closest pair of points, recursion not stopping at the correct area

Question

Working on a closest point algorithm here. I am taking a 2d array of ints in. For some reason I am not getting the right answer. What is wrong with my recursion in conquer? I think that is the problem.

package ClosestPair;

import java.awt.Point;
import java.util.Arrays;

public class ClosestPair{   




public double divide(int[][] data){


    int size = data.length;


    int[][] X_L = new int[size/2][2];
    int[][] X_R = new int[size/2][2];
    int[][] Y_L = new int[size/2][2];
    int[][] Y_R = new int[size/2][2];
    int[][] P_L = new int[size][2];
    int[][] P_R = new int[size][2];






    ////////// X Portion ////////////////////////////
    //sort the points
    int[][] temp = data;
    Arrays.sort(temp, new ColumnComparator(0));

    //split the points
    for (int i = 0; i < temp.length/2; i++){
        X_L[i][0] = temp[i][0];
        X_L[i][1] = temp[i+1][1];
        X_R[i][0] = temp[i+temp.length/2][0];
        X_R[i][1] = temp[i+temp.length/2][1];
    }




    ////////// Y Portion ////////////////////////////
    Arrays.sort(temp, new ColumnComparator(1));
            //split the points
            for (int i = 0; i < temp.length/2; i++){
                Y_L[i][0] = temp[i][0];
                Y_L[i][1] = temp[i+1][1];
                Y_R[i][0] = temp[i+temp.length/2][0];
                Y_R[i][1] = temp[i+temp.length/2][1];
            }


            //P_L
            P_L= appendArrays(X_L, Y_L);

            //P_R
            P_R= appendArrays(X_R, Y_R);

            if(conquer(P_L)< conquer(P_R)) return conquer(P_L);

            return conquer(P_R);        
}

public double conquer(int[][] array){
    if(array.length == 2){
        return distance(array);
    }

    int[][] temp1 = new int[array.length/2][2];
    int[][] temp2 = new int[array.length/2][2];
    int length = array.length/4;
    for (int i = 0; i < length; i++){
        temp1[i][0] = array[i][0];
        temp1[i][1] = array[i+1][1];
        temp2[i][0] = array[i+array.length/2][0];
        temp2[i][1] = array[i+array.length/2][1];
    }       
    return conquer(temp1);
}


 private static int[][] appendArrays(int[][] array1, int[][] array2) {
        int[][] ret = new int[array1.length + array2.length][];
        int i = 0;
        for (;i<array1.length;i++) {
            ret[i] = array1[i];
        }
        for (int j = 0;j<array2.length;j++) {
            ret[i++] = array2[j];
        }
        return ret;
    }



public double distance(int[][] points){
    //pair distance
    int a = (points[0][0]+points[1][0]);
    int b = (points[1][1]+points[1][1]);
    return Math.sqrt(  Math.pow(a, 2) + Math.pow(b, 2)    );
}

}

score 0 · Answer 1 · answered May 10 '16 at 15:28

You have wrong formula for calculation distance. It should be

int a = (points[0][0]-points[1][0]);
int b = (points[1][1]-points[1][1]);
return Math.sqrt(  Math.pow(a, 2) + Math.pow(b, 2)    );

Or you could use special function:

int x = (points[0][0]-points[1][0]);
int y = (points[1][1]-points[1][1]);
return Math.hypot(x,y);

closest pair of points, recursion not stopping at the correct area

1 Answers1