Lecture 9 Linked Lists - CCVCLvisionlab.engr.ccny.cuny.edu/~fhu/Lecture09-LinkedLists.pdf ·...

CSC212 Data Structure

- Section FG

Lecture9LinkedLists

Instructor:FengHUDepartmentofComputerScience

CityCollegeofNewYork

Motivation

• Inasequenceusinganarray,insertinganewitemneedstomoveothersback...

10 20 30 ? ?

18

Motivation


10 20 20 30 ?

18

Motivation


• SotheBig-OoftheinsertisO(n)

10 18 20 30 ?

Motivation

• Howcanweinsertanewitemwithoutmovingothers?

10 20 30 ? ?

18

We need a new data structure

Motivation


18

??302010

break an array into a linked chain...

Motivation


18

??302010

and then put the new item into the chain

Motivation


18

??302010

But the links (->) need some way to build up

LinkedListsinAction

•Chapter5introducestheoften-useddatastructureoflinkedlists.

• Thispresentationshowshowtoimplementthemostcommonoperationsonlinkedlists.

CHAPTER 5Data Structures and Other Objects

DeclarationsforLinkedLists

• Eachnode inthelinkedlistisaclass,asshownhere.

data

link

10

data

link

15

data

link

7

null

class node{public:

typedef int value_type;...

private:value_type data;node *link;

};

data

link

7


• Thedataportionofeachnodeisatypecalledvalue_type,definedbyatypedef.

link

null

data

link

15

data10

class node{public:



};


• Eachnodealsocontainsalinkfieldwhichisapointertoanothernode.

data15

data7

data10

link

linknull

link

class node{public:



};


• Aprogramcankeeptrackofthefirstnodebyusingapointervariablesuchashead_ptr inthisexample.

• Noticethathead_ptr itselfisnotanode-- itisapointertoanode.

data

link

10

data

link

15

data

link

7

nullhead_ptr

node * head_ptr;


• Aprogramcanalsokeeptrackofthelastnodebyusingapointervariablesuchastail_ptr inthisexample.

• Noticethattail_ptr itselfisnotanode-- itisapointertoanode.

data

link

10

data

link

15

data

link

7

nullhead_ptr

node * head_ptr;node * tail_ptr; tail_ptr


• Aprogramcankeeptrackofthefirstandthelastnodesbyusingpointervariablessuchashead_ptr,tail_ptr.

• Noticethatneitherhead_ptrnortail_ptrisanode-- itisapointertoanode.

• Foranemptylist,null isstoredinboththeheadandthetailpointers.

head_ptrnull

tail_ptrnull

node * head_ptr;node * tail_ptr;head_ptr = NULL;tail_ptr = NULL;// NULL can be used for any pointers!

TheComplete node ClassDefinition

• Thenodeclassisfundamentaltolinkedlists• Theprivatemembervariables

• data:avalue_typevariable• link:apointertothenextnode

• Thememberfunctionsinclude:• Aconstructor• Setdataandsetlink• Retrievedataandretrievelink



• data_field• link_field


class node{public:

// TYPEDEFtypedef double value_type;

// CONSTRUCTORnode(

const value_type& init_data = value_type( ),node* init_link = NULL

){ data = init_data; link = init_link; }

// Member functions to set the data and link fields:void set_data(const value_type& new_data) { data = new_data; }void set_link(node* new_link) { link = new_link; }

// Constant member function to retrieve the current data:value_type data( ) const { return data; }

// Two slightly different member functions to retrieve// the current link:const node* link( ) const { return link; }node* link( ) { return link;}

private:value_type data;node* link;

};

default argument given by the value_type default constructor

Why TWO? p. 213-4

ASmallQuiz-

• Supposeaaprogramhasbuiltthelinkedlistasshown,andhead_ptrisapointertoanode.

• What is the data type of *head_ptr?• cout << (*head_ptr). data();• cout << head_ptr->data();

data

link

10

data

link

15

data

link

7

nullhead_ptr

LinkedListToolkit

• DesignContainerClassesusingLinkedLists• Theuseofalinked listissimilartoourprevioususeofanarrayinacontainerclass

• Butstoringandretrievingneedsmoreworksincewedonothavethathandyindexing

• =>LinkedListToolbox• usingnodeclass

TheWorkingsoffourfunctions

• Thislecturewillshowfourfunctions:• Computethelengthofalinkedlist(code)• Insertanewnodeatthehead(code)• Insertanodeatanylocation(pseudo-code)• Deleteanodefromthehead(pseudo-code)

•ReadSection5.2forotherfunctionsintheToolbox• willbeusedincontainerclassesbagandsequence

size_t list_length(const node* head_ptr);

LengthofaLinkedList

Wesimplywanttocomputethelength ofthelinkedlist,forexampletheoneshownhere.

Notethatlist_lengthisnotamemberfunctionofthenodeclass

10

15

7

nullhead_ptr


Pseudo-codeoflist_length

1. Initializethecount tozero.2. Makecursorpointtoeachnode,

startingatthehead.Eachtimecursorpointstoanewnode,add1tocount.

3. returncount.

10

15

7

nullhead_ptr

count0





3. returncount.

10

15

7

nullhead_ptr

count1

cursor





3. returncount.

10

15

7

nullhead_ptr

count2

cursor





3. returncount.

10

15

7

nullhead_ptr

count3

cursor

size_t list_length(const node* head_ptr){

const node *cursor;size_t count = 0; // step 1for (cursor = head_ptr; cursor != NULL; cursor = cursor->link())

count++; // step 2return count; // step 3

}

Realcodeoflist_length:ListTraverse

1. Initializethecount tozero.2. Eachtimecursor pointstoanew

node,add1tocount.3. returncount.

10

15

7

nullhead_ptrcount

0

cursor




}




10

15

7

nullhead_ptrcount

1

cursor




}




10

15

7

nullhead_ptrcount

2cursor




}




10

15

7

nullhead_ptrcount

3

cursor


const node *cursor;size_t count = 0;for (cursor = head_ptr; cursor != NULL; cursor = cursor->link())

count++;return count; // step 3

}

Big-Ooflist_length

Big-O:O(n)iflengthisn10

15

7

nullhead_ptrcount

3

cursor



count++;return count;

}

Islist_lengthworksforanemptylist?

cursor=head_ptr=NULLcount=0

count0

head_ptr

nullcursor

null

void list_head_insert(node*& head_ptr, const node::value_type& entry);

InsertinganodeattheHead

Wewanttoaddanewentry,13,tothehead ofthelinkedlistshownhere.

Notethathead_ptrisareferencenodepointer

10

15

7

nullhead_ptr

entry13

InsertingaNodeattheHead

❶ Createanewnode,pointedtobyalocalvariableinsert_ptr.

10

15

7

nullhead_ptr

entry13

insert_ptr



❶ insert_ptr = new node;

10

15

7

nullhead_ptr

entry13

insert_ptr



❶ insert_ptr = new node;❷ Placethedatainthenewnode'sdata

field.

10

15

7

nullhead_ptr

entry13

insert_ptr13



❶ insert_ptr = new node;❷ insert_ptr->data = entry;

What expression appears on the left side of the assignment statement ?

10

15

7

nullhead_ptr

entry13

insert_ptr13

?




But data is a private variable, so cannot be accessed by a non-member function

10

15

7

nullhead_ptr

entry13

insert_ptr13




But data is a private variable, so cannot be accessed by a non-member function

10

15

7

nullhead_ptr

entry13

insert_ptr13


X


❶ insert_ptr = new node;❷ insert_ptr->set_data(entry);

Instead, Set_data function is used since data_field is a private variable of the node class

10

15

7

nullhead_ptr

entry13

insert_ptr13



10

15

7

nullhead_ptr

entry13

insert_ptr13

❶ insert_ptr = new node;❷ insert_ptr->set_data(entry);❸ Connect the new node to the

front of the list.



What will be the parameter ?

10

15

7

nullhead_ptr

entry13

insert_ptr13

❶ insert_ptr = new node;❷ insert_ptr->set_data(entry);➌ insert_ptr->set_link(head_ptr);?



The new node is linked to the node that head_ptr is pointing to. 10

15

7

nullhead_ptr

entry13

insert_ptr13

❶ insert_ptr = new node;❷ insert_ptr->set_data(entry);➌ insert_ptr->set_link(head_ptr);



10

15

7

nullhead_ptr

entry13

insert_ptr13

❶ insert_ptr = new node;❷ insert_ptr->set_data(entry);➌ insert_ptr->set_link(head_ptr);❹ Make the head_ptr point to the new

head of the linked list.



10

15

7

nullhead_ptr

entry13

insert_ptr13

❶ insert_ptr = new node;❷ insert_ptr->set_data(entry);➌ insert_ptr->set_link(head_ptr);➍ head_ptr = insert_ptr;



❶ insert_ptr = new node;❷ insert_ptr->set_data(entry);➌ insert_ptr->set_link(head_ptr);➍ head_ptr = insert_ptr;

10

15

7

nullhead_ptr

13

When the function returns, thelinked list has a new node at thehead, containing 13.


void list_head_insert(node*& head_ptr, const node::value_type& entry){

node *insert_ptr;

insert_ptr = new node;insert_ptr->set_data(entry);insert_ptr->set_link(head_ptr);head_ptr = insert_ptr;

}


Linked List: O(1)

- cmp: Array: O(n)

What is the Big-O of

the head_insert function?


node *insert_ptr;


}


Does the function workcorrectly for the empty

list ?


node *insert_ptr;


}


head_ptrentry13 null

Does the function workcorrectly for the empty

list ?


node *insert_ptr;


}

InsertingaNodeattheFront


insert_ptr13


node *insert_ptr;


}



insert_ptr13

null


node *insert_ptr;


}


head_ptrentry13

insert_ptr13

null


node *insert_ptr;


}


head_ptr

13

null

When the functionreturns, the linked list

has one node,containing 13.

Caution!

•Alwaysmakesurethatyourlinkedlistfunctionsworkcorrectlywithanemptylist.

EMPTY LIST


node *insert_ptr;


}


Q: Can you give an implementation with ONLY a single statement?


node *insert_ptr;

insert_ptr = new node(entry, head_ptr);

head_ptr = insert_ptr;}


YES, we can use the constructor with parameters!


head_ptr = new node(entry, head_ptr);

}


and assign the return pointer of new directly to the head pointer !

• Break


• Eachnodealsocontainsalinkfieldwhichisapointertoanothernode.

data15

data7

data10

link

linknull

link

class node{public:



};



• data_field• link_field


class node{public:

// TYPEDEFtypedef double value_type;

// CONSTRUCTORnode(

const value_type& init_data = value_type( ),node* init_link = NULL

){ data = init_data; link = init_link; }

// Member functions to set the data and link fields:void set_data(const value_type& new_data) { data = new_data; }void set_link(node* new_link) { link = new_link; }

// Constant member function to retrieve the current data:value_type data( ) const { return data; }

// Two slightly different member functions to retrieve// the current link:const node* link( ) const { return link; }node* link( ) { return link;}

private:value_type data;node* link;

};

default argument given by the value_type default constructor

Why TWO? p. 213-4



count++;return count; // step 3

}

Big-Ooflist_length

Big-O:O(n)iflengthisn10

15

7

nullhead_ptrcount

3

cursor

PseudocodeforInsertingNodes

• Nodesareofteninsertedatplacesotherthanthefrontofalinkedlist.• Thereisageneralpseudocodethatyoucanfollowforanyinsertion

function...


❶Determinewhetherthenewnodewillbethefirstnodeinthelinkedlist.Ifso,thenthereisonlyonestep:

list_head_insert(head_ptr, entry);


➊Determinewhetherthenewnodewillbethefirstnodeinthelinkedlist.Ifso,thenthereisonlyonestep:





A pointerto the

head ofthe list


❷ Otherwise (if the new node will not be first):❐ Start by setting a pointer named previous_ptr to point to the

node which is just before the new node's position.


15

10

7

nullhead_ptr



In this example, thenew node will bethe second node

previous_ptr


What is the name of this pointer ?

15

10

7

nullhead_ptr


node which is just before the new node's position

Look at the pointerwhich is in the node

*previous_ptrprevious_ptr


Always remember how can you access link

15

10

7

nullhead_ptr



This pointer is calledprevious_ptr->link

previous_ptr


15

10

7

nullhead_ptr

➋ Otherwise (if the new node will not be first):❐ Start by setting a pointer named previous_ptr to point to the


previous_ptr->linkpoints to the headof a smaller linkedlist, with 10 and 7

previous_ptr


Write one C++ statement which will do the insertion.

15

10

7

nullhead_ptr



The new node mustbe inserted at thehead of this small

linked list.

13

previous_ptr


Write one C++ statement which will do the insertion.

15

10

7

nullhead_ptr


node which is just before the new node's position. 13

previous_ptrlist_head_insert(previous_ptr->link, entry);

X


More precisely, you need to use member function link() , and have three lines of code

15

10

7

nullhead_ptr


node which is just before the new node's position.13

previous_ptr

node *sl_head_ptr;sl_head_ptr = previous_ptr->link();list_head_insert(sl_head_ptr, entry);previous_ptr->set_link(sl_head_ptr);

sl_head_ptr


More precisely, you need to use member function link() , and have three lines of code

15

10

7

nullhead_ptr


node which is just before the new node's position. 13

previous_ptr

node *sl_head_ptr;sl_head_ptr = previous_ptr->link();list_head_insert(sl_head_ptr, entry);previous_ptr->set_link(sl_head_ptr);

sl_head_ptr




❷ Otherwise (if the new node will not be first):❐ Set a pointer named previous_ptr to point to the node

which is just before the new node's position.❐ Do the following :

node *sl_head_ptr;sl_head_ptr = previous_ptr->link(); list_head_insert(sl_head_ptr, entry);previous_ptr->set_link(sl_head_ptr);


• Theprocessofaddinganewnodeinthemiddleofalist(only the step after previous_ptr has been set)canalsobeincorporatedasaseparatefunction.Thisfunctioniscalledlist_insert inthelinkedlisttoolkitofSection5.2.

• Challengeyourself:• Thetextbookactuallygivesyouadifferentimplementation(p235,4linesofcode)

• Canyouimplementlist_insert withjustonelineofcode?• Don’tuselist_head_insert• SeeSelf-TestEx16

PseudocodeforRemovingNodes

• Nodesoftenneedtoberemovedfromalinkedlist.• Aswithinsertion,thereisatechniqueforremovinganodefromthe

frontofalist,andatechniqueforremovinganodefromelsewhere.• We’lllookatthepseudocodeforremovinganodefromtheheadofa

linkedlist.

RemovingtheHeadNode

10 15

7

nullhead_ptr

13

❶ Start by setting up a temporary pointer named remove_ptrto the head node.

remove_ptr

RemovingtheHeadNode

10 15

7

nullhead_ptr

13

❶ Set up remove_ptr.❷ head_ptr = remove_ptr->link();

remove_ptr

Draw the change that this statement will make to the linked list.

RemovingtheHeadNode

10 15

7

nullhead_ptr

13

➊ Set up remove_ptr.➋ head_ptr = remove_ptr->link();

remove_ptr

RemovingtheHeadNode

➊ Set up remove_ptr.➋ head_ptr = remove_ptr->link;❸ delete remove_ptr; // Return the node's memory to heap.

10 15

7

nullhead_ptr

13

remove_ptr

RemovingtheHeadNode

Here’s what the linked list looks like after the removal finishes.

10 15

7

nullhead_ptr

Summary

• Itiseasytoinsertanodeatthefrontofalist.• Thelinkedlisttoolkitalsoprovidesafunctionforinsertinganewnodeelsewhere

• Itiseasytoremoveanodeatthefrontofalist.• Thelinkedlisttoolkitalsoprovidesafunctionforremovinganodeelsewhere--youshouldreadaboutthisfunctionandtheotherfunctionsofthetoolkit.

Keypointsyouneedtoknow

• LinkedListToolkitusesthenodeclasswhichhas• setandretrievefunctions

• ThefunctionsintheToolkitarenotmemberfunctionsofthenodeclass• length,insert(2),remove(2),search,locate,copy,...• comparetheirBig-Oswithsimilarfunctionsforanarray

• Theycanbeusedinvariouscontainerclasses,suchasbag,sequence,etc.

Toolkit Code

Homework...

• Self-TestExercises(node)• 1-12

•Readafterclass• LinkedListToolKit (Section5.2)• DoSelf-TestEx13-25

•Readbeforethenextlecture• Section5.3- 5.4

THE END

Presentation copyright 1997, Addison Wesley Longman,For use with Data Structures and Other Objects Using C++by Michael Main and Walter Savitch.

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Students and instructors who use Data Structures and Other Objects Using C++ are welcometo use this presentation however they see fit, so long as this copyright notice remainsintact.

Lecture 9 Linked Lists - CCVCLvisionlab.engr.ccny.cuny.edu/~fhu/Lecture09-LinkedLists.pdf ·...

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