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Friday, 20 January 2017

Chapter 22 Exercise 9, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

22.9 (Geometry: gift-wrapping algorithm for finding a convex hull) Section 22.10.1 introduced the gift-wrapping algorithm for finding a convex hull for a set of points. Assume that the Java’s coordinate system is used for the points. Imple-
ment the algorithm using the following method:

/** Return the points that form a convex hull */
public static ArrayList<Point2D> getConvexHull(double[][] s)

Point2D is defined in Section 9.6.
Write a test program that prompts the user to enter the set size and the points
and displays the points that form a convex hull.


import java.util.ArrayList;

public class Exercise09 {

 public static void main(String[] args) {
  double[][] points = new double[6][2];
  for (int i = 0; i < points.length; i++) {
   points[i][0] = (int)(Math.random() * 100);
   points[i][1] = (int)(Math.random() * 100);
  }
  /*
  points[0][0] = 1;
  points[0][1] = 2.4;
  points[1][0] = 2.5;
  points[1][1] = 2;
  points[2][0] = 1.5;
  points[2][1] = 34.5;
  points[3][0] = 5.5;
  points[3][1] = 6;
  points[4][0] = 6;
  points[4][1] = 2.4;
  points[5][0] = 5.5;
  points[5][1] = 9;
  */          
  ArrayList<MyPoint> hull = getConvexHull(points);
  System.out.println("There are " + points.length + " points:");
  for (int i = 0; i < points.length; i++) {
   System.out.print("(" + points[i][0] + ", " + points[i][1] + ")  ");
  }
  System.out.println("\nThe convex hull is:");
  for (MyPoint myPoint : hull) {
   System.out.print("(" + myPoint.x + ", " + myPoint.y + ")  ");
  }
 }

 public static ArrayList<MyPoint> getConvexHull(double[][] s) {
  ArrayList<MyPoint> oldPoints = new ArrayList<>();
  for (int i = 0; i < s.length; i++) {
   oldPoints.add(new MyPoint(s[i][0], s[i][1]));
  }
  
  ArrayList<MyPoint> points = new ArrayList<>();
  
  //first point
  MyPoint h0 = oldPoints.get(0);
  for (int i = 1; i < oldPoints.size(); i++) {
   if(oldPoints.get(i).y > h0.y) {
    h0 = oldPoints.get(i);
   } else if(oldPoints.get(i).y == h0.y) {
    if(oldPoints.get(i).x > h0.x) {
     h0 = oldPoints.get(i);
    }
   }
  }
  points.add(h0);
  
  //second point
  MyPoint virtualPoint = new MyPoint(h0.x + 1, h0.y);
  MyPoint h1 = oldPoints.get(0);
  double tmpAngle = getAngle(h0, virtualPoint, h1);
  if(Double.isNaN(tmpAngle)) {
   tmpAngle = Double.MAX_VALUE;
  }
  for (int j = 1; j < oldPoints.size(); j++) {
   double newAngle = getAngle(h0, virtualPoint, oldPoints.get(j));
   if(Double.isNaN(newAngle)) {
    newAngle = Double.MAX_VALUE;
   }
   if(newAngle < tmpAngle) {
    tmpAngle = newAngle;
    h1 = oldPoints.get(j);
   }
  }
  points.add(h1);
  
  //other points
  while(h1 != points.get(0)) {
   MyPoint tmpPoint = oldPoints.get(0);
   double tmpAngle2 = getAngle(h1, tmpPoint, h0);
   if(Double.isNaN(tmpAngle2)) {
    tmpAngle2 = 0;
   }
   for (int j = 1; j < oldPoints.size(); j++) {
    double newAngle = getAngle(h1, oldPoints.get(j), h0);
    if(Double.isNaN(newAngle)) {
     continue;
    }
    if(newAngle > tmpAngle2) {
     tmpAngle2 = newAngle;
     tmpPoint = oldPoints.get(j);
    } else if(newAngle == tmpAngle2) {
     if(getSide(h1, oldPoints.get(j)) > getSide(h1, tmpPoint)) {
      tmpAngle2 = newAngle;
      tmpPoint = oldPoints.get(j);
     }
    }
   }
   if(tmpPoint != points.get(0)) {
    points.add(tmpPoint);
   }
   h0 = h1;
   h1 = tmpPoint;
  }
  return points;
 }
 
 public static double getAngle(MyPoint p1, MyPoint p2, MyPoint p3) {
  double a = getSide(p2, p3);
  double b = getSide(p1, p3);
  double c = getSide(p1, p2);
  return Math.toDegrees(Math.acos((a * a - b * b - c * c) / (-2 * b * c)));
 }
 
 public static double getSide(MyPoint p1, MyPoint p2) {
  return Math.sqrt((p2.x - p1.x) * (p2.x - p1.x) + (p2.y - p1.y) * (p2.y - p1.y));
 }
 
 static class MyPoint {  
  double x, y;  
  MyPoint(double x, double y) {
   this.x = x; this.y = y;
  }
 }
}

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