1. COSC2006 - Data Structures I Chapter 4 Data Abstraction The Walls

2. Topics • Introduction • ADT Examples • Specify ADT • Implement ADT

3. Information Hiding: Why? • “You don’t want to know.” … but also … • “If I told you, I’d have to kill you!” • … OK, not quite that extreme, but … • “A little knowledge is a dangerous thing.”

4. ADT • Abstract data type (ADT): • A collection of data together with a set of operations on that data

5. ADT Why? • During a design of a solution, we need to support operations on data (Specification) • Design an ADT • Specify the operations (Interface) that enable communication with the data • After defining ADT operations (specification), we consider their implementation (Implementation) • Specify the data structure • Specify the details of how operations work

6. Wall Request to perform operation Slit in Wall Implementation of method s Program using method s Result of operation ADT

7. Out of ice indicator Chilled water Crushed ice Ice cubes Water ADT Example: Ice Dispenser • Specification • Data • Input: water • Output: chilled water, crushed ice, ice cubes, or red light • Operations: • Chill, Crush, Cube, IsEmpty • Don’t care how operations are done inside

8. ADT Example: Ice Dispenser • Implementation: • Internal structure of the dispenser • Manual • Mechanical • Electronic • We can improve an operation by modifying its module • We can add another operation by adding another module to the machine without affecting the other modules

9. ADT List • Specification • Data characteristics • Items appear in sequence • Has one first element • One last element • The first item is called the head (front) • The last item is called the tail (rear/end) • Items are of the same type

10. ADT List • Specification • Operations (Behavior): • Create an empty list • Destroy a list • Determine whether a list is empty • Determine the number of items in the list • Insert an item in a given position • Delete an item at a given position • Retrieve (look at) an item at a given position

11. CreateList DestroyList Retrieve item at index ListInsert Implementation of ADT-List Method DisplayList ListDelete DataItem ListRetrieve Client ADT Wall Server ADT List Specification

12. Using ADT List • Example: Display list data items displayList(in aList: List) // Displays the items on the list aLIst for (Position = 1, through aList.getLength ( ) ) { aList . retrieve (position, dataItem, success) Display aataItem } / / end for • Example: Replace an item with another replace (in aList: List, in i: integer, in newItem: ListItemType, out success: boolean) // Replaces the ith item on the list aList with newItem. // success flag indicates whether the replacement was successful aList . remove (i, success) if (success) aList . insert (i, newItem, success)

13. Designing an ADT • Case Study: List Holidays • Problem: • Determine the dates of all holidays in a given year • Specification: • Data: • Month • Day • Year • Operations: • Determine date of fist day of a given year firstDay( in year: integer) • Determine whether a date is before another date isBefore(in Date1: Date, in Date2: Date) • Determine whether a date is a holiday isHoliday(in aDate: Date) • Determine the date following a given date nextDay(in aDate: Date) • Assumption: • Days in a year form a sorted list • The list is accessed by giving the year number

14. Designing an ADT • Case Study: List Holidays • Using ADT List-Holidays Operations: • After specifying the operations, we can use them to solve other problems independent of the implementation details of the ADT. listHolydays ( in year : integer ) // Displays the dates of of all holidays in a given year. { date = firstDay ( year ) while ( isBefore ( date, firstDay ( year + 1 ))) { if ( isHoliday ( date )) write ( date, “ is a holiday “ ) date = nextDay ( date ) } // end while } // end listHolidays

15. Implementing ADT • Choose the data structure • Implement operations: • Write the functions definition that access the data according to the ADT operations • Both the data structures & the functions should be hidden from the client

16. ADT List Array-Based Implementation • Data Structure: private final int MAX_LIST = 100; // max length of list private listItemType items [MAX_LIST] ; // array of list items private int numItems; // length of list • Each operation will need access to both array Items & the list's length Size , They should be made as private data members of the class • Implementation of Operations • Extra function needed • Index (Position) • Defined to return the index value, since the client can't access private data members

17. Array indexes 0 1 2 3 k-1 MAX_LIST - 1 K 12 3 44 19 10 . . . 5 10 18 . . . ? . . . ? 12 3 19 10 . . . 5 10 18 . . . ? ? . . . ? 1 2 3 4 k MAX_LIST Size Items ADT list positions New item Array indexes 0 1 2 3 k MAX_LIST - 1 K+1 1 2 3 4 k+1 MAX_LIST Items Size ADT list positions ADT List Array-Based Implementation • Insert Item • To insert an item at a given position • Shift items from the position on to the right, then insert the new item

18. Array indexes Array indexes 0 1 2 3 k-1 MAX_LIST - 1 0 1 2 3 k MAX_LIST - 1 K 12 3 44 10 . . . 5 10 18 . . . ? . . . ? 12 3 44 19 10 . . . 5 10 18 . . . ? . . . ? K+1 1 2 3 4 k MAX_LIST Size 1 2 3 4 k+1 MAX_LIST Items Items Size ADT list positions ADT list positions ADT List Array-Based Implementation • Delete Item • To delete an item • Shift the elements to the left to fill the gap left by the deleted item

19. Array Implementation of List: Issues • Array has fixed physical size; List ADT has variable logical size • ADT-specific exceptions are informative • The array and its logical size are private data fields • Gaps are bad: must shift elements when adding or deleting

20. Array Implementation (page 1) / ******************************************************** // Array-based implementation of the ADT list. // ********************************************************* public class ListArrayBased implements ListInterface { private static final int MAX_LIST = 50; private Object items[]; // an array of list items private int numItems; // number of items in list public ListArrayBased() { items = new Object[MAX_LIST]; numItems = 0; } // end default constructor private int translate(int position) { return position - 1; } // end translate

21. Array Implementation (page 1) / ******************************************************** // Array-based implementation of the ADT list. // ********************************************************* public class ListArrayBased implements ListInterface { private static final int MAX_LIST = 50; private Object items[]; // an array of list items private int numItems; // number of items in list public ListArrayBased() { items = new Object[MAX_LIST]; numItems = 0; } // end default constructor private int translate(int position) { return position - 1; } // end translate

22. Array Implementation (page 2) public boolean isEmpty() { return (numItems == 0); } // end isEmpty public int size() { return numItems; } // end size public void removeAll() { // Creates a new array; marks old array for // garbage collection. items = new Object[MAX_LIST]; numItems = 0; } // end removeAll

23. Array Implementation (page 2) public boolean isEmpty() { return (numItems == 0); } // end isEmpty public int size() { return numItems; } // end size public void removeAll() { // Creates a new array; marks old array for // garbage collection. items = new Object[MAX_LIST]; numItems = 0; } // end removeAll

24. Array Implementation (page 3) if (numItems >= MAX_LIST) { throw new ListException("ListException on add"); } public void add(int index, Object item) throws ListIndexOutOfBoundsException { if (index >= 1 && index <= numItems+1) { // make room for new element by shifting all items at // positions >= index toward the end of the // list (no shift if index == numItems+1) for (int pos = numItems; pos >= index; pos--) { items[translate(pos+1)] = items[translate(pos)]; } // insert new item items[translate(index)] = item; numItems++; } else { // index out of range throw new ListIndexOutOfBoundsException( "ListIndexOutOfBoundsException on add"); } } //end add

25. Array Implementation (page 4) public void remove(int index) throws ListIndexOutOfBoundsException { if (index >= 1 && index <= numItems) { // delete item by shifting all items at // positions > index toward the beginning of the list // (no shift if index == size) for (int pos = index+1; pos <= size(); pos++) { items[translate(pos-1)] = items[translate(pos)]; } numItems--; } else { // index out of range throw new ListIndexOutOfBoundsException( "ListIndexOutOfBoundsException on remove"); } } //end remove Trace this … notice something funny at the end?

26. Array Implementation (page 5) public Object get(int index) throws ListIndexOutOfBoundsException { if (index >= 1 && index <= numItems) { return items[translate(index)]; } else { // index out of range throw new ListIndexOutOfBoundsException( "ListIndexOutOfBoundsException on get"); } } // end get } // end ListArrayBased

27. Review • The specifications of an ADT’s operations indicate ______. • what the operations do • how to implement the operations • how to store the data in the ADT • how to carry out the operations

28. Review • A(n) ______ allows two modules to communicate with each other. • data structure • axiom • Interface • client

29. Review • In the following list: • John, Kate, Fred, Mark, Jon, Adam, Drew • which element is the tail of the list? • John • Mark • Drew • Adam

30. Review • The items in the ADT list are referenced by ______. • name • value • position number • position name

31. Review • The insertion operation of the ADT list can insert new items ______. • only at the front of the list • only at the end of the list • only in the middle of the list • into any position of the list

32. Review • In the ADT list, when an item is deleted from position i of the list, ______. • the position of all items is decreased by 1 • the position of each item that was at a position smaller than i is decreased by 1 • the position of each item that was at a position greater than i is decreased by 1 • the position of each item that was at a position smaller than i is increased by 1 while the position of each item that was at a position greater than i is decreased by 1

33. Review • Which of the following operations of the ADT list changes the list? • remove • isEmpty • Size • get