursive call,
the method is said to be tail recursive
void displayNodes (ListNode node)
if (node null)
System.out.println("End")
else
System.out.println(node.myNum)
displayNodes(node.nextNumber)

55
Head recursion vs. Tail recursion

If we change the order, the method is called head
recursive
void displayNodes (ListNode node)
if (node null)
System.out.println("End")
else
displayNodes(node.nextNumber)
System.out.println(node.myNum)

56
Head recursion vs. Tail recursion

In assignment 14, experiment with the display
method using head and then tail recursion and see
what happens
void displayNodes (ListNode node)
if (node null)
System.out.println("End")
else
displayNodes(node.nextNumber)
System.out.println(node.myNum)

57
Assignment 14 file20.txt