CSC%20205%20

1
CSC 205 Java Programming II

• Lecture 24
• March 6, 2002

2
The Application Interface

• import java.util.
•  
• public interface Application
•  
• // Postcondition true has been returned if pos
  could be on a //path to a goal position.
  Otherwise, false has been returned.
• public boolean valid (Position pos)
•   
• // Postcondition the position specified by pos
  has been //marked as being on a path to a goal
  position.
• public void record (Position pos)
•  
• // Postcondition true has been returned if pos
  is a goal //position. Otherwise, false has been
  returned.
• public boolean done (Position pos)
•  

3
The Application Interface

• // Postcondition the position specified by pos
  has been //marked as not being on a path to a
  goal position.
• public void undo (Position pos)
•  
• // Postcondition a string representation of
  this Application has //been returned.
• public String toString()
• // Postcondition an iterator over the positions
  directly //accessible from pos has been returned.
• public Iterator iterator (Position pos)
•  
• // interface Application

4
The BackTrack Class

• import java.util.
•  
• public class BackTrack
• Application app
•  
• // Postcondition this BackTrack has been
  initialized from app.
• public BackTrack (Application app)
• this.app app
• // constructor
• // Postcondition a solution going through
  pos has been attempted.
• public boolean tryToSolve (Position pos)
• // method tryToSolve
• // class BackTrack

5
The tryToSolve Method

• boolean success false
• Iterator itr app.iterator (pos)
• while (!success itr.hasNext())
• pos (Position)itr.next()
• if (app.valid (pos))
• app.record (pos)
• if (app.done (pos))
• success true
• else
• success tryToSolve (pos)
• if (!success)
• app.undo (pos)
• // not done
• // a valid position
• // while
• return success

6
The Eight Queens Problem

• Place eight queens on a chessboard
• No queen is under attack from any other queen

7
Discussions
8
Binary Searching

• public interface Comparable
•   int compareTo(Object o)
•  
•  The int returned by x.compareTo (y) is
•  lt 0, x is less than y
•   0, if x.equals (y)
•  gt 0, if x is greater than y.

9
Sequential Search

• SEQUENTIAL SEARCH OF ARRAY a FOR ELEMENT key
• START AT INDEX 0
• COMPARE EACH ELEMENT IN a TO Key
• UNTIL
• SUCCESS (key FOUND) OR
• FAILURE (END OF A REACHED).

10
Sequential Search

• // Precondition all the elements in the array a
  are in the
• // same class, and that
  class implements the
• // Comparable interface.
• // Postcondition if there is an index i such
  that
• // ((Comparable) a
  i).compareTo (key)
• // returns 0, then such
  an index has been
• // returned. Otherwise,
  -1 has been returned.
• // The worstTime (n) is O
  (n).
• public static int sequentialSearch (Object a,
  Object key)
•   for (int i 0 i lt a.length i)
• if ((Comparable) a i.compareTo (key)
  0)
• return i
• return 1
•   // sequentialSearch

11
A Sorted Array

• a 0 ANDREW
• a 1 BRANDON
• a 2 CHRIS
• a 3 CHUCK
• a 4 GEOFF
• a 5 JASON
• a 6 MARGARET
• a 7 MARK
• a 8 MATT
• a 9 ROB
• a 10 SAMIRA

12
Binary Search (recursive)

• public static int binarySearch(Object a, int
  first, int last,
• 
  Object key)
• if (first lt last)
•   int mid (first last) / 2
• Object midVal a mid
• int comp ((Comparable)midVal).compar
  eTo (key)
• if (comp lt 0)
• return binarySearch (a, mid 1,
  last, key)
• if (comp gt 0)
• return binarySearch (a, first,
  mid - 1, key)
• return mid // key found
•   // if first lt last
• return -first - 1 // key not found
  belongs at a first
• // method binarySearch