Lecture 23: Pointers. 2 Lecture Contents: t Pointers and addresses t Pointers and function arguments t Pointers and arrays t Pointer arrays t Demo programs.

Lecture 23: Pointers

2 Lecture Contents: t Pointers and addresses t Pointers and function arguments t Pointers and arrays t Pointer arrays t Demo programs t 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: t based on direct addressing t 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 operator ptr = 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++ notation cout<< "\n" << alpha << " " << *ptr; // C notation printf(“\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 t 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]; } t 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; }

21 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 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 call cout << fact(5); pf = fact; cout << (*pf)(5);

25 More on Pointers Extract from Friedman/Koffman, chapter 13

Pointers & Dynamic Data Structures Chapter 13

27 Dynamic Data Structures t Arrays & structs are static (compile time) t Dynamic expand as program executes t Linked list is example of dynamic data structure Node Pointer Linked list

28 13.1 Pointers and the “new” Operator t Pointer Declarations –pointer variable of type “pointer to float” –can store the address of a float in p float*p; t The new operator creates a variable of type float and puts the address of the variable in pointer p p = new float; t Dynamic allocation - program execution

29 Pointers t Actual address has no meaning t Form:type*variable; t Example:float *p; ? P

30 new Operator t Actually allocates storage t Form:new type; new type [n]; t Example:new float;

31 Accessing Data with Pointers t * - indirection operator *p = 15.5; t Stores floating value 15.5 in memory location *p - the location pointed to by p 15.5 p

32 Pointer Statements float*p; p = new float; *p = 15.5; cout << “The contents of the memory cell pointed to by p is “ << *p << endl; Output The contents of memory cell pointed to by p is 15.5

33 Pointer Operations t Pointers can only contain addresses t So the following are errors: –p = 1000; –p = 15.5; t Assignment of pointers if q & p are the same pointer type –q = p; t Also relational operations == and !=

34 13.2 Manipulating the Heap t When new executes where is struct stored ? t Heap –C++ storage pool available to new operator t Effect of p = new node; t Figure 14.1 shows Heap before and after executing new operator

35 Effect on new on the Heap

36 Returning Cells to the Heap t Operation –delete p; t Returns cells back to heap for re-use t When finished with a pointer delete it t Watch dual assignments and initialization t Form:deletevariable; t Example:deletep;

37 13.3 Linked Lists t Arrange dynamically allocated structures into a new structure called a linked list t Think of a set of children’s pop beads t Connecting beads to make a chain t You can move things around and re-connect the chain t We use pointers to create the same effect

38 Children’s Beads

39 Declaring Nodes t If a pointer is included in a struct we can connect nodes struct node { string word; int count; node *link; }; node*p, *q, *r;

40 Declaring Nodes t Each var p, q and r can point to a struct of type node –word(string) –count(int) –link(pointer to a node address) wordcountlink Struct of type node String Integer Address

41 Connecting Nodes t Allocate storage of 2 nodes p = new node; q = new node; t Assignment Statements p->word = “hat”; p->count = 2; q->word = “top”; q->count = 3;

42 Figure 13.3

43 Connecting Nodes t Link fields undefined until assignment p->link = q; t Address of q is stored in link field pointed to by p t Access elements as follows q->word or p->link->word t Null stored at last link field q->link = NULL; or p->link->link = NULL;

44 Connecting Nodes

45 Inserting a Node t Create and initialize node r = new node; r->word = “the”; r->count = 5; t Connect node pointed to by p to node pointed to by r p->link = r; t Connect node pointed to by r to node pointed to t by q r->link = q;

46 Inserting a New Node in a List

47 Insertion at Head of List t OldHead points to original list head oldHead = p; t Point p to a new node p = new node; t Connect new node to old list head p->link = oldHead;

48 Insertion at Head of List

49 Insertion at End of List t Typically less efficient (no pointer) t Attach new node to end of list last->link = new node; t Mark end with a NULL last->link->link = NULL;

50 Insertion at End of List

51 Deleting a Node t Adjust the link field to remove a node t Disconnect the node pointed to by r p->link = r->link; t Disconnect the node pointed to by r from its successor r->link = NULL; t Return node to Heap delete r;

52 Deleting a Node

53 Traversing a List t Often need to traverse a list t Start at head and move down a trail of pointers t Typically displaying the various nodes contents as the traversing continues t Advance node pointer head = head->link; t 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 word count << endl; head = head->link; }

55 Circular Lists - Two Way Option t A list where the last node points back to the first node t 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 t Implement Stack as a dynamic structure –Earlier we used arrays (chps 12, 13) t Use a linked list t The first element is s.top t New nodes are inserted at head of list t LIFO (Last-In First-Out) t StackLis.h

57 StackList.h //FILE: StackList.h #ifndef STACK_LIST_H #define STACK_LIST_H template 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 // for NULL using namespace std; // Member functions... template stackList ::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 bool stackList ::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 bool stackList ::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 bool stackList ::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 bool stackList ::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 bool stackList ::isFull() const { return false; } // end isFull

68 13.5 Queue ADT t List structure where items are added to one end and removed from the opposite end t FIFO (First-In First-Out) t Bank service line, car wash or check-out are examples of a queue t Implementing a queue as a list we added elements to the end and remove from the front t 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 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 // for NULL using namespace std; // Member functions // constructor - create an empty queue template queue ::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 bool queue ::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 bool queue ::remove(queueElement& x) { queueNode* oldFront; // Local data if (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 bool queue ::isEmpty() { return (numItems == 0); }

79 Queue.cpp // Returns queue size template int queue ::getSize() { return numItems; }

80 13.6 Binary Trees t List with additional pointer t 2 pointers –right pointer –left pointer t Binary Tree –0 - 1 or 2 successor nodes –empty –root –left and right sub-trees

81 Binary Tree

82 Binary Search Tree t Efficient data retrieval t Data stored by unique key t Each node has 1 data component t Values stored in right sub-tree are greater than the values stored in the left sub-tree t 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 t Tree created from root downward t Item 1 stored in root t Next item is attached to left tree if value is smaller or right tree if value is larger t 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 t Recursive algorithm if (tree is not empty) display left sub-tree display root display right sub-tree t In-order traversal t Pre and post order traversals

89 Example of traversal t 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 t Specification for a Binary Search Tree –rootpointer to the tree root –binaryTreea constructor –insertinserts an item –retrieveretrieves an item –searchlocates a node for a key –displaydisplays 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 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 using namespace std; // Member functions... // constructor - create an empty tree template binaryTree ::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 bool binaryTree ::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 bool binaryTree ::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 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 bool binaryTree ::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 bool binaryTree ::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 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 void binaryTree ::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 void binaryTree ::display (treeNode* aRoot) const

106 BinaryTree.cpp { if (aRoot != NULL) { // recursive step display(aRoot->left); cout info << endl; display(aRoot->right); } } // display

107 BinaryTree.cpp // Insert member functions retrieve. template bool binaryTree ::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 bool binaryTree ::retrieve (treeNode* aRoot, treeElement& el) const { return true; }

109 13.8 Efficiency of a Binary Search Tree t Searching for a target in a list is O(N) t Time is proportional to the size of the list t Binary Tree more efficient –cutting in half process t Possibly not have nodes matched evenly t Efficiency is O(log  N)

110 13.9 Common Programming Errors t Use the * de-referencing operator t Operator -> member t *p refers to the entire node t p->x refers to member x t new operator to allocate storage t delete de-allocates storage t Watch out for run-time errors with loops t Don’t try to access a node returned to heap

111 Exercise 25.1-25.6 Build programs based on pointers: t Exchange values of two integer variables (function swap); t Display a character string symbol by symbol on separate lines in forward and backward order; t Define the length of a character string (own version of strlen function); t Catenate two character strings (own version of strcat function); t Define a function returning the name of a month as a character string; t Operate as demo programs for pointers to functions.

112 Exercise 25.1-25.6 Build programs based on pointers: t 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: t 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: t 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

123 Copyright © 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 Pointer Linked list

124 Copyright © 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

127 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

128 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Pointer Statements float *p; p = new float; *p = 15.5; cout << “The contents of the memory cell pointed to by p is “ << *p << endl; Output The contents of memory cell pointed to by p is 15.5

129 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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 !=

130 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Pointers to Structs struct electric { string current; int volts; }; electric *p, *q; p and q are pointers to a struct of type electric

131 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Pointers to Structs p = new electric; Allocates storage for struct of type electric and places address into pointer p currentvolts p ??

132 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley struct Member Access through a Pointer p ->current = “AC”; p ->volts = 115; Could also be referenced as (*p).current = “AC”; (*p).volts = 115; currentvolts p AC115

134 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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; currentvolts p AC115 currentvolts q AC115

135 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Pointers and Structs q ->volts = 220; q = p; currentvolts AC220 q AC 115 AC220 pq->currentq->volts p->currentp->volts q

136 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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;

138 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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;

139 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

141 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Declaring Nodes If a pointer is included in a struct we can connect nodes struct node { string word; int count; node *link; }; node *p, *q, *r;

142 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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 countlink Struct of type node String Integer Address

143 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Connecting Nodes Allocate storage of 2 nodes p = new node; q = new node; Assignment Statements p->word = “hat”; p->count = 2; q->word = “top”; q->count = 3;

145 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Connecting Nodes Link fields are undefined until assignment p->link = q; –Address of q is stored in link field pointed to by p Accessing elements q->word or p->link->word Null stored at last link field q->link = NULL; or p->link->link = NULL;

147 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Inserting a Node Create and initialize node r = 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;

149 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Insertion at Head of List OldHead points to original list head oldHead = p; Point p to a new node p = new node; Connect new list head to old list head p->link = oldHead;

151 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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 list last->link = new node; Mark end with a NULL (from cstdlib) last->link->link = NULL;

153 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Deleting a Node Adjust the link fields to remove a node Disconnect the node pointed to by r from its predecessor p->link = r->link; Disconnect the node pointed to by r from its successor r->link = NULL; Return node to heap delete r;

155 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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 traverse head = head->link; Watch use of reference parameters

156 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Listing 13.1 Function printList // File: printList.cpp // Display the list pointed to by head void printList (listNode *head) { while (head != NULL) { cout word count << endl; head = head->link; }

157 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

158 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

159 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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&)

163 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

165 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley The C++ stack Class Must include stack library #include Declare the stack stack stack-name; E.g. stack nameStack; stack s;

167 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Example x = s.top( ); // stores ‘*’ into x, stack unchanged s.pop( ); // removes top of stack s.push(‘/’); // adds ‘/’ to top of stack *C+2*C+2 s C+2C+2 s /C+2/C+2 s

168 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Implementing a stack Template Class Use linked list or vector –all insertions/deletions from same end only *C+2*C+2 C+2 * s.top stack after insertion of “*” C+2C+2 C+2 s.top stack before insertion

175 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

179 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

188 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

191 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

192 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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 else search right subtree

194 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

195 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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 else insert new item in right subtree

197 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

198 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 13.7 Binary Search Tree ADT Attributes –rootpointer to the tree root Member Functions –binaryTreea constructor –insertinserts an item –retrieveretrieves an item –searchlocates a node for a key –displaydisplays a tree

207 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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)

209 Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 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

210 Thank You For Your Attention

Lecture 23: Pointers. 2 Lecture Contents: t Pointers and addresses t Pointers and function arguments t Pointers and arrays t Pointer arrays t Demo programs.

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Lecture 23: Pointers. 2 Lecture Contents: t Pointers and addresses t Pointers and function arguments t Pointers and arrays t Pointer arrays t Demo programs.

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Presentation on theme: "Lecture 23: Pointers. 2 Lecture Contents: t Pointers and addresses t Pointers and function arguments t Pointers and arrays t Pointer arrays t Demo programs."— Presentation transcript:

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