Rossella Lau Lecture 3, DCO20105, Semester A, DCO Data structures and algorithms  Lecture 3: Basics of Linked List  C++ pointer revision  Basics of a linked list -- By Rossella Lau

Rossella Lau Lecture 3, DCO20105, Semester A, A reason for using a Linked List  Array  It allows storing of multiple occurrences of items consecutively  The number of items should be specified ahead  Linked list  As an array may occupy too many spaces or may be too small to store all items, a linked list allows storing of the exact number of items  Storage for an item is allocated dynamically on demand  Example Program List.h

Rossella Lau Lecture 3, DCO20105, Semester A, C++ pointer  Revision: slide 6-21, Lecture 8 in DCO10105  Another point of view: Ford’s Slides: 5: 2-14

Rossella Lau Lecture 3, DCO20105, Semester A, An example of pointer and array  Ford’s revision exercise: 5.2: What are the final values in the array arr?  int arr[] = {45, 24, 16, 3, 2, 8};  int *parr = arr;  *parr = 3;  parr +=4;  *parr = 5;  parr = parr - 3;  *parr = 15;  parr[4] *= 2; Final values in the array arr = { ____, ____, ____, ____, ____, ____}

Rossella Lau Lecture 3, DCO20105, Semester A, Dynamic storage requests for items  When an item needs to be stored, new is used, and a pointer is needed to point to the new item  When a second item is required to be stored, new is used again. An additional pointer is needed.  It is unwise to use an array to store such pointers – as we cannot determine the size of the array ahead item1 ptr item2next  Therefore, the pointer which points to the second element is better stored with the first element.

Rossella Lau Lecture 3, DCO20105, Semester A, Nodes of a Linked list  When more items are requested, one can see elements are linked as a list.  To formalize all items in the linked list, a null pointer (value 0) is added to the last item. 0 nextitem1 ptr item2next itemn … itemnext  Now, each node (item + next) has the same format

Rossella Lau Lecture 3, DCO20105, Semester A, Typical structures of a Linked List template class Node { T item; Node *next; friend class List ; }; template class List { Node *head, *tail; }; head tail nextitem10itemN ……

Rossella Lau Lecture 3, DCO20105, Semester A, C++ Notes – friends  Friend classes can access the private members of the class which is declaring  Node and List are hand by hand classes but should be implemented as two independent classes  In order to allow for class List to direct use the private members in Node, List is declared as a friend class inside class Node  Similarly, a class may declare its friend function(s)  However, friends should be avoided as much as possible because they are contradicted with encapsulation!

Rossella Lau Lecture 3, DCO20105, Semester A, C++ notes on header and implementation source  Traditional C uses a header file (.h) to define data members and function prototypes and implementation codes (.cpp) will be written in another file  The “.h” should be included if a program uses the class, and lines in “.h” file should be compiled again and again  The “.cpp” is only required to be compiled once and allow for “link” up during link time after the code compilation.  It saves time to compile for the “.cpp” file  C++ uses only one header file (.h) to define a template classe since a template’s type is not determined until the compilation time.  It cannot save time by separating the header and implementation  It sacrifices compilation time for flexibility

Rossella Lau Lecture 3, DCO20105, Semester A, Typical operations of a linked list node  Creation of a node: nodePtr = new Node(item);  Member identification of a node: nodePtr  item  Care should be taken to avoid identification from a null pointer or a pointer without properly being assigned a value  Nodes “linked” up: node1Ptr  next = node2Ptr;  List pointer assignment: head = firstNodePtr;

Rossella Lau Lecture 3, DCO20105, Semester A, Typical operations of a linked list  push_back() -- append an item at the end  push_front() -- insert an item at the front  find() -- a linear search  insert() -- insert an item before or after a node and usually performed after a find()  remove() -- remove an item

Rossella Lau Lecture 3, DCO20105, Semester A, Examples of push_back() and push_front() push_back(10), next10 headtail next20 Initial : headtail (null) Initial : headtail (null) push_front(10), next10 tailhead next20 push_back(20) push_front(20),

Rossella Lau Lecture 3, DCO20105, Semester A, An example of insert_before() next10 headtail next20 Initial : Insert_before(20, 15) next10 head tail next20 next15 prev curr

Rossella Lau Lecture 3, DCO20105, Semester A, Examples of remove() next 7 9 head next 4 6 delete 9: head next delete 6: next 7 head next 4 6 prevcurr next 9 curr next 6

Rossella Lau Lecture 3, DCO20105, Semester A, Pointer traversal on a linked list  A typical traversal of an array: for (size_t i =0; i != SIZE; i++) { array[i]......}  As head points to the first element, all the items can be traversed through the link, i.e., Node *next. Treating head similar to a[0], iteration through  next similar to i++, null pointer similar to the end of an array, a typical traversal on linked list: for (Node * it=head; it; it=it->next) { it->item......} Example in List.h: search()

Rossella Lau Lecture 3, DCO20105, Semester A, Pointer “look ahead” traversal  The above loop is simple but would end up with the iterating pointer pointing to null  We may also use a look ahead traversal method to examine contents of a node from its previous node on the linked list for (Node * it=head; it->next; it=it->next) { it->next->item } It is used more often and examples are List.h: print(), find()

Rossella Lau Lecture 3, DCO20105, Semester A, Summary  A pointer stores an address and can apply de- referencing and object member identification  Pointer is a powerful but also a dangerous feature  Linked list is a typical dynamic memory structure in which it uses pointers to link up its nodes  Nodes on a linked list are created on demand and are more space efficient than an array in which unwanted space is usually presented

Rossella Lau Lecture 3, DCO20105, Semester A, Reference  Ford:  STL online references    Example programs: List.h, testList.cpp -- END --