Savitch Ch 13

Overview 13.1 Nodes and Linked Lists 13.2 Stacks and Queues Slide 13-

Nodes and Linked Lists A linked list is a list that can grow and shrink while the program is running A linked list is constructed using pointers A linked list often consists of structs or classes that contain a pointer variable connecting them to other dynamic variables A linked list can be visualized as items, drawn as boxes, connected to other items by arrows Slide 13- 10 12 14 end head

The boxes in the previous drawing represent the nodes of a linked list Nodes contain the data item(s) and a pointer that can point to another node of the same type The pointers point to the entire node, not an individual item that might be in the node The arrows in the drawing represent pointers Nodes Slide 13- Display 13.1

Nodes are implemented in C++ as structs or classes Example: A structure to store two data items and a pointer to another node of the same type, along with a type definition might be: struct ListNode { string item; int count; ListNode *link; }; typedef ListNode* ListNodePtr; Implementing Nodes Slide 13- This circular definition is allowed in C++

The head of a List The box labeled head, in display 13.1, is not a node, but a pointer variable that points to a node Pointer variable head is declared as: ListNodePtr head; Slide 13-

Accessing Items in a Node Using the diagram of 13.1, this is one way to change the number in the first node from 10 to 12: (*head).count = 12; head is a pointer variable so *head is the node that head points to The parentheses are necessary because the dot operator . has higher precedence than the dereference operator * Slide 13-

The arrow operator -> combines the actions of the dereferencing operator * and the dot operator to specify a member of a struct or object pointed to by a pointer (*head).count = 12; can be written as head->count = 12; The arrow operator is more commonly used The Arrow Operator Slide 13- Display 13.2

NULL The defined constant NULL is used as… An end marker for a linked list A program can step through a list of nodes by following the pointers, but when it finds a node containing NULL, it knows it has come to the end of the list The value of a pointer that has nothing to point to The value of NULL is 0 Any pointer can be assigned the value NULL: double* there = NULL; Slide 13-

To Use NULL A definition of NULL is found in several libraries, including <iostream> and <cstddef> A using directive is not needed for NULL Slide 13-

Linked Lists The diagram in Display 13.2 depicts a linked list A linked list is a list of nodes in which each node has a member variable that is a pointer that points to the next node in the list The first node is called the head The pointer variable head, points to the first node The pointer named head is not the head of the list…it points to the head of the list The last node contains a pointer set to NULL Slide 13-

Building a Linked List: The node definition Let's begin with a simple node definition: struct Node { int data; Node *link; }; typedef Node* NodePtr; Slide 13-

Building a Linked List: Declaring Pointer Variable head With the node defined and a type definition to make or code easier to understand, we can declare the pointer variable head: NodePtr head; head is a pointer variable that will point to the head node when the node is created Slide 13-

Building a Linked List: Creating the First Node To create the first node, the operator new is used to create a new dynamic variable: head = new Node; Now head points to the first, and only, node in the list Slide 13-

Building a Linked List: Initializing the Node Now that head points to a node, we need to give values to the member variables of the node: head->data = 3; head->link = NULL; Since this node is the last node, the link is set to NULL Slide 13-

Function head_insert It would be better to create a function to insert nodes at the head of a list, such as: void head_insert(NodePtr& head, int the_number); The first parameter is a NodePtr parameter that points to the first node in the linked list The second parameter is the number to store in the list head_insert will create a new node for the number The number will be copied to the new node The new node will be inserted in the list as the new head node Slide 13-

Create a new dynamic variable pointed to by temp_ptr Place the data in the new node called *temp_ptr Make temp_ptr's link variable point to the head node Make the head pointer point to temp_ptr Pseudocode for head_insert Slide 13- Display 13.3

The pseudocode for head_insert can be written in C++ using these lines in place of the lines of pseudocode: NodePtr temp_ptr; //create the temporary pointer temp_ptr = new Node; // create the new node temp_ptr->data = the_number; //copy the number temp_ptr->link = head; //new node points to first node head = temp_ptr; // head points to new // first node Translating head_insert to C++ Slide 13- Display 13.4

An Empty List A list with nothing in it is called an empty list An empty linked list has no head node The head pointer of an empty list is NULL head = NULL; Any functions written to manipulate a linked list should check to see if it works on the empty list Slide 13-

You might be tempted to write head_insert using the head pointer to construct the new node: head = new Node; head->data = the_number; Now to attach the new node to the list The node that head used to point to is now lost! Losing Nodes Slide 13- Display 13.5

Memory Leaks Nodes that are lost by assigning their pointers a new address are not accessible any longer The program has no way to refer to the nodes and cannot delete them to return their memory to the freestore Programs that lose nodes have a memory leak Significant memory leaks can cause system crashes Slide 13-

Searching a Linked List To design a function that will locate a particular node in a linked list: We want the function to return a pointer to the node so we can use the data if we find it, else return NULL The linked list is one argument to the function The data we wish to find is the other argument This declaration will work: NodePtr search(NodePtr head, int target); Slide 13-

Refining our function We will use a local pointer variable, named here, to move through the list checking for the target The only way to move around a linked list is to follow pointers We will start with here pointing to the first node and move the pointer from node to node following the pointer out of each node Function search Slide 13- Display 13.6

Pseudocode for search Make pointer variable here point to the head node while(here does not point to a node containing target AND here does not point to the last node) { make here point to the next node } If (here points to a node containing the target) return here; else return NULL; Slide 13-

Moving Through the List The pseudocode for search requires that pointer here step through the list How does here follow the pointers from node to node? When here points to a node, here->link is the address of the next node To make here point to the next node, make the assignment: here = here->link; Slide 13-

A Refinement of search The search function can be refined in this way: here = head; while(here->data != target && here->link != NULL) { here = here->next; } if (here->data = = target) return here; else return NULL; Slide 13- Check for last node

Our search algorithm has a problem If the list is empty, here equals NULL before the while loop so… here->data is undefined here->link is undefined The empty list requires a special case in our search function A refined search function that handles an empty list is shown in Searching an Empty List Slide 13- Display 13.7

Pointers as Iterators An iterator is a construct that allows you to cycle through the data items in a data structure to perform an action on each item An iterator can be an object of an iterator class, an array index, or simply a pointer A general outline using a pointer as an iterator: Node_Type *iter; for (iter = Head; iter != NULL; iter = iter->Link) //perform the action on the node iter points to Head is a pointer to the head node of the list Slide 13-

Iterator Example Using the previous outline of an iterator we can display the contents of a linked list in this way: NodePtr iter; for (iter = Head; iter != NULL; iter = iter->Link) cout << (iter->data); Slide 13-

To insert a node after a specified node in the linked list: Use another function to obtain a pointer to the node after which the new node will be inserted Call the pointer after_me Use function insert, declared here to insert the node: void insert(NodePtr after_me, int the_number); Inserting a Node Inside a List Slide 13- Display 13.8

Inserting the New Node Function insert creates the new node just as head_insert did We do not want our new node at the head of the list however, so… We use the pointer after_me to insert the new node Slide 13-

Inserting the New Node This code will accomplish the insertion of the new node, pointed to by temp_ptr, after the node pointed to by after_me: temp_ptr->link = after_me->link; after_me->link = temp_ptr; Slide 13- head after_me temp_ptr 2 2 3 2 7 2 9 0 5 2

The order of pointer assignments is critical If we changed after_me->link to point to temp_ptr first, we would loose the rest of the list! The complete insert function is shown in Caution! Slide 13- Display 13.9

Function insert Again Notice that inserting into a linked list requires that you only change two pointers This is true regardless of the length of the list Using an array for the list would involve copying as many as all of the array elements to new locations to make room for the new item Inserting into a linked list is often more efficient than inserting into an array Slide 13-

To remove a node from a linked list Position a pointer, before, to point at the node prior to the node to remove Position a pointer, discard, to point at the node to remove Perform: before->link = discard->link; The node is removed from the list, but is still in memory Return *discard to the freestore: delete discard; Removing a Node Slide 13- Display 13.10

Assignment With Pointers If head1 and head2 are pointer variables and head1 points to the head node of a list: head2 = head1; causes head2 and head1 to point to the same list There is only one list! If you want head2 to point to a separate copy, you must copy the list node by node or overload the assignment operator appropriately Slide 13-

Many other data structures can be constructed using nodes and pointers Doubly-Linked List Each node has two links, one to the next node and one to the previous node Allows easy traversal of the list in both directions struct Node { int data; Node *forward_link; Node *back_link; }; Variations on Linked Lists Slide 13- Display 13.11

A tree is a data structure that looks like an upside-down tree with the root at the top No cycles In a binary tree each node has at most two links Binary Tree Slide 13- struct TreeNode { int data; TreeNode *left_link; TreeNode *right_link; }; Display 13.12

The preceding examples created linked lists of structs. We can also create linked lists using classes. Logic to use a class is identical except the syntax of using and defining a class should be substituted in place of that for a struct Interface and Definition for a Node Class Linked List of Classes Slide 13- Display 13.13 Display 13.14 (1-2)

Program using the Node class We can create a linked list of numbers using the Node class. Slide 13- Display 13.15 (1-3)

Section 13.1 Conclusion Can you Write type definitions for the nodes and pointers in a linked list? Call the node type NodeType and call the pointer type PointerType. The linked lists will be lists of letters. Explain why inserting into an array can be less efficient than inserting into a linked list? Slide 13-

A stack is a data structure that retrieves data in the reverse order the data was stored If 'A', 'B', and then 'C' are placed in a stack, they will be removed in the order 'C', 'B', and then 'A' A stack is a last-in/first-out data structure like the stack of plates in a cafeteria; adding a plate pushes down the stack and the top plate is the first one removed A Linked List Application Slide 13- Display 13.16

We will create a stack class to store characters Adding an item to a stack is pushing onto the stack Member function push will perform this task Removing an item from the stack is popping the the item off the stack Member function pop will perform this task contains the stack class interface Program Example: A Stack Class Slide 13- Display 13.17

demonstrates the use of the stack class Using the stack Class Slide 13- Display 13.18 (1-2)

Function push The push function adds an item to the stack It uses a parameter of the type stored in the stack void push(char the_symbol); Pushing an item onto the stack is precisely the same task accomplished by function head_insert of the linked list For a stack, a pointer named top is used instead of a pointer named head Slide 13-

Function pop The pop function returns the item that was at the top of the stack char pop( ); Before popping an item from a stack, pop checks that the stack is not empty pop stores the top item in a local variable result, and the item is "popped" by: top = top->link; A temporary pointer must point to the old top item so it can be "deleted" to prevent a memory leak pop then returns variable result Slide 13-

Empty Stack An empty stack is identified by setting the top pointer to NULL top = NULL; Slide 13-

The Copy Constructor Because the stack class uses a pointer and creates new nodes using new, a copy constructor is needed The copy constructor (a self-test exercise) must make a copy of each item in the stack and store the copies in a new stack Items in the new stack must be in the same position in the stack as in the original Slide 13-

The stack destructor Because function pop calls delete each time an item is popped off the stack, ~stack only needs to call pop until the stack is empty char next; while( ! empty ( ) ) { next = pop( ); } Slide 13-

The stack class implementation is found in stack Class Implementation Slide 13- Display 13.19 (1) Display 13.19 (2)

Section 13.2 Conclusion Can you Give the definition of member function push? Create a definition for the stack class copy constructor? Slide 13-

Savitch Ch 13

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Savitch Ch 13