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POmerging convex hulls in java
 

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  1. POmerging convex hulls in java
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    <p>I've written a java program which which uses the divide and conquer algorithm to find the convex hull of a polygon in a cartesian coordination. I have a class Coord which has two "double" fields X and y and "this" which I'm using the method on, is a collection of coordinates (set). my method should return the hull (Collection) of the polygon</p> <p>My code is like this:</p> <pre><code> public Collection&lt;Coord&gt; territoire() { Collection&lt;Coord&gt; sommets = new ArrayList&lt;Coord&gt;(); ArrayList&lt;Coord&gt; thisToArrList = new ArrayList&lt;Coord&gt;(); for(Coord c : this) thisToArrList.add(c); ArrayList&lt;Coord&gt; sortedPointsByX = new ArrayList&lt;Coord&gt;(); int n = this.size(); if (n &lt;= 2) return this; //sorting the points by their X coordinates sortedPointsByX = sortedArrayByX(thisToArrList); //&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt; works good till here &lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt; // now sortedPointsByX array contains the points with increasing X // splitting the sortedPointsByX into two arrays ArrayList&lt;Coord&gt; firstPart = new ArrayList&lt;Coord&gt;(); ArrayList&lt;Coord&gt; secondPart = new ArrayList&lt;Coord&gt;(); // if the number of the points is prime, the leftmost and the rightmost half // both have same number of points if(sortedPointsByX.size() % 2 == 0) { for(int i = 0; i &lt; sortedPointsByX.size()/2; i++) { firstPart.add(sortedPointsByX.get(i)); } for(int i = sortedPointsByX.size()/2; i &lt; sortedPointsByX.size(); i++) { secondPart.add(sortedPointsByX.get(i)); } } //&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;works good till here&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt; // if the number of points is odd, the leftmost half have the extra points else { for(int i = 0; i &lt; sortedPointsByX.size()/2+1; i++) { firstPart.add(sortedPointsByX.get(i)); } for(int i = sortedPointsByX.size()/2+1; i &lt; sortedPointsByX.size(); i++) { secondPart.add(sortedPointsByX.get(i)); } } //&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;works good till here&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt; CoordSet firstSet = new CoordSet(firstPart); CoordSet secondSet = new CoordSet(secondPart); // converting the arrays to list of coordinates in order to use recursion over them //recursion for sub coordsets Collection&lt;Coord&gt; firstSetSommet = firstSet.territoire(); Collection&lt;Coord&gt; secondSetSommet = secondSet.territoire(); ArrayList&lt;Coord&gt; firstHull = new ArrayList&lt;Coord&gt;(firstSetSommet); ArrayList&lt;Coord&gt; secondHull = new ArrayList&lt;Coord&gt;(secondSetSommet); sommets = mergeHulls(firstHull, secondHull); return sommets; } public Collection&lt;Coord&gt; mergeHulls(ArrayList&lt;Coord&gt; firstHull, ArrayList&lt;Coord&gt; secondHull) { Collection&lt;Coord&gt; pointsInside = new ArrayList&lt;Coord&gt;(); Collection&lt;Coord&gt; sommets = new ArrayList&lt;Coord&gt;(); //********************upper tangent*************** //find the highest point of the leftmost part Coord firstPartHighestPoint = getMaxY(firstHull); //find the highest point of the rightmost part Coord secondPartHighestPoint = getMaxY(secondHull); for(int i = 0; i&lt; firstHull.size(); i++) { // check if the points lie on the line between highest point in leftmost and in rightmost // if true, the current point is above the line if(isCollinear(firstPartHighestPoint, secondPartHighestPoint, firstHull.get(i))&gt;0) { // the current point is above the line firstPartHighestPoint = firstHull.get(i); } pointsInside.add(firstPartHighestPoint); } for(int i = 0; i &lt; secondHull.size(); i++) { if(isCollinear(firstPartHighestPoint, secondPartHighestPoint, secondHull.get(i))&gt;0) { // the current point is above the line secondPartHighestPoint = secondHull.get(i); } pointsInside.add(secondPartHighestPoint); } //******************lower tangent*************** //find the lowest point of the leftmost part Coord firstPartLowestPoint = getMinY(firstHull); // find the lowest point of the rightmost part Coord secondPartLowestPoint = getMinY(secondHull); for(int i = 0; i&lt; firstHull.size(); i++) { // check if the points lie on the line between highest point in leftmost and in rightmost // if true, the current point is above the line if(isCollinear(firstPartLowestPoint, secondPartLowestPoint, firstHull.get(i)) &lt; 0) { // the current point is above the line firstPartLowestPoint = firstHull.get(i); } pointsInside.add(firstPartLowestPoint); } for(int i = 0; i &lt; secondHull.size(); i++) { if(isCollinear(firstPartLowestPoint, secondPartLowestPoint, secondHull.get(i)) &lt; 0) { // the current point is above the line secondPartLowestPoint = secondHull.get(i); } pointsInside.add(firstPartLowestPoint); } sommets.addAll(firstHull); sommets.addAll(secondHull); sommets.removeAll(pointsInside); return sommets; } //**********************************Auxiliary méthods**************************************************** // if the equation is equal to 0, the points are collinear // the method returns the determinant of the point matrix // This determinant tells how far point 'c' is from vector ab and on which side // it is // &lt; 0 if the point 'c' is below the line (assumption : horizontal line) // &gt; 0 if the point 'c' is above the line public double isCollinear(Coord a, Coord b, Coord c) { return ((b.x - a.x)*(c.y - a.y) - (b.y - a.y)*(c.x - a.x)); } //************************************** line equation ************************************************ // find the slope of the line between two points public static double findSlope(Coord point1, Coord point2) { return (point2.y - point1.y)/(point2.x-point1.x); } // finding the constant 'b' of the line equation y = xm + b public static double constantB(Double slope, Coord point) { return point.y - slope* point.x; } //*************************************** Minimum and Maximum "Y" ***************************************** // the point with maximum Y public static Coord getMaxY(ArrayList&lt;Coord&gt; points) { double maxY = points.get(0).y; // start with the first value Coord maxPoint = points.get(0); for (int i=1; i&lt;points.size(); i++) { if (points.get(i).y &gt; maxY) { maxY = points.get(i).y; // new maximum maxPoint = points.get(i); } } return maxPoint; } // a method to find the Point with the minimum y public static Coord getMinY(ArrayList&lt;Coord&gt; points) { double minValue = points.get(0).y; Coord minPoint = points.get(0); for(int i=1;i&lt;points.size();i++){ if(points.get(i).y &lt; minValue) { minPoint = points.get(i); minValue = points.get(i).y; } } return minPoint; } //************************************** sorting the points ******************************************** //sorting the points by their x in ascending order public static ArrayList&lt;Coord&gt; sortedArrayByX(ArrayList&lt;Coord&gt; arrayOfPoints) { //double minval = arrayOfPoints[0].x; Coord temp = null; for(int i = 0; i&lt; arrayOfPoints.size(); i++) { for(int j = 0; j&lt; arrayOfPoints.size()-1; j++) { if(arrayOfPoints.get(j+1).x &lt; arrayOfPoints.get(j).x) { temp = arrayOfPoints.get(j+1); arrayOfPoints.set(j+1, arrayOfPoints.get(j)); arrayOfPoints.set(j, temp); } } } return arrayOfPoints; } </code></pre> <p>I can't get why when I run the program, the following message shows up:</p> <pre><code>Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0 at java.util.ArrayList.rangeCheck(ArrayList.java:604) at java.util.ArrayList.get(ArrayList.java:382) at miniprojet2.CoordSet.getMaxY(CoordSet.java:270) at miniprojet2.CoordSet.mergeHulls(CoordSet.java:154) at miniprojet2.CoordSet.territoire(CoordSet.java:139) at miniprojet2.CalculeTerritoire.main(CalculeTerritoire.java:36) </code></pre> <p>I'll be so glad if you tell me where I've made a mistake</p>
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    1. PTQuestion
    1. USNavid Koochooloo
    1. USNavid Koochooloo
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
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    1. VO
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      1. POmerging convex hulls in java
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
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      1. POmerging convex hulls in java
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
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      1. POmerging convex hulls in java
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      1. VTUpMod
    1. COThat's a runtime error, not a compile-time error.
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      1. POmerging convex hulls in java
      1. USwchargin
    2. COyes, you are right. my bad :) but I don't even know why I got this error
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      1. POmerging convex hulls in java
      1. USNavid Koochooloo
 

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