lecture 23: pointers. 2 lecture contents: t pointers and addresses t pointers and function arguments...
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Lecture 23:Pointers
2
Lecture Contents:
Pointers and addresses Pointers and function arguments Pointers and arrays Pointer arrays Demo programs Exercises
3
Pointer basics
Pointers are variables that contain address values.
Pointers are variables used to store address values.
The basic concept of a pointer is:
indirect access to data values
4
Pointer basics
Given two integer variables alpha and beta. Variable alpha is defined, declared, initialized or assigned the value of 5.
int alpha=5, beta; Problem: To copy the value of alpha(5) to beta
Possible Solutions: based on direct addressing based on indirect addressing
(pointers)
5
Pointer basics
Direct access to data values:
int alpha=5; int beta;
beta = alpha;
6
Pointer basics
Indirect access to data values:
int alpha=5, beta, *ptr;
ptr = α
beta = *ptr;
7
Pointer basics
Indirect access to data values:int alpha=5, beta, *ptr;
// & - address of operatorptr = α
//indirection or dereferencing operator
beta = *ptr;
8
Declaration of pointers
How to define (declare) pointers as variables?
int *p1;
p1 is a variable whose memory space will be used to store addresses of integer data.
9
Declaration of pointers
How to define (declare) pointers as variables?
char *p2;
p2 is a variable whose memory space will be used to store addresses of character data.
10
Declaration of pointers
How to define (declare) pointers as variables?
double *p3;
p3 is a variable whose memory space will be used to store addresses of double data.
11
How to use pointers?
int alpha=5, *ptr;ptr = α
// display alpha value/contents by direct/indirect addressing// C++ notationcout<< "\n" << alpha << " " << *ptr;
// C notationprintf(“\n%d %d”, alpha, *ptr);
12
How to use pointers?
int alpha=5, *ptr=α
// display address of alpha, I.e. contents of ptr
// C++ notation
cout << "\n " << &alpha << " " << ptr;
// C notation
printf(“\n%d %u %o %x %X %p”, ptr,ptr,ptr,ptr,ptr,ptr);
13
More on Pointers
Pointers and Addresses
14
More on Pointers and Arrays
loop to traverse all array elements using direct access based on array subscripting expressions
int a[10]; for (I=0;I<10;I++) { a[I]=I; cout << endl << a[I]; }
loop to traverse all array elements using indirect access based on pointers
int a[10]; int *pa; pa = &a[0]; for (I=0;I<10;I++) { *pa=I; cout << endl << *pa; pa++; }
15
More on Pointers and Arrays
char amessage[] = “Now is the time!”;
char *pmessage;
pmessage = “Now is the time!”;
16
More on Pointers and Arrays
char amessage[] = “Now is the time!”;
int I=0;
while(amessage[I] != ‘\0’)
{
cout << endl << amessage[I];
I++;
}
17
More on Pointers and Arrays
char *pmessage = “Now is the time!”; while(*pmessage != ‘\0’) {
cout << *pmessage; pmessage++; }
=================================================== char *pmessage = “Now is the time!”; char *q; q = pmessage + strlen(pmessage); while( pmessage < q ) {
cout << *pmessage; pmessage++; }
18
More on Pointers
Array of pointers
char *pname[] = { “Illegal”, “Jan”, “Feb”, . . .
“Nov”, “Dec” };
char aname[][15] ={ “Illegal”, “Jan”, “Feb”,
. . . “Nov”, “Dec” };
19
More on Pointers
Multidimensional arrays and pointers
int a[10][20];
int *b[10];
20
Pointers and function arguments
Problem: function to swap contents of two variables: void swap(int, int);
swap(a, b);
void swap(int p1, int p2){
int temp; temp=p1; p1=p2; p2=temp;}
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Pointers and function arguments
The solution: addresses specified as actual arguments void swap(int *, int *);
swap(&a, &b);
void swap(int *p1, int *p2){
int temp; temp=*p1; *p1=*p2; *p2=temp;}
22
More on Pointers
Address arithmetic:
char a[10];
a ≡ a + 0 ≡ &a[0]
a + I ≡ &a[I]
*(a+I) ≡ *&a[I] ≡ a[I]
23
More on Pointers
Address arithmetic: int a[10], *p, *q;
Assigning initial value to a pointer: p=q=a;
Increment/decrement pointer: p++; p++; p--;
Add a constant to pointer: q=q+8;
Subtract constant from a pointer: q-=4;
Comparison of two pointers: if(p<q) while(q>p)
Subtraction of two pointers: p=a; q=a+10; q-p is 10
24
More on Pointers
Pointers to functions int fact(int n) { if(n==0) return 1; return n*fact(n-1); }
int (*pf)(int); // pointer to function that has// one int param and returns int
Direct function call Indirect function callcout << fact(5); pf = fact;
cout << (*pf)(5);
25
More on Pointers
Extract from Friedman/Koffman, chapter 13
Pointers & Dynamic Data Structures
Chapter 13
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Dynamic Data Structures
Arrays & structs are static (compile time) Dynamic expand as program executes Linked list is example of dynamic data
structure
Node Node Node
PointerPointer
Linked list
28
13.1 Pointers and the “new” Operator
Pointer Declarations– pointer variable of type “pointer to float”
– can store the address of a float in p
float *p; The new operator creates a variable of type float
and puts the address of the variable in pointer pp = new float;
Dynamic allocation - program execution
29
Pointers
Actual address has no meaning
Form: type *variable; Example: float *p;
?
P
30
new Operator
Actually allocates storage
Form: new type;
new type [n];
Example: new float;
31
Accessing Data with Pointers
* - indirection operator*p = 15.5;
Stores floating value 15.5 in memory location *p - the location pointed to by p
15.5
p
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Pointer Statements
float *p;p = new float;*p = 15.5;cout << “The contents of the memory cell pointed to
by p is “ << *p << endl;
OutputThe contents of memory cell pointed to by p is 15.5
33
Pointer Operations
Pointers can only contain addresses So the following are errors:
– p = 1000;– p = 15.5;
Assignment of pointers if q & p are the same pointer type– q = p;
Also relational operations == and !=
34
13.2 Manipulating the Heap
When new executes where is struct stored ?
Heap– C++ storage pool available to new operator
Effect of p = new node; Figure 14.1 shows Heap before and after
executing new operator
35
Effect on new on the Heap
36
Returning Cells to the Heap
Operation– delete p;
Returns cells back to heap for re-use When finished with a pointer delete it Watch dual assignments and initialization
Form: delete variable; Example: delete p;
37
13.3 Linked Lists
Arrange dynamically allocated structures into a new structure called a linked list
Think of a set of children’s pop beads Connecting beads to make a chain You can move things around and re-connect
the chain We use pointers to create the same effect
38
Children’s Beads
39
Declaring Nodes
If a pointer is included in a struct we can connect nodesstruct node
{
string word;
int count;
node *link;
};
node *p, *q, *r;
40
Declaring Nodes
Each var p, q and r can point to a struct of type node– word (string)– count (int)– link (pointer to a node address)
word count link
Struct of type node
String Integer Address
41
Connecting Nodes
Allocate storage of 2 nodesp = new node;q = new node;
Assignment Statementsp->word = “hat”;p->count = 2;q->word = “top”;q->count = 3;
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Figure 13.3
43
Connecting Nodes
Link fields undefined until assignmentp->link = q;
Address of q is stored in link field pointed to by p
Access elements as followsq->word or p->link->word
Null stored at last link fieldq->link = NULL; or p->link->link = NULL;
44
Connecting Nodes
45
Inserting a Node
Create and initialize noder = new node;r->word = “the”;r->count = 5;
Connect node pointed to by p to node pointed to by rp->link = r;
Connect node pointed to by r to node pointed to by q
r->link = q;
46
Inserting a New Node in a List
47
Insertion at Head of List
OldHead points to original list headoldHead = p;
Point p to a new nodep = new node;
Connect new node to old list headp->link = oldHead;
48
Insertion at Head of List
49
Insertion at End of List
Typically less efficient (no pointer) Attach new node to end of list
last->link = new node; Mark end with a NULL
last->link->link = NULL;
50
Insertion at End of List
51
Deleting a Node
Adjust the link field to remove a node Disconnect the node pointed to by r
p->link = r->link; Disconnect the node pointed to by r from its
successorr->link = NULL;
Return node to Heapdelete r;
52
Deleting a Node
53
Traversing a List
Often need to traverse a list Start at head and move down a trail of
pointers Typically displaying the various nodes
contents as the traversing continues Advance node pointer
head = head->link; Watch use of reference parameters
54
PrintList.cpp
// FILE: PrintList.cpp
// DISPLAY THE LIST POINTED TO BY HEAD
void printList (listNode *head)
{
while (head != NULL)
{
cout << head->word << " " << head ->count << endl;
head = head->link;
}
}
55
Circular Lists - Two Way Option
A list where the last node points back to the first node
Two way list is a list that contains two pointers– pointer to next node– pointer to previous node
56
13.4 Stacks as Linked Lists
Implement Stack as a dynamic structure– Earlier we used arrays (chps 12, 13)
Use a linked list The first element is s.top New nodes are inserted at head of list LIFO (Last-In First-Out) StackLis.h
57
StackList.h
//FILE: StackList.h
#ifndef STACK_LIST_H#define STACK_LIST_H
template <class stackElement>class stackList{ public: // Member functions ... // CONSTRUCTOR TO CREATE AN EMPTY STACK stackList ();
58
StackList.h
bool push (const stackElement& x); bool pop (stackElement& x); bool peek (stackElement& x) const; bool isEmpty () const; bool isFull () const;private: struct stackNode { stackElement item;
stackNode* next; };// Data member
stackNode* top;};
#endif // STACK_LIST_H
59
StackList.cpp
//Implementation of template class stack as a linked list
#include "stackList.h"
#include <cstdlib> // for NULL
using namespace std;
// Member functions ...
template <class stackElement>
stackList<stackElement>::stackList()
{
top = NULL;
} // end stackList
60
StackList.cpp
// Push an element onto the stack
// Pre: The element x is defined.
// Post: If there is space on the heap,
// the item is pushed onto the stack and
// true is returned. Otherwise, the
// stack is unchanged and false is
// returned.
61
StackList.cpp
template <class stackElement>bool stackList<stackElement>::push(const stackElement& x) {
stackNode* oldTop; bool success; // Local data oldTop = top; top = new stackNode; if (top == NULL) { top = oldTop; success = false; } else { top->next = oldTop;
top->item = x; success = true; } return success;} // end push
62
StackList.cpp
// Pop an element off the stack
// Pre: none
// Post: If the stack is not empty, the value
// at the top of the stack is removed, its
// value is placed in x, and true is returned.
// If stack empty, x is not defined and false returned.
63
StackList.cpp
template <class stackElement>bool stackList<stackElement>::pop(stackElement& x) { stackNode* oldTop; bool success; if (top == NULL) success = false; else { x = top->item; oldTop = top; top = oldTop->next; delete oldTop; success = true; } return success; } // end pop
64
StackList.cpp
// Get top element from stack without popping
// Pre: none
// Post: If the stack is not empty, the value
// at the top is copied into x and true is
// returned. If the stack is empty, x is not
// defined and false is returned. In
// either case, the stack is not changed.
65
StackList.cpp
template <class stackElement>bool stackList<stackElement>::peek(stackElement& x) const{ bool success; // Local data if (top == NULL) success = false; else { x = top->item; success = true;
} return success; } // end peek
66
StackList.cpp
// Test to see if stack is empty
// Pre : none
// Post: Returns true if the stack is empty;
// otherwise, returns false.
template <class stackElement>
bool stackList<stackElement>::isEmpty() const
{
return top == NULL;
} // end isEmpty
67
StackList.cpp
// Test to see if stack is full
// Pre : none
// Post: Returns false. List stacks are never
// full. (Does not check heap availability.)
template <class stackElement>
bool stackList<stackElement>::isFull() const
{
return false;
} // end isFull
68
13.5 Queue ADT
List structure where items are added to one end and removed from the opposite end
FIFO (First-In First-Out) Bank service line, car wash or check-out are
examples of a queue Implementing a queue as a list we added elements
to the end and remove from the front Queue.h
69
Queue of Customers
70
Queue.h
// FILE: Queue.h// DEFINITION AND IMPLEMENTATION OF A TEMPLATE// CLASS QUEUE USING A LINKED LIST
#ifndef QUEUE_H#define QUEUE_H
template<class queueElement>class queue{ public: queue ();
71
Queue.h
bool insert (const queueElement& x); bool remove (queueElement& x);
bool isEmpty ();int getSize ();
private: struct queueNode { queueElement item;
queueNode* next; };queueNode* front;
queueNode* rear; int numItems; };#endif // QUEUE_H
72
Queue.cpp
// File: queue.cpp// Implementation of template class queue
#include "queue.h"#include <cstdlib> // for NULLusing namespace std;
// Member functions // constructor - create an empty queuetemplate<class queueElement>queue<queueElement>::queue(){ numItems = 0; front = NULL; rear = NULL;}
73
Queue.cpp
{
numItems = 0;
front = NULL;
rear = NULL;
}
// Insert an element into the queue
// Pre : none
// Post: If heap space is available, the
// value x is inserted at the rear of the queue
// and true is returned. Otherwise, the queue is
// not changed and false is returned.
74
Queue.cpp
// Insert an element into the queue
// Pre : none
// Post: If heap space is available, the
// value x is inserted at the rear of the queue
// and true is returned. Otherwise, the queue is
// not changed and false is returned.
75
Queue.cpp
template<class queueElement>bool queue<queueElement>::insert(const queueElement& x) { if (numItems == 0){ rear = new queueNode; if (rear == NULL)return false; else front = rear; }
else { rear->next = new queueNode; if (rear->next == NULL) return false; else rear = rear->next; } rear->item = x; numItems++; return true; } // end insert
76
Queue.cpp
// Remove an element from the queue
// Pre : none
// Post: If the queue is not empty, the value at
// the front of the queue is removed, its value
// is placed in x, and true is returned. If the
// queue is empty, x is not defined and false
// is returned.
77
Queue.cpp
template<class queueElement>bool queue<queueElement>::remove(queueElement& x){
queueNode* oldFront; // Local dataif (numItems == 0) {
return false; } else {
oldFront = front; x = front->item; front = front->next; oldFront->next = NULL; delete oldFront; numItems--; return true;
}} // end remove
78
Queue.cpp
// Test whether queue is empty
template<class queueElement>
bool queue<queueElement>::isEmpty()
{
return (numItems == 0);
}
79
Queue.cpp
// Returns queue size
template<class queueElement>
int queue<queueElement>::getSize()
{
return numItems;
}
80
13.6 Binary Trees
List with additional pointer 2 pointers
– right pointer– left pointer
Binary Tree – 0 - 1 or 2 successor nodes– empty– root – left and right sub-trees
81
Binary Tree
82
Binary Search Tree
Efficient data retrieval Data stored by unique key Each node has 1 data component Values stored in right sub-tree are greater
than the values stored in the left sub-tree Above must be true for all nodes in the
binary search tree
83
Searching Algorithm
if (tree is empty)
target is not in the tree
else if (the target key is the root)
target found in root
else if (target key smaller than the root’s key)
search left sub-tree
else
search right sub-tree
84
Searching for Key 42
85
Building a Binary Search Tree
Tree created from root downward Item 1 stored in root Next item is attached to left tree if value is
smaller or right tree if value is larger When inserting an item into existing tree
must locate the items parent and then insert
86
Algorithm for Insertion
if (tree is empty)
insert new item as root
else if (root key matches item)
skip insertion duplicate key
else if (new key is smaller than root)
insert in left sub-tree
else
insert in right sub-tree
87
Figure 13.18 Building a Tree
88
Displaying a Binary Search Tree
Recursive algorithm
if (tree is not empty)
display left sub-tree
display root
display right sub-tree In-order traversal Pre and post order traversals
89
Example of traversal
Trace of Figure 13.18– Display left sub-tree of node 40– Display left sub-tree of node 20– Display left sub-tree of node 10– Tree is empty - return left sub-tree node is 10– Display item with key 10– Display right sub-tree of node 10
90
Example of traversal
– Tree is empty - return from displaying right sub-tree node is 10
– Return from displaying left sub-tree of node 20– Display item with key 20– Display right sub-tree of node 20– Display left sub-tree of node 30– Tree is empty - return from displaying left sub-
tree of node 30– Display item with key 30
91
Example of traversal
– Display right sub-tree of node 30– Tree is empty - return from displaying right
sub-tree of node 30– Return from displaying right sub-tree of node
20– Return from displaying left sub-tree of node 40– Display item with key 40– Display right sub-tree of node 40
92
13.7 Binary Search Tree ADT
Specification for a Binary Search Tree– root pointer to the tree root– binaryTree a constructor– insert inserts an item– retrieve retrieves an item– search locates a node for a key– display displays a tree
93
BinaryTree.h
// FILE: BinaryTree.h
// DEFINITION OF TEMPLATE CLASS BINARY SEARCH TREE
#ifndef BIN_TREE_H
#define BIN_TREE_H
// Specification of the class
template<class treeElement>
class binTree
{
94
BinaryTree.h
public: // Member functions ... // CREATE AN EMPTY TREE binTree (); // INSERT AN ELEMENT INTO THE TREE bool insert (const treeElement& el );
// RETRIEVE AN ELEMENT FROM THE TREE bool retrieve (treeElement& el ) const;
// SEARCH FOR AN ELEMENT IN THE TREE bool search (const treeElement& el ) const;
// DISPLAY A TREE void display () const;
95
BinaryTree.h
private: // Data type ... struct treeNode { treeElement info; treeNode* left; treeNode* right;
};
96
BinaryTree.h
// Data member .... treeNode* root; // Member functions ... // Searches a subtree for a key bool search (treeNode*, const treeElement&) const; // Inserts an item in a subtree bool insert (treeNode*&, const treeElement&) const; // Retrieves an item in a subtree bool retrieve (treeNode*, treeElement&) const;
// Displays a subtree void display (treeNode*) const; };#endif // BIN_TREE_H
97
BinaryTree.cpp
// File: binaryTree.cpp// Implementation of template class binary search tree#include "binaryTree.h"#include <iostream>using namespace std;// Member functions ...// constructor - create an empty treetemplate<class treeElement>binaryTree<treeElement>::binaryTree(){ root = NULL;}
98
BinaryTree.cpp
// Searches for the item with same key as el// in a binary search tree.// Pre : el is defined.// Returns true if el's key is located,// otherwise, returns false.template<class treeElement>bool binaryTree<treeElement>::search (const treeElement& el) const { return search(root, el); } // search
99
BinaryTree.cpp
// Searches for the item with same key as el // in the subtree pointed to by aRoot. Called// by public search.// Pre : el and aRoot are defined.// Returns true if el's key is located,// otherwise, returns false.template<class treeElement>bool binaryTree<treeElement>::search (treeNode* aRoot,const treeElement& el)
const{ if (aRoot == NULL)
100
BinaryTree.cpp
return false;
else if (el == aRoot->info)
return true;
else if (el <= aRoot->info)
return search(aRoot->left, el);
else
return search(aRoot->right, el);
} // search
101
BinaryTree.cpp
// Inserts item el into a binary search tree.// Pre : el is defined.// Post: Inserts el if el is not in the tree.// Returns true if the insertion is performed.// If there is a node with the same key value // as el, returns false.template<class treeElement>bool binaryTree<treeElement>::insert (const treeElement& el){ return insert(root, el);} // insert
102
BinaryTree.cpp
// Inserts item el in the tree pointed to by // aRoot.// Called by public insert.// Pre : aRoot and el are defined.// Post: If a node with same key as el is found,// returns false. If an empty tree is reached,// inserts el as a leaf node and returns true.template<class treeElement>bool binaryTree<treeElement>::insert (treeNode*& aRoot, const treeElement& el){
103
BinaryTree.cpp
// Check for empty tree. if (aRoot == NULL) { // Attach new node aRoot = new treeNode; aRoot->left = NULL; aRoot->right = NULL; aRoot->info = el; return true; } else if (el == aRoot->info) return false;
104
BinaryTree.cpp
else if (el <= aRoot->info)
return insert(aRoot->left, el);
else
return insert(aRoot->right, el);
} // insert
// Displays a binary search tree in key order.
// Pre : none
// Post: Each element of the tree is displayed.
// Elements are displayed in key order.
105
BinaryTree.cpp
template<class treeElement>void binaryTree<treeElement>::display() const{ display(root);} // display// Displays the binary search tree pointed to// by aRoot in key order. Called by display.// Pre : aRoot is defined.// Post: displays each node in key order.template<class treeElement>void binaryTree<treeElement>::display (treeNode* aRoot) const
106
BinaryTree.cpp
{
if (aRoot != NULL)
{ // recursive step
display(aRoot->left);
cout << aRoot->info << endl;
display(aRoot->right);
}
} // display
107
BinaryTree.cpp
// Insert member functions retrieve.
template<class treeElement>
bool binaryTree<treeElement>::retrieve
(const treeElement& el) const
{
return retrieve(root, el);
} // retrieve
108
BinaryTree.cpp
// Retrieves for the item with same key as el // in the subtree pointed to by aRoot. Called // by public search.// Pre : el and aRoot are defined.// Returns true if el's key is located,// otherwise, returns false.template<class treeElement>bool binaryTree<treeElement>::retrieve (treeNode* aRoot, treeElement& el) const{ return true;}
109
13.8 Efficiency of a Binary Search Tree
Searching for a target in a list is O(N) Time is proportional to the size of the list Binary Tree more efficient
– cutting in half process Possibly not have nodes matched evenly Efficiency is O(log N)
110
13.9 Common Programming Errors
Use the * de-referencing operator Operator -> member *p refers to the entire node p->x refers to member x new operator to allocate storage delete de-allocates storage Watch out for run-time errors with loops Don’t try to access a node returned to heap
111
Exercise 25.1-25.6
Build programs based on pointers: Exchange values of two integer variables (function swap); Display a character string symbol by symbol on separate
lines in forward and backward order; Define the length of a character string (own version of
strlen function); Catenate two character strings (own version of strcat
function); Define a function returning the name of a month as a
character string; Operate as demo programs for pointers to functions.
112
Exercise 25.1-25.6
Build programs based on pointers:
Display a character string symbol by symbol on separate lines in forward and backward order;
113
Exercise 23.1
char str[] = “AUBG Blagoevgrad”;
void main(){ int I=0;
cout << endl << str << endl;while (str[I] != ‘\0’){
cout << endl << str[I];I++;
}}
114
Exercise 23.1
char str[] = “AUBG Blagoevgrad”;
void main(){ char *p = str;
cout << endl << str << endl << p << endl;while ( *p != ‘\0’){
cout << endl << *p;p++;
}}
115
Exercise 25.1-25.6
Build programs based on pointers:
Define the length of a character string (own version of strlen function);
116
Exercise 23.1
char str[] = “AUBG Blagoevgrad”; int strlenm(char m[]);
void main(){
cout << endl << strlenm(str) << endl;}int strlenm(char m[]){
int I=0, len;while (m[I] != 0x00) I++;len = I;return len;
}
117
Exercise 23.1
char str[] = “AUBG Blagoevgrad”; int strlenm(char *pm);
void main(){
char *p = str;cout << endl << strlenm(str) << “ “ << strlenm(p) << endl;
}int strlenm(char *pm){
int len = 0;while (*pm != 0x00) { Ien++; pm++; )return len;
}
118
Exercise 25.1-25.6
Build programs based on pointers:
Copy a character string (own version of strcpy function);
119
Exercise 23.1
char str[] = “AUBG Blagoevgrad”;
void copym(char dst[], char src[]);
void main()
{ char newstr[20];
copym(newstr, str);
cout << endl << newstr << endl;
}
void copym(char dst[], char src[])
{
int I=0;
while( ( dst[I] = src[I] ) != ‘\0’ ) I++;
}
120
Exercise 23.1
char str[] = “AUBG Blagoevgrad”;
void copym(char *dst, char *src);
void main()
{ char *newstr; newstr = new char[20];
copym(newstr, str);
cout << endl << newstr << endl;
}
void copym(char *dst, char *src)
{
while( ( *dst = *src ) != ‘\0’ ) { dst++; src++; }
}
121
Before lecture end
Lecture:Pointers
More to read:
Friedman/Koffman, Chapter 13
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Chapter 13:Pointers and Dynamic Data Structures
Problem Solving,
Abstraction, and Design using C++ 5e
by Frank L. Friedman and Elliot B. Koffman
123Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Dynamic Data Structures
• Arrays & structs are static (compile time)
• Dynamic structures expand as program executes
• Linked list is example of dynamic data structure
Node Node Node
PointerPointer
Linked list
124Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
13.1 Pointers and the new Operator• Pointer Variable Declarations
– pointer variable of type “pointer to float”– can store the address of a float in p
float *p;
• The new operator creates (allocates memory for) a variable of type float & puts the address of the variable in pointer p
p = new float;
• Dynamic allocation occurs during program execution
125Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Pointers
• Actual address has no meaning for us
• Form: type *variable;
• Example: float *p;
?
P
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new Operator
• Actually allocates storage
• Form: new type;
new type [n];
• Example: new float;
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Accessing Data with Pointers
• indirection operator **p = 15.5;
• Stores floating value 15.5 in memory location *p - the location pointed to by p
15.5
p
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Pointer Statements
float *p;
p = new float;
*p = 15.5;
cout << “The contents of the memory cell pointed to by p is “
<< *p << endl;
OutputThe contents of memory cell pointed to by p is 15.5
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Pointer Operations
• Pointers can only contain memory addresses
• So the following are errors:p = 1000;p = 15.5;
• Assignment of pointers is valid if q & p are the same pointer type
q = p;
• Also relational operations == and !=
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Pointers to Structs
struct electric{ string current; int volts;};electric *p, *q;• p and q are pointers to a struct of type
electric
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Pointers to Structs
p = new electric;
• Allocates storage for struct of type electric and places address into pointer p
current voltsp? ?
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struct Member Access through a Pointer
p ->current = “AC”;p ->volts = 115;
• Could also be referenced as(*p).current = “AC”;(*p).volts = 115;
current voltspAC 115
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struct Member Access through a Pointer
• Form: p ->m
• Example: p ->voltscout << p->current << p->volts << endl;
• OutputAC115
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Pointers and Structs
q = new electric;
• Allocates storage for struct of type electric and places address into pointer q
• Copy contents of p struct to q struct
*q = *p;current voltsp
AC 115
current voltsqAC 115
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Pointers and Structs
q ->volts = 220;
q = p;
current voltsAC 220
q
AC 115
AC 220
p q->current q->voltsp->current p->volts
q
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13.2 Manipulating the Heap
• When new executes where is struct stored ?
• Heap– C++ storage pool available to new operator
• Effect of
p = new electric;
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Figure 13.1 Heap before and after execution of p - new node;
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Returning Cells to the Heap• Operation
delete p;
• Returns cells back to heap for re-use• When finished with a pointer, delete it• Watch
– multiple pointers pointing to same address– only pointers created with new are deleted
• Form: delete variable;• Example: delete p;
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13.3 Linked Lists and the list Class
• Arrange dynamically allocated structures into a new structure called a linked list
• Think of a set of children’s pop beads– Connecting beads to make a chain– You can move things around and re-connect the
chain
• We use pointers to create the same effect
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Figure 13.2 Children’s pop beads in a chain
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Declaring Nodes
• If a pointer is included in a struct we can connect nodesstruct node{
string word;int count;node *link;
};node *p, *q, *r;
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Declaring Nodes
• Each variable p, q and r can point to a struct of type node, containing members– word (string)– count (int)– link (pointer to a node address)
word count link
Struct of type node
String Integer Address
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Connecting Nodes
• Allocate storage of 2 nodesp = new node;q = new node;
• Assignment Statementsp->word = “hat”;p->count = 2;q->word = “top”;q->count = 3;
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Figure 13.3 Nodes pointed to by p and q
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Connecting Nodes
• Link fields are undefined until assignmentp->link = q;
– Address of q is stored in link field pointed to by p
• Accessing elementsq->word or p->link->word
• Null stored at last link fieldq->link = NULL; or p->link->link = NULL;
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Figure 13.4 List with two elements
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Inserting a Node
• Create and initialize noder = new node;r->word = “the”;r->count = 5;
• Connect node pointed to by p to node pointed to by r
p->link = r;
• Connect node pointed to by r to node pointed to by q
r->link = q;
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Inserting a New Node in a List
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Insertion at Head of List
• OldHead points to original list headoldHead = p;
• Point p to a new nodep = new node;
• Connect new list head to old list headp->link = oldHead;
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Figure 13.6 Insertion at the head of a list
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Insertion at End of List
• Typically less efficient (usually no pointer to end of the list), but sometimes necessary
• Attach new node to end of listlast->link = new node;
• Mark end with a NULL (from cstdlib)last->link->link = NULL;
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Figure 13.7 Insertion at the end of a list
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Deleting a Node• Adjust the link fields to remove a node• Disconnect the node pointed to by r from its
predecessorp->link = r->link;
• Disconnect the node pointed to by r from its successor
r->link = NULL;
• Return node to heapdelete r;
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Figure 13.8 Deleting a list node
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Traversing a List• Often need to traverse a list
– E.g. print contents of list
• Start at head and move down a trail of pointers– Typically displaying the various nodes contents as the
traversing continues
• Advance node pointer as you traversehead = head->link;
• Watch use of reference parameters
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Listing 13.1 Function printList
// File: printList.cpp
// Display the list pointed to by head
void printList (listNode *head)
{
while (head != NULL)
{
cout << head->word << " " << head->count << endl;
head = head->link;
}
}
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Circular Lists and Two Way Lists
• A circular list is where the last node points back to the first node
• Two way (doubly linked) list contains two pointers– pointer to next node– pointer to previous node
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The list Class
• C++ STL provides a list container class– Two-way– Can use instead of implementing your own– insert, remove from either end– traverse in either direction– use iterator to traverse
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Member Functions of list Class
int size( ) const
T front( )
T back( )
void push_back(const T&)
void push_front(const T&)
void pop_back(int)
void pop_front(int)
void insert(iterator, const T&)
void remove(const T&)
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Listing 13.2 Using list class
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Listing 13.2 Using list class (continued)
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Listing 13.2 Using list class (continued)
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13.4 The Stack Abstract Data Type
• A stack is a data structure in which only the top element can be accessed
• LIFO (Last-In First-Out)
• Operations– push– pop
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A Stack of Characters
*
C
+
2
s
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The C++ stack Class
• Must include stack library#include <stack>
• Declare the stackstack <type> stack-name;
• E.g.stack <string> nameStack;stack <char> s;
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Some stack Member Functions
void push(const T&)
T top( ) const
void pop( )
bool empty( ) const
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Example
x = s.top( ); // stores ‘*’ into x, stack unchanged
s.pop( ); // removes top of stack
s.push(‘/’); // adds ‘/’ to top of stack
*
C
+
2
s
C
+
2
s
/
C
+
2
s
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Implementing a stack Template Class• Use linked list or vector
– all insertions/deletions from same end only
*
C
+
2
C + 2*
s.top
stack after insertion of “*”
C
+
2C + 2
s.top
stack before insertion
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Listing 13.4 Header file for template class stackList
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Listing 13.4 Header file for template class stackList (continued)
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Listing 13.5 Implementation file for template class stackList
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Listing 13.5 Implementation file for template class stackList (continued)
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Listing 13.5 Implementation file for template class stackList (continued)
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Listing 13.5 Implementation file for template class stackList (continued)
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13.5 The Queue ADT
• List-like structure where items are inserted at one end and removed from the other
• First-In-First-Out (FIFO)
• E.g. a customer waiting line
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Figure 13.12 Queue of customers
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The C++ queue Class
• Must include queue library#include <queue>
• Declare the stackstack <type> queue-name;
• E.g.stack <string> customers;
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Member Functions of queue Class
void push(const T&)
T top( ) const
void pop( )
bool empty( ) const
int size( ) const
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Implementing a Queue ADT
• Implement as linked list
• Same as stack, except that element at the front of the queue is removed first– need pointer to first list element
• New elements inserted at rear– need pointer to last list element
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Listing 13.6 Header file for queue template class
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Listing 13.6 Header file for queue template class (continued)
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Listing 13.6 Header file for queue template class (continued)
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Listing 13.7 Implementation file for queue template class
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Listing 13.7 Implementation file for queue template class (continued)
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Listing 13.7 Implementation file for queue template class (continued)
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Listing 13.7 Implementation file for queue template class (continued)
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Listing 13.7 Implementation file for queue template class (continued)
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13.6 Binary Trees
• Like a list with additional pointer
• Nodes contain 2 pointers– right pointer– left pointer– 0 (leaf nodes), 1, or 2 successor nodes
• Binary Tree – empty– root
• left and right sub-trees
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Figure 13.13 Binary trees
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Additional Tree Terminology
• Disjoint subtrees
• parent
• children
• siblings
• ancestor
• descendant
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Binary Search Tree
• Efficient data retrieval
• Data stored by unique key
• Each node has 1 data component
• Each node has value that is less than all values in right subtree are greater than all values stored in the left subtree– Must be true for all nodes in the binary search
tree
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Searching a Binary Search Tree
if (tree is empty)
target is not in the tree
else if (the target key is in the root)
target found in root
else if (target key smaller than the root’s key)
search left subtree
elsesearch right subtree
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Figure 13.14 Searching for key 42
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Building a Binary Search Tree
• Process items in no particular order
• Tree created from root downward
• Item 1 stored in root
• Next item is attached to left tree if value is smaller or right tree if value is larger
• When inserting an item into existing tree must locate the item’s parent and then insert
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Algorithm for Insertion
if (tree is empty)
insert new item in tree’s root node
else if (root’s key matches new item’s key)
skip insertion - duplicate key
else if (new key is smaller than root’s key)
insert new item in left subtree
elseinsert new item in right subtree
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Figure 13.15 Building a binary search tree
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Displaying a Binary Search Tree
• Recursive algorithm
1. if (tree is not empty)
2. display left subtree
3. display root item
4. display right subtree
• Inorder traversal
• Also preorder and postorder traversals
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13.7 Binary Search Tree ADT
• Attributes– root pointer to the tree root
• Member Functions– binaryTree a constructor– insert inserts an item– retrieve retrieves an item– search locates a node for a key– display displays a tree
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Listing 13.8 Template class specification for tree<treeElement>
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Listing 13.8 Template class specification for tree<treeElement> (continued)
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Listing 13.8 Template class specification for tree<treeElement> (continued)
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Listing 13.9 Member functions binaryTree and search
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Listing 13.9 Member functions binaryTree and search (continued)
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Listing 13.10 Member functions insert
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Listing 13.10 Member functions insert (continued)
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Listing 13.11 Member functions display
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13.8 Efficiency of a Binary Search Tree
• Searching for a target in a linked list is O(N)– Time is proportional to the size of the list
• Binary Tree more efficient– because of cutting in half process
• Possibly not have nodes matched evenly• Best case efficiency is O(log N)
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Values of N versus log2N
N log2N
32 5
64 6
128 7
256 8
512 9
1024 10
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13.9 Common Programming Errors
• Syntax Errors– Misuse of * and ->– Misuse of new and delete
• Run-Time Errors– Missing braces– NULL pointer reference– Pointers as reference parameters– Heap overflow and underflow– Referencing a node on the heap
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