03/31/03 Lab 7. Linked Lists

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1 03/31/03 Lab 7 Lists are a collection of items in which each item has a specific position. The specification for positioning the items provides some rules of order so this data structure is called an ordered list. Some common ordered lists are: chronologically ordered- in which items are inserted in the order in which they arrive for insertion frequency ordered- in which the items used most often are closest to the beginning of the list sorted- in which the items are placed in ascending or descending order. By choosing a data structure we can specify the ordered sequence and specify what needs to be done to maintain the sequence. There are two common ways of implementing lists using native data types. These are array implementations and linked list implementations. We have discussed array implementations in detail over stacks and queues. Linked Lists In this representation, the structure does not necessarily reflect the logical organization. Items may appear in any order. The logical organization is provided through pointers. Each item in the list, called a node, consists of a data portion containing the item information and a pointer, which contains the location (address) of the next item in the logical sequence that has to be maintained. As with all ADT s we can define a set of operations on linked lists Operation Initialize IsEmpty Insert Remove Empty Display Description Initialize internal structure; create an empty list True iff the list has no elements Insert an Element into the list Remove an element from the list Delete all elements in the list and free the memory Display all elements in the list

2 A singly linked list of nodes (records) with fields element and next can be visualized as shown below: Element Next Null We specify a node as follows: private type Listnode; type Listptr is access Listnode; --we define an access type or pointer to the node type Listnode is record Element : Elementtype; --we define one field in the record to be of type element Next : Listptr; --we define the other field as a pointer to the next node end record; type List is record Head : Listptr; -- we define the head to be of type record. end record; Initialize Post-Conditions: List with head pointer set to null. IsEmpty List.Head := Null; Return List to the user Post-Conditions: Returns a Boolean variable determining if the list is empty. Insert If List.Head = Null then Return True Return False The insertion operation can take place anywhere in the list. We will list the pseudo code for inserting at the beginning and the end of the list. Is it hard to insert anywhere in the list?

3 Insert at the beginning of the list Preconditions : Element to be inserted and the List Post-Conditions: List with element inserted Create a new node and store the pointer to the node in NodePtr Set NodePtr.Element to Element Set NodePtr.Next to List.Head List.Head is set to NodePtr Return List to the user Insert at the end of the list Preconditions : Element to be inserted and the List Post-Conditions: List with element inserted Create a new node and store the pointer to the node in NodePtr Set NodePtr.Element to Element Set NodePtr.Next to Null If the list is empty then List.Head := NodePtr Set Temp to List.Head Traverse to the end of the list as follows: While Temp.Next /= Null Temp:= Temp.Next Temp.Next := NodePtr Return List to the user Removal The removal operation can take place anywhere in the list. We will list the pseudo code for removing from the beginning and the end of the list. Is it hard to remove from anywhere in the list? Removal from the beginning of the list Preconditions : Non-Empty List Post-Conditions: List with the first element removed, and the element removed If the list is empty then Display cannot delete from an empty list

4 Set Temp to List.Head If Temp.Next = Null then List.Head := Null List.Head := Temp.Next Element:= Temp.Element Free Temp Return List and Element to the user Remove from the end of the list Preconditions : A non-empty List Post-Conditions: List with element removed from the end and the element removed If the list is empty then Display cannot delete from an empty list Create two temporary pointers called Temp and Prev Set Temp to List.Head Set Prev to null Traverse to the end of the list as follows: While Temp.Next /= Null Prev :=Temp Temp:= Temp.Next If Prev = Null then List.Head:= Null Prev.Next := Null Element := Temp.Element Free Temp Return List and Element to the user Empty Post-Conditions: frees all the allocated nodes in the list. While List.Head /= Null loop Temp := List.Head List.Head := Temp.Next Free Temp

5 Display Post-Conditions: displays all the elements in the list. Temp := List.Head While Temp /= Null loop Display Temp.Element Temp:= Temp.Next Given the package specification below, GNAT 3.13p ( ) Copyright Free Software Foundation, Inc. Checking: c:/docume~2/jk/desktop/16070c~1/lab6~1/linked_list.ads (source file time stamp: :22:44) 1. package Linked_List is Specification for singly-linked lists Specified: Jayakanth Srinivasan Last Modified: March 30, type List is limited private; 9. subtype Elementtype is Integer; procedure Makeempty ( 13. L : in out List ); Pre: L is defined Post: L is empty function Isempty ( 18. L : in List ) 19. return Boolean; Pre: L is defined Post: returns True if L is empty, False otherwise procedure Addtofront ( 24. L : in out List; 25. Element : in Elementtype ); Pre: Element is defined; L may be empty Post: Element is inserted at the beginning of L procedure Removefront ( 30. L : in out List; 31. Element : out Elementtype ); Pre: L is defined; L may be empty Post: The first node of L is removed Raises: ListEmpty if the list is empty before the removal

6 procedure Addtoend ( 37. L : in out List; 38. Element : in Elementtype ); Pre: Element is defined; L may be empty Post: Element is appended to the end of L procedure Display ( 43. L : in List ); Pre: L may be empty Post: displays the contents of L's Element fields, in the order in which they appear in L procedure Initialize ( 49. L : in out List ); private type Listnode; 54. type Listptr is access Listnode; 55. type Listnode is 56. record 57. Element : Elementtype; 58. Next : Listptr; 59. end record; type List is 62. record 63. Head : Listptr; 64. end record; end Linked_List; 66 lines: No errors Complete the.adb file for the implementation of o Addtofront o Addtoend o RemoveFront Modify the.ads and.adb file to enable removing from the end of the list o RemoveEnd Use the test file supplied to see if your program works.

A collision is said to occur when two or more keys hash to the same index location.

A collision is said to occur when two or more keys hash to the same index location. Problem Set #8 Solutions Problem 1-9 points Given the tree below, traverse the tree a. Inorder : a/b c * d*e / f + g b. Preorder: * - / a b c / * d e + f g c. Postorder a b / c d e * f g + / * * - / /