java.lang.NullPointerException

I'm geting java.lang.NullPointerException for the distance variable



package closestPair;


import java.util.*;


/** The  implementation of Shamos's Algorithm. */

public class Code {


	/** Find the square distance of the closest pairs in the point set. This static function is the preparation step for the recursive part of the algorithm defined in the method closestPairAux. */

    public static int closestPair(PointSet P)

            throws TrivialClosestPairException, UnknownSortOptionException

    {

        int distance = 0;//

        Point[] X = P.sort('x');

        Point[] Y = P.sort('y');

    

     distance = closestPairAux(X, Y);[B]->here[/B]

 

 return distance;


  }


    /** The recursive part of Shamos's Algorithm. The parameter X is an array of points sorted by the X axis and the parameter Y is the same array of points sorted by the Y axis. The burden of work is going on here. Good luck! */

    public static int closestPairAux(Point[] X, Point[] Y)

            throws TrivialClosestPairException, UnknownSortOptionException

    {//trivial case

         if (X.length<4){

             return PointSet.naiveClosestPair(new PointSet (X));


          }

        //get the middle element of the array here

         int centerIndex = X.length/2;

         int W = X[centerIndex].getX();


        //the two disjoint sets devided by the conceptual sweep line

        Point[] PL = Arrays.copyOfRange(X,(int) 0,(int) Math.ceil(centerIndex));

        Point[] PR = Arrays.copyOfRange(X,(int) Math.floor(centerIndex), (int) X.length-1);

      //the conquering part,taking the min from the two sets

       int distance = Math.min(closestPairAux(PL,Y),closestPairAux(PR,Y));[B]->here[/B]

       Point[] shortDist = new Point[Y.length];

       //part 4.b of the shamos algorithm

       int n = 0;

       for (int i = 0; i <Y.length; i++)

    {

          int Z = Y[i].getY();

          if ((W-Z)*(W-Z) <= distance)

      {

          shortDist[n] = Y[i];


           }

        }

       //part 4.c of the shamos algorithm

       // collecting those elements which are near the divider in the shortDist-array.

     for (int i =0; i< shortDist.length -1; i++)               

     {

      for (int j = i+1; j <shortDist.length-1 && j< i + 7; j ++){

         distance = Math.min(distance, shortDist[i].sqrDist(shortDist[j]));[B]->here[/B]

     }

      }


     return distance;


     }


 }

package closestPair;


public class Tests {

	public static void main(String [ ] args) throws InvalidNumberOfTestsException

	{

		ClosestPair.getRuntimes(10, 20, "f");

	}


}

• public static void getRuntimes(int p, int t, String f)
does the following:
(a) Generates p sets of points for various sizes starting from 10 (the sets are not random so
that experiments are repeatable).
(b) Finds a closest pair of points using naiveClosestPair and closestPair, and
takes the cpu times for these.
© Records the worst case times for each algorithm implementation taken over the p sets
(note that it is pefectly possible that the worst case runtimes for the two algorithm implementations
occur for different sets).
(d) Repeats the above with sets of size 20, 30, . . . , 10t.
(e) Outputs the result for each iteration on the file named f in the format
input-set-size naiveClosestPair-worst-case-runtime
ClosestPair-worst-case-runtime
What should I do?Thanks in advance

Attached Files

PointComparatorByX.java 197bytes 49 downloads
TrivialClosestPairException.java 85bytes 49 downloads
Point.java 1.58K 56 downloads
ClosestPair.java 9.89K 62 downloads
UnreadablePointSetException.java 85bytes 50 downloads
InvalidNumberOfTestsException.java 87bytes 49 downloads
GraphingData.java 13.41K 73 downloads
StudentCode.java 2.31K 56 downloads
UnknownSortOptionException.java 84bytes 50 downloads
PointSet.java 4.66K 56 downloads
PointComparatorByY.java 197bytes 50 downloads

Edited by PatrickM, 28 February 2011 - 12:17 AM.

for (int i = 0; i <Y.length; i++) { int Z = Y[i].getY(); if ((W-Z)*(W-Z) <= distance) { [COLOR=red]shortDist[n] = Y[i];[/COLOR] //Did you to mean to always write to shortDist[0]? } } //part 4.c of the shamos algorithm // collecting those elements which are near the divider in the shortDist-array. for (int i =0; i< [COLOR=red]shortDist.length -1[/COLOR]; i++) { for (int j = i+1; j <[COLOR=red]shortDist.length-1[/COLOR] && j< i + 7; j ++){ distance = Math.min(distance, shortDist[i].sqrDist(shortDist[j]));->here } } return distance; }