Java Find Closest Pair in Dataset gives NullPointerException

Question

I am trying to find closest pair from a few different datasets. So far it works with the SmallerSet which has 12 points but when I change dataset which has 100 points . It's giving NullPointer Excepiton at the line that I add "--->" before the loop .I dont't figure it out to solve.

public static PointPair closestPair(Point2D.Double Px[], Point2D.Double Py[],int n) {
    PointPair closestpairLeft;
    PointPair closestpairRight;

    if (n <= 3) {
        if(n==2)
            return new PointPair(Px[0],Px[1]);
        else{
        PointPair p1=new PointPair(Px[0],Px[1]);
        PointPair p2=new PointPair(Px[0],Px[2]);
        PointPair p3=new PointPair(Px[1],Px[2]);

        if(p1.closerThan(p2)<0){
            if(p1.closerThan(p3)<0)
                return p1;
            else
                return p3;
        }
        else{
            if(p2.closerThan(p3)<0)
                return p2;
            else
                return p3;
        }
        }
    } else {
        int mid = n / 2;


        Point2D.Double Xl[] = Arrays.copyOfRange(Px, 0, mid);
        Point2D.Double Xr[] = Arrays.copyOfRange(Px, mid, n);

        Point2D.Double Yl[] = new Point2D.Double[Xl.length];
        Point2D.Double Yr[] = new Point2D.Double[Xr.length];


       ---> for (int i = 0, k = 0, j = 0; i < n; i++) {
            if (Py[i].getX() <= Xl[mid-1].getX()&& j<mid) {
                Yl[j++] = Py[i];
            } else if (k<mid){
                Yr[k++] = Py[i];

            }

        }

        closestpairLeft=closestPair(Xl,Yl,mid);
        closestpairRight=closestPair(Xr,Yr,n-mid);
    }

score 1 · Accepted Answer · answered Apr 20 '20 at 11:34

1

Do it as follows:

for (int i = 0, k = 0, j = 0; i < n && i < Py.length && (mid - 1) < Xl.length && j < Yl.length
        && k < Yr.length; i++) {
    if (Py[i] != null && Xl[mid - 1] != null && Py[i].getX() <= Xl[mid - 1].getX() && j < mid) {
        Yl[j++] = Py[i];
    } else if (k < mid) {
        Yr[k++] = Py[i];
    }
}

Note that I have checked not just Py[i] and Xl[mid-1] for null but also the bounds of the arrays to avoid ArrayIndexOutOfBoundsException.

answered Apr 20 '20 at 11:34

Arvind Kumar Avinash

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Thanks a lot , it worked but i don't understand that, why we should check all boundaries. – Erhan Kilic Apr 20 '20 at 11:52
You are most welcome. You should check all boundaries to ensure the code doesn't try to access an element from an index which is out of bounds. Such checks are always recommended to avoid accidental mistakes. – Arvind Kumar Avinash Apr 20 '20 at 12:01

Java Find Closest Pair in Dataset gives NullPointerException

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