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1154 Data Structures and Collections Chapter 23 145 // loop while current node is not lastNode 146 while ( current.Next != lastNode ) 147 current = current.Next; // move to next node 148 149 // current is new lastNode 150 lastNode = current; 151 current.Next = null; 152 } 153 154 return removeItem; // return removed data 155 } 156 } 157 158 // return true if List is empty 159 public bool IsEmpty() 160 { 161 lock ( this ) 162 { 163 return firstNode == null; 164 } 165 } 166 167 // output List contents 168 virtual public void Print() 169 { 170 lock ( this ) 171 { 172 if ( IsEmpty() ) 173 { 174 Console.WriteLine( “Empty ” + name ); 175 return; 176 } 177 178 Console.Write( “The ” + name + ” is: ” ); 179 180 ListNode current = firstNode; 181 182 // output current node data while not at end of list 183 while ( current != null ) 184 { 185 Console.Write( current.Data + ” ” ); 186 current = current.Next; 187 } 188 189 Console.WriteLine( “n” ); 190 } 191 } 192 193 } // end class List 194 Fig. 23.4 Fig. 23.4Fig. 23FiFi.4g. 23.4g.23.4Definitions of classes ListNode, Listand EmptyListException. (Part 4 of 5.)
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Chapter 23 Data Structures and Collections 1153 94 lock ( this ) 95 { 96 if ( IsEmpty() ) 97 firstNode = lastNode = 98 new ListNode( insertItem ); 99 100 else 101 lastNode = lastNode.Next = 102 new ListNode( insertItem ); 103 } 104 } 105 106 // remove first node from List 107 public object RemoveFromFront() 108 { 109 lock ( this ) 110 { 111 if ( IsEmpty() ) 112 throw new EmptyListException( name ); 113 114 object removeItem = firstNode.Data; // retrieve data 115 116 // reset firstNode and lastNode references 117 if ( firstNode == lastNode ) 118 firstNode = lastNode = null; 119 120 else 121 firstNode = firstNode.Next; 122 123 return removeItem; // return removed data 124 } 125 } 126 127 // remove last node from List 128 public object RemoveFromBack() 129 { 130 lock ( this ) 131 { 132 if ( IsEmpty() ) 133 throw new EmptyListException( name ); 134 135 object removeItem = lastNode.Data; // retrieve data 136 137 // reset firstNode and lastNode references 138 if ( firstNode == lastNode ) 139 firstNode = lastNode = null; 140 141 else 142 { 143 ListNode current = firstNode; 144 Fig. 23.4 Fig. 23.4Fig. 23FiFi.4g. 23.4g.23.4Definitions of classes ListNode, Listand EmptyListException. (Part 3 of 5.)
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1152 Data Structures and Collections Chapter 23 42 43 // property Data 44 public object Data 45 { 46 get 47 { 48 return data; 49 } 50 } 51 52 } // end class ListNode 53 54 // class List definition 55 public class List 56 { 57 private ListNode firstNode; 58 private ListNode lastNode; 59 private string name; // string like “list” to display 60 61 // construct empty List with specified name 62 public List( string listName ) 63 { 64 name = listName; 65 firstNode = lastNode = null; 66 } 67 68 // construct empty List with “list” as its name 69 public List() : this( “list” ) 70 { 71 } 72 73 // Insert object at front of List. If List is empty, 74 // firstNode and lastNode will refer to same object. 75 // Otherwise, firstNode refers to new node. 76 public void InsertAtFront( object insertItem ) 77 { 78 lock ( this ) 79 { 80 if ( IsEmpty() ) 81 firstNode = lastNode = 82 new ListNode( insertItem ); 83 else 84 firstNode = 85 new ListNode( insertItem, firstNode ); 86 } 87 } 88 89 // Insert object at end of List. If List is empty, 90 // firstNode and lastNode will refer to same object. 91 // Otherwise, lastNode’s Next property refers to new node. 92 public void InsertAtBack( object insertItem ) 93 { Fig. 23.4 FiFiFiFig.g.g.g. 23232323.4.4.4.4Definitions of classes ListNode, Listand EmptyListException. (Part 2 of 5.)
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Chapter 23 Data Structures and Collections 1151 (Fig. 23.4) to set instance variable datato refer to the objectpassed as the first argument and sets the nextreference to null. 3. If the list is not empty, the new node is threaded (not to be confused with multithreading) into the list by setting firstNodeto refer to a new ListNodeobject initialized with insertItem and firstNode (lines 84 85). When the List- Nodeconstructor (lines 23 27 of Fig. 23.4) executes, it sets instance variable data to refer to the object passed as the first argument and performs the insertion by setting the nextreference to the ListNodepassed as the second argument. 1 // Fig. 23.4: LinkedListLibrary.cs 2 // Class ListNode and class List definitions. 3 4 using System; 5 6 namespace LinkedListLibrary 7 { 8 // class to represent one node in a list 9 class ListNode 10 { 11 private object data; 12 private ListNode next; 13 14 // constructor to create ListNode that refers to dataValue 15 // and is last node in list 16 public ListNode( object dataValue ) 17 : this( dataValue, null ) 18 { 19 } 20 21 // constructor to create ListNode that refers to dataValue 22 // and refers to next ListNode in List 23 public ListNode( object dataValue, ListNode nextNode ) 24 { 25 data = dataValue; 26 next = nextNode; 27 } 28 29 // property Next 30 public ListNode Next 31 { 32 get 33 { 34 return next; 35 } 36 37 set 38 { 39 next = value; 40 } 41 } Fig. 23.4 Fig. 23.4Fig. 23FiFi.4g. 23.4g.23.4Definitions of classes ListNode, Listand EmptyListException. (Part 1 of 5.)
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1150 Data Structures and Collections Chapter 23 Performance Tip 23.5 Insertion and deletion in a sorted array can be time consuming all the elements following the inserted or deleted element must be shifted appropriately. The program consists of four classes ListNode (Fig. 23.4, lines 9 52), List (Fig. 23.4, lines 55 193), EmptyListException (Fig. 23.4, lines 196 203) and class ListTest (Fig. 23.5). The classes in Fig. 23.4 create a linked-list library (defined in namespace LinkedListLibrary) that can be reused throughout this chapter. Encapsulated in each List object is a linked list of ListNode objects. Class List- Node (Fig. 23.4, lines 9 52) consists of two member variables data and next. Member data can refer to any object. Member next stores a reference to the next List- Nodeobject in the linked list. A List accesses the ListNodemember variables via the properties Data (lines 44 50) and Next (lines 30 41), respectively. Class List contains private members firstNode (a reference to the first ListNode in a List) and lastNode (a reference to the last ListNode in a List). The constructors (lines 62 66 and 69 71) initialize both references to null. InsertAt- Front (lines 76 87), InsertAtBack (lines 92 104), RemoveFromFront (lines 107 125) and RemoveFromBack (lines 128 156) are the primary methods of class List. Each method uses a lock block to ensure that List objects are multithread safe when used in a multithreaded program. If one thread is modifying the contents of a List object, no other thread can modify the same List object at the same time. Method IsEmpty (lines 159 165) is a predicate method that determines whether the list is empty (i.e., the reference to the first node of the list is null). Predicate methods typically test a condition and do not modify the object on which they are called. If the list is empty, method IsEmpty returns true; otherwise, it returns false. Method Print (lines 168 191) displays the list s contents. Both IsEmpty and Print also use lock blocks so that the state of the list does not change while those methods are performing their tasks. Class EmptyListException (lines 196 203) defines an exception class to handle illegal operations on an empty List. Class ListTest (Fig. 23.5) uses the linked-list library to create and manipulate a linked list. Line 14 creates a new instance of type List named list. Lines 17 20 create data to add to the list. Lines 23 30 use List insertion methods to insert these objects and use List method Print to output the contents of list after each insertion. The code inside the try block (lines 36 53) removes objects via List deletion methods, outputs the object removed and outputs list after every deletion. If there is an attempt to remove an object from an empty list, this try block catches the EmptyListException. Note that class ListTest uses namespace LinkedListLibrary (Fig. 23.4); thus, the solution for class ListTest must have a reference to the LinkedListLibraryclass library. Over the next several pages, we discuss each of the methods of class List in detail. Method InsertAtFront (Fig. 23.4, lines 76 87) places a new node at the front of the list. The method consists of three steps (illustrated in Fig. 23.6): 1. Call IsEmpty to determine whether the list is empty (line 80). 2. If the list is empty, set both firstNode and lastNode to refer to a new ListNode initialized with insertItem (lines 81 82). The ListNode constructor at lines 16 19 (Fig. 23.4) calls the ListNode constructor at lines 23 27
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Chapter 23 Data Structures and Collections 1149 Performance Tip 23.1 An array can be declared to contain more elements than the number of items expected, at the expense of wasting memory. Linked lists provide better memory utilization in these situations and they allow the program to adapt at run time. Performance Tip 23.2 After locating the insertion point for a new item in a sorted linked list, inserting an element in the list is fast only two references have to be modified. All existing nodes remain at their current locations in memory. Programmers can maintain linked lists in sorted order simply by inserting each new element at the proper point in the list (locating the proper insertion point does take time). They do not need to move existing list elements. Performance Tip 23.3 The elements of an array are stored contiguously in memory to allow immediate access to any array element the address of any element can be calculated directly from its offset from the beginning of the array. Linked lists do not afford such immediate access to their elements an element can be accessed only by traversing the list from the front. Memory does not normally store linked list nodes contiguously. Rather, the nodes are logically contiguous. Figure 23.3 illustrates a linked list with several nodes. Performance Tip 23.4 Using dynamic memory allocation (instead of arrays) for data structures that grow and shrink at execution time can save memory. Keep in mind, however, that references occupy space, and that dynamic memory allocation incurs the overhead of method calls. The program of Fig. 23.4 Fig. 23.5 uses an object of class List to manipulate a list of miscellaneous object types. The Mainmethod of class ListTest(Fig. 23.5) creates a list of objects, inserts objects at the beginning of the list using Listmethod InsertAt- Front, inserts objects at the end of the list using Listmethod InsertAtBack, deletes objects from the front of the list using List method RemoveFromFront and deletes objects from the end of the list using List method RemoveFromBack. Each insertion and deletion operation invokes Listmethod Printto display the current list contents. A detailed discussion of the program follows. If an attempt is made to remove an item from an empty list, an EmptyListExceptionoccurs. firstNode lastNode …H D Q Fig. 23.3 Fig. 23.Fig. 23.FF3ig. 23.3ig.3A graphical representation of a linked list. 23.
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1148 Data Structures and Collections Chapter 23 15 10 Fig. 23.2 Fig. 23.Fig. 23.FF2ig. 23.2ig.2Two self-referential class objects linked together. 23. Creating and maintaining dynamic data structures requires dynamic memory allocation a program s ability to obtain more memory space at execution time to hold new nodes and to release space no longer needed. As we have already learned, C# programs do not explicitly release dynamically allocated memory. Rather, C# performs automatic garbage collection. The limit for dynamic memory allocation can be as large as the amount of available disk space in a virtual-memory system. Often, the limits are much smaller, because the computer s available memory must be shared among many users. Operator new is essential to dynamic memory allocation. Operator new takes as an operand the type of the object being dynamically allocated and returns a reference to a newly created object of that type. For example, the statement Node nodeToAdd = new Node( 10 ); allocates the appropriate amount of memory to store a Nodeand stores a reference to this object in nodeToAdd. If no memory is available, new throws an OutOfMemoryException. The 10 is the Nodeobject s data. The following sections discuss lists, stacks, queues and trees. These data structures are created and maintained with dynamic memory allocation and self-referential classes. Good Programming Practice 23.1 When creating a very large number of objects, test for an OutOfMemoryException. Perform appropriate error processing if the requested memory is not allocated. 23.3 Linked Lists A linked list is a linear collection (i.e., a sequence) of self-referential class objects, called nodes, connected by reference links hence, the term linked list. A program accesses a linked list via a reference to the first node of the list. Each subsequent node is accessed via the link-reference member stored in the previous node. By convention, the link reference in the last node of a list is set to nullto mark the end of the list. Data are stored in a linked list dynamically that is, each node is created as necessary. A node can contain data of any type, including objects of other classes. Stacks and queues are also linear data structures, and they are constrained versions of linked lists. Trees are nonlinear data structures. Lists of data can be stored in arrays, but linked lists provide several advantages. A linked list is appropriate when the number of data elements to be represented in the data structure is unpredictable. Unlike a linked list, the size of a conventional C# array cannot be altered, because the array size is fixed at creation time. Conventional arrays can become full, but linked lists become full only when the system has insufficient memory to satisfy dynamic storage allocation requests.
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Chapter 23 Data Structures and Collections 1147 clared here hence, the term self-referential class. Member nextis referred to as a link (i.e., nextcan be used to tie an object of type Nodeto another object of the same type). Class Node also has two properties: One for variable data (named Data), and another for variable next(named Next). Self-referential objects can be linked together to form useful data structures, such as lists, queues, stacks and trees. Figure 23.2 illustrates two self-referential objects linked together to form a list. A backslash (representing a null reference) is placed in the link member of the second self-referential object to indicate that the link does not refer to another object. The slash is for illustration purposes; it does not correspond to the backslash character in C#. A nullreference normally indicates the end of a data structure. Common Programming Error 23.1 Not setting the link in the last node of a list (or other linear data structure) to null is a common logic error. 1 class Node 2 { 3 private int data; 4 private Node next; 5 6 public Node( int d ) 7 { 8 // constructor body 9 } 10 11 public int Data 12 { 13 get 14 { 15 // get body 16 } 17 18 set 19 { 20 // set body 21 } 22 } 23 24 public Node Next 25 { 26 get 27 { 28 // get body 29 } 30 31 set 32 { 33 // set body 34 } 35 } 36 } Fig. 23.1 Fig. 23.FiFig. 23.1g. 23.1Sample self-referential Nodeclass definition. Fig. 23.1
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1146 Data Structures and Collections Chapter 23 Outline 23.1 Introduction 23.2 Self-Referential Classes 23.3 Linked Lists 23.4 Stacks 23.5 Queues 23.6 Trees 23.6.1 Binary Search Tree of Integer Values 23.6.2 Binary Search Tree of IComparable Objects 23.7 Collection Classes 23.7.1 Class Array 23.7.2 Class ArrayList 23.7.3 Class Stack 23.7.4 Class Hashtable Summary Terminology Self-Review Exercises Answers to Self-Review Exercises Exercises 23.1 Introduction The data structures that we have studied thus far have had fixed size, such as single- and double-subscripted arrays. This chapter introduces dynamic data structures that grow and shrink at execution time. Linked lists are collections of data items lined up in a row users can make insertions and deletions anywhere in a linked list. Stacks are important in compilers and operating systems because insertions and deletions are made at only one end its top. Queues represent waiting lines; insertions are made at the back (also referred to as the tail) of a queue, and deletions are made from the front (also referred to as the head) of a queue. Binary trees facilitate high-speed searching and sorting of data, efficient elimination of duplicate data items, representation of file system directories and compilation of expressions into machine language. These data structures have many other interesting applications as well. We will discuss each of the major types of data structures and implement programs that create and manipulate them. We use classes, inheritance and composition to create and package these data structures for reusability and maintainability. The chapter examples are practical programs that will be useful in more advanced courses and in industrial applications. The programs devote special attention to and focus on reference manipulation. The exercises offer a rich collection of useful applications. 23.2 Self-Referential Classes A self-referential class contains a reference member that refers to an object of the same class type. For example, the class definition in Fig. 23.1 defines a type, Node. This type has two private instance variables integer data and Node reference next. Member next references an object of type Node, an object of the same type as the one being de
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23 Data Structures and Collections Objectives To be able to form linked data structures using references, self-referential classes and recursion. To be able to create and manipulate dynamic data structures such as linked lists, queues, stacks and binary trees. To understand various important applications of linked data structures. To understand how to create reusable data structures with classes, inheritance and composition. Much that I bound, I could not free; Much that I freed returned to me. Lee Wilson Dodd Will you walk a little faster? said a whiting to a snail, There s a porpoise close behind us, and he s treading on my tail. Lewis Carroll There is always room at the top. Daniel Webster Push on keep moving. Thomas Morton I think that I shall never see A poem lovely as a tree. Joyce Kilmer
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