Ch17

Starting Out with C++, 3rd
Edition
1
Linked Lists

Edition
2
17.1 Introduction to the Linked
List ADT
• A linked list is a series of connected nodes,
where each node is a data structure.
• A linked list can grow or shrink in size as
the program runs

Edition
3
Advantages of Linked Lists
over Arrays and vectors
• A linked list can easily grow or shrink in
size.
• Insertion and deletion of nodes is quicker
with linked lists than with vectors.

Edition
4
The composition of a Linked
List
• Each node in a linked list contains one or
more members that represent data.
• In addition to the data, each node contains a
pointer, which can point to another node.

Edition
5
The composition of a Linked
List
• A linked list is called "linked" because each
node in the series has a pointer that points
to the next node in the list.

Edition
6
Declarations
• First you must declare a data structure that
will be used for the nodes. For example, the
following struct could be used to create
a list where each node holds a float:
struct ListNode
{
float value;
struct ListNode *next;
};

Edition
7
Declarations
• The next step is to declare a pointer to serve
as the list head, as shown below.
ListNode *head;
• Once you have declared a node data
structure and have created a NULL head
pointer, you have an empty linked list.
• The next step is to implement operations
with the list.

Edition
8
17.2 Linked List Operations
• We will use the following class declaration (on the
next slide), which is stored in FloatList.h.

Edition
9
class FloatList
{
private:
// Declare a structure for the list
struct ListNode
{
float value;
};
ListNode *head; // List head pointer
public:
FloatList(void) // Constructor
{ head = NULL; }
~FloatList(void); // Destructor
void appendNode(float);
void insertNode(float);
void deleteNode(float);
void displayList(void);
};

Edition
10
Appending a Node to the List
• To append a node to a linked list means to add the node to
the end of the list.
• The pseudocode is shown below. The C++ code follows.
Create a new node.
Store data in the new node.
If there are no nodes in the list
Make the new node the first node.
Else
Traverse the List to Find the last node.
Add the new node to the end of the list.
End If.

Edition
11
void FloatList::appendNode(float num)
{
ListNode *newNode, *nodePtr;
// Allocate a new node & store num
newNode = new ListNode;
newNode->value = num;
newNode->next = NULL;
// If there are no nodes in the list
// make newNode the first node
if (!head)
head = newNode;
else // Otherwise, insert newNode at end
{
// Initialize nodePtr to head of list
nodePtr = head;
// Find the last node in the list
while (nodePtr->next)
nodePtr = nodePtr->next;
// Insert newNode as the last node
nodePtr->next = newNode;
}
}

Edition
12
Program 17-1
// This program demonstrates a simple append
// operation on a linked list.
#include <iostream.h>
#include "FloatList.h”
void main(void)
{
FloatList List;
list.appendNode(2.5);
}
(This program displays no output.)

Edition
13
Stepping Through the Program
• The head pointer is declared as a global variable.
head is automatically initialized to 0 (NULL),
which indicates that the list is empty.
• The first call to appendNode passes 2.5 as the
argument. In the following statements, a new node
is allocated in memory, 2.5 is copied into its
value member, and NULL is assigned to the
node's next pointer.

Edition
14
newNode->next = nULL;

Edition
15
The next statement to execute is the following if statement.
if (!head)
head = newNode;
There are no more statements to execute, so control returns to
function main.

Edition
16
In the second call to appendNode, 7.9 is passed as the argument.
Once again, the first three statements in the function create a new
node, store the argument in the node's value member, and assign
its next pointer to NULL.

Edition
17
Since head no longer points to NULL, the else part of the if statement executes:
{
nodePtr = head;
}

Edition
18
nodePtr is already at the end of the list, so the while loop
immediately terminates. The last statement, nodePtr->next =
newNode; causes nodePtr->next to point to the new node.
This inserts newNode at the end of the list.

Edition
19
The third time appendNode is called, 12.6 is passed as the
argument. Once again, the first three statements create a node with
the argument stored in the value member.

Edition
20
next, the else part of the if statement executes. As before,
nodePtr is made to point to the same node as head.

Edition
21
Since nodePtr->next is not NULL, the while loop will
execute. After its first iteration, nodePtr will point to the second
node in the list.

Edition
22
The while loop's conditional test will fail after the first iteration
because nodePtr->next now points to NULL. The last
statement, nodePtr->next = newNode; causes
nodePtr->next to point to the new node. This inserts newNode
at the end of the list
The figure above depicts the final state of the linked list.

Edition
23
Traversing the List
• The displayList member function traverses the list,
displaying the value member of each node. The
following pseudocode represents the algorithm. The C++
code for the member function follows on the next slide.
Assign List head to node pointer.
While node pointer is not NULL
Display the value member of the node pointed to by node pointer.
Assign node pointer to its own next member.
End While.

Edition
24
void FloatList::displayList(void)
{
ListNode *nodePtr;
nodePtr = head;
while (nodePtr)
{
cout << nodePtr->value << endl;
}
}

Edition
25
Program 17-2
// This program calls the displayList member function.
// The funcion traverses the linked list displaying
// the value stored in each node.
#include "FloatList.h"
void main(void)
{
FloatList List;
list.displayList();
}

Edition
26
Program 17-2 Output
2.5
7.9
12.6

Edition
27
Inserting a Node
• Using the listNode structure again, the
pseudocode on the next slide shows an
algorithm for finding a new node’s proper
position in the list and inserting there.
• The algorithm assumes the nodes in the list
are already in order.

Edition
28
Create a new node.
Store data in the new node.
If there are no nodes in the list
Make the new node the first node.
Else
Find the first node whose value is greater than or equal
the new value, or the end of the list (whichever is first).
Insert the new node before the found node, or at the end of
the list if no node was found.
End If.

Edition
29
The code for the traversal algorithm is shown below. (As before, num
holds the value being inserted into the list.)
nodePtr = head;
// Skip all nodes whose value member is less
// than num.
while (nodePtr != NULL && nodePtr->value < num)
{
previousNode = nodePtr;
}
The entire insertNode function begins on the next slide.

Edition
30
void FloatList::insertNode(float num)
{
ListNode *newNode, *nodePtr, *previousNode;
// Allocate a new node & store Num
if (!head)
{
head = newNode;
}
else // Otherwise, insert newNode.
{
nodePtr = head;
// than num.
{
}
Continued on next slide…

Edition
31
// If the new mode is to be the 1st in the list,
// insert it before all other nodes.
if (previousNode == NULL)
{
head = newNode;
newNode-> = nodePtr;
}
else
{
previousNode->next = newNode;
newNode->next = nodePtr;
}
}
}
Continued from previous slide.

Edition
32
Program 17-3
// This program calls the displayList member function.
// The function traverses the linked list displaying
// the value stored in each node.
#include "FloatList.h”
void main(void)
{
FloatList list;
// Build the list
// Insert a node in the middle
// of the list.
list.insertNode(10.5);
// Dispay the list
list.displayList();
}

Edition
33
Program 17-3 Output
2.5
7.9
10.5
12.6

Edition
34
In insertNode, a new node is created and the function argument is
copied to its value member. Since the list already has nodes stored
in it, the else part of the if statement will execute. It begins by
assigning nodePtr to head.

Edition
35
Since nodePtr is not NULL and nodePtr->value is less than
num, the while loop will iterate. During the iteration,
previousNode will be made to point to the node that nodePtr is
pointing to. nodePtr will then be advanced to point to the next
node.

Edition
36
Once again, the loop performs its test. Since nodePtr is not NULL
and nodePtr->value is less than num, the loop will iterate a
second time. During the second iteration, both previousNode and
nodePtr are advanced by one node in the list.

Edition
37
This time, the loop's test will fail because nodePtr is not less than
num. The statements after the loop will execute, which cause
previousNode->next to point to newNode, and
newNode->next to point to nodePtr.
If you follow the links, from the head pointer to the NULL, you will
see that the nodes are stored in the order of their value members.

Edition
38
Deleting a Node
• Deleting a node from a linked list requires
two steps:
– Remove the node from the list without breaking
the links created by the next pointers
– Deleting the node from memory
• The deleteNode function begins on the
next slide.

Edition
39
void FloatList::deleteNode(float num)
{
ListNode *nodePtr, *previousNode;
// If the list is empty, do nothing.
if (!head)
return;
// Determine if the first node is the one.
if (head->value == num)
{
nodePtr = head->next;
delete head;
head = nodePtr;
}

Edition
40
else
{
nodePtr = head;
// Skip all nodes whose value member is
// not equal to num.
while (nodePtr != NULL && nodePtr->value != num)
{
}
// Link the previous node to the node after
// nodePtr, then delete nodePtr.
previousNode->next = nodePtr->next;
delete nodePtr;
}
}

Edition
41
Program 17-4
// This program demonstrates the deleteNode member function
#include "FloatList.h“
void main(void)
{
FloatList list;
// Build the list
cout << "Here are the initial values:n";
list.displayList();
cout << endl;
cout << "Now deleting the node in the middle.n";
cout << "Here are the nodes left.n";
list.deleteNode(7.9);
list.displayList();
cout << endl;

Edition
42
cout << "Now deleting the last node.n";
list.displayList();
cout << endl;
cout << "Now deleting the only remaining node.n";
list.displayList();
}

Edition
43
Program Output
Here are the initial values:
2.5
7.9
12.6
Now deleting the node in the middle.
Here are the nodes left.
2.5
12.6
Now deleting the last node.
2.5
Now deleting the only remaining node.

Edition
44
Look at the else part of the second if statement. This is where the
function will perform its action since the list is not empty, and the
first node does not contain the value 7.9. Just like insertNode,
this function uses nodePtr and previousNode to traverse the
list. The while loop terminates when the value 7.9 is located. At this
point, the list and the other pointers will be in the state depicted in
the figure below.

Edition
45
next, the following statement executes.
The statement above causes the links in the list to bypass the node
that nodePtr points to. Although the node still exists in memory, this
removes it from the list.
The last statement uses the delete operator to complete the total
deletion of the node.

Edition
46
Destroying the List
• The class's destructor should release all the
memory used by the list.
• It does so by stepping through the list,
deleting each node one-by-one. The code is
shown on the next slide.

Edition
47
FloatList::~FloatList(void)
{
ListNode *nodePtr, *nextNode;
nodePtr = head;
while (nodePtr != NULL)
{
nextNode = nodePtr->next;
delete nodePtr;
nodePtr = nextNode;
}
}
Notice the use of nextNode instead of previousNode. The
nextNode pointer is used to hold the position of the next node in
the list, so it will be available after the node pointed to by nodePtr
is deleted.

Edition
48
17.3 A Linked List Template
#ifndef LINKEDLIST_H
#define LINKEDLIST_H
template <class T>
class LinkedList
{
private:
// Declare a structure for the list
struct ListNode
{
T value;
};
ListNode *head; // List head pointer

Edition
49
public:
LinkedList(void) // Constructor
{ head = NULL; }
~LinkedList(void); // Destructor
void appendNode(T);
void insertNode(T);
void deleteNode(T);
void displayList(void);
};
// appendNode appends a node containing the
// value pased into num, to the end of the list.
template <class T>
void LinkedList<T>::AppendNode(T num)
{
ListNode *newNode, *nodePtr;
// Allocate a new node & store num

Edition
50
if (!head)
head = newNode;
{
nodePtr = head;
}
}

Edition
51
// DisplayList shows the value
// stored in each node of the linked list
// pointed to by head.
template <class T>
void LinkedList<T>::DisplayList(void)
{
ListNode *nodePtr;
nodePtr = head;
while (nodePtr)
{
cout << nodePtr->value << endl;
}
}

Edition
52
// The insertNode function inserts a node with
// num copied to its value member.
template <class T>
void LinkedList<T>::insertNode(T num)
{
ListNode *newNode, *nodePtr, *previousNode;
// Allocate a new node & store Num
if (!head)
{
head = newNode;
}

Edition
53
{
nodePtr = head;
// than num.
{
}
// Insert the node after the one pointed to
// by previousNode and before the one pointed to
// by nodePtr.
previousNode->next = newNode;
newNode->next = nodePtr;
}
}

Edition
54
// The deleteNode function searches for a node
// with Num as its value. The node, if found, is
// deleted from the list and from memory.
template <class T>
void LinkedList<T>::deleteNode(T num)
{
ListNode *nodePtr, *previousNode;
// If the list is empty, do nothing.
if (!head)
return;
// Determine if the first node is the one.
if (head->value == num)
{
nodePtr = head->next;
delete head;
head = nodePtr;
} Continued on next slide…

Edition
55
else
{
nodePtr = head;
// Skip all nodes whose value member is
// not equal to num.
while (nodePtr != NULL && nodePtr->value != num)
{
}
// Link the previous node to the node after
// nodePtr, then delete nodePtr.
delete nodePtr;
}
}

Edition
56
// Destructor
// This function deletes every node in the list.
template <class T>
LinkedList<T>::~LinkedList(void)
{
ListNode *nodePtr, *nextNode;
nodePtr = head;
while (nodePtr != NULL)
{
nextNode = nodePtr->next;
delete nodePtr;
nodePtr = nextNode;
}
}
#endif

Edition
57
Program 17-5
// This program demonstrates the linked list template.
#include "LinkedList.h“
void main(void)
{
LinkedList<int> list;
// Build the list
list.appendNode(2);
list.appendNode(4);
list.appendNode(6);
cout << "Here are the initial values:n";
list.displayList();
cout << endl;

Edition
58
cout << "Now inserting the value 5.n";
list.insertNode(5);
cout << "Here are the nodes now.n";
list.displayList();
cout << endl;
cout << "Now deleting the last node.n";
list.deleteNode(6);
list.displayList();
}

Edition
59
Program Output
Here are the initial values:
2
4
6
Now inserting the value 5.
Here are the nodes now.
2
4
5
6
Now deleting the last node.
2
4
5

Edition
60
17.4 Variations of the Linked List
The Doubly-Linked List

Edition
61
17.4 Variations of the Linked
List
The Circular Linked List

Edition
62
17.5 The STL list Container
• The list container, found in the Standard
Template Library, is a template version of a
doubly linked list.
• STL lists can insert elements, or add elements
to their front quicker than vectors can, because
lists do not have to shift the other elements.
• lists are also efficient at adding elements at
their back because they have a built-in pointer to
the last element in the list (no traversal
required).

Edition
63
Member Function Examples & Description
back cout << list.back() << endl;
The back member function returns a reference to the
last element in the list.
erase list.erase(iter);
list.erase(firstIter, lastIter)
The first example causes the list element pointed to by the iterator
iter to be removed. The second example causes all of the list
elements from firstIter to lastIter to be removed.
empty if (list.empty())
The empty member function returns true if the list is empty. If
the list has elements, it returns false.

Edition
64
end iter = list.end();
end returns a bi-directional iterator to the end of the list.
front cout << list.front() << endl;
front returns a reference to the first element of the list.
insert list.insert(iter, x)
The insert member function inserts an element into the list. The
example shown above inserts an element with the value x, just before
the element pointed to by iter.
merge list1.merge(list2);
merge inserts all the items in list2 into list1. list1 is
expanded to accommodate the new elements plus any elements
already stored in list1. merge expects both lists to be sorted.
When list2 is inserted into list1, the elements are inserted into
their correct position, so the resulting list is also sorted.

Edition
65
pop_back list.pop_back();
pop_back removes the last element of the list.
pop_front list.pop_front();
pop_front removes the first element of the list.
push_back list.push_back(x);
push_back inserts an element with value x at the end of
the list.
push_front list.push_front(x);
push_front inserts an element with value x at the beginning of the
list.
reverse list.reverse();
reverse reverses the order in which the elements appear in the list.

Edition
66
size() Returns the number of elements in the list.
swap list1.swap(List2)
The swap member function swaps the elements stored in two
lists. For example, assuming list1 and list2 are lists, the
statement shown above will exchange the values in the two.
unique list.unique();
unique removes any element that has the same value as the element
before it.

Edition
67
Program 17-6
// This program demonstrates the STL list container.
#include <list> // Include the list header
using namespace std; // Required by some compilers
void main(void)
{
list<int> myList;
list<int>::iterator iter;
// Add values to the list
for (int x = 0; x < 100; x += 10)
myList.push_back(x);
// Display the values
for (iter = myList.begin(); iter != myList.end(); iter++)
cout << *iter << " ";
cout << endl;

Edition
68
// Now reverse the order of the elements
myList.reverse();
// Display the values again
for (iter = myList.begin(); iter != myList.end(); iter++)
cout << *iter << " ";
cout << endl;
}
Program Output
0 10 20 30 40 50 60 70 80 90
90 80 70 60 50 40 30 20 10 0

Ch17

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