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Thread: I think I need an algorith

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    Question I think I need an algorith

    I'm trying to get it to compare rectangles in an array.

    I want it to compare Rectangle 0 and Rectangle 1 and Rectangle 0 and Rectangle 2, etc.

    The number of rectangles is being specified by the user.

    I want it to check like for 6 rectangles,

    1 and 2, 1 and 3, 1 and 4, 1 and 5, 1 and 6
    2 and 3, 2 and 4, 2 and 5, 2 and 6
    3 and 4, 3 and 5, 3 and 6
    4 and 5, 4 and 6
    5 and 6

    Where 1 is the first rectangle.

    Note: Rectangle 1 corresponds to Rectangle[0] where each value in Rectangle[0] through Rectangle[n] is a Rectangle(my defined Rectangle, not the already written Rectangle class.)

    I'd like to compare each Rectangle with each other to see if it overlaps but I don't want to do check the same thing, like 1 and 1, 2 and 3 and also checking 3 and 2.


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    Default Re: I think I need an algorith

    Ok, so if you are checking to see if they are overlapping. You want to see if any given point in each edge of Rectangle A is Less than the X value and the Y value of the top-left edge of Rectangle B and greater than the X value and the Y value of the bottom-right edge of Rectangle B.

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    Default Re: I think I need an algorith

    First, write a method which compares the rectangles. In the java.awt.Rectangle class, there is a method intersects which returns a boolean, perhaps implement something like this (any reason why you are not using or extending that class?). You then need a double loop to compare the rectangles...something like this

    int size = 10;
    Rectangle[] rects = new Rectangle[size];
    //instantiate the rectangles
    for ( int i = 0; i < size; i++ ){
        for ( int j = i+1; j < size; j++ ){
             if ( rects[i].intersects(rects[j]) ){
                //do something
            }
        }
    }

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    Default Re: I think I need an algorith

    intersects() can also be used for collision detection, right?

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    Default Re: I think I need an algorith

    If you're not sure, write a small program, define two rectangles, and execute the intersects() method to see.

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    Default Re: I think I need an algorith

    ok, I will try it

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    Default Re: I think I need an algorith

    Kind of. You need to figure out whether intersects will return true if the two objects share an edge, or even just a vertex. If it does, then that would be take care of collision checking between two rectangles, and you'd just need to check for collision between all rectangles or use an accelerator structure to reduce the number of potential collisions you need to check.

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    Default Re: I think I need an algorith

    Trying not to be annoying, but what is an "accelerator structure?"

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    Default Re: I think I need an algorith

    It's a way of dividing up your rectangles/objects in a way that allows you to quickly decide if it's even possible that any two objects/rectangles will be in collision. It's kind of a general term, but there are many accelerator structures for collision detection and they operate in different ways.

    A simple one is using a grid:

    Divide your data into small grids, then for every rectangle that lies in that region, add it to a list of objects for that grid (note: each rectangle can belong to several grid boxes).

    Then, when you're checking for collisions, start by figuring out which grid boxes the rectangle you're interested in lies in. You will only need to check collisions against any rectangle which lies in these grids because your rectangle doesn't lie in any other grid.

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    Unhappy Re: I think I need an algorith

    Here's what they want me to do. I'm not sure how to do a switch structure.

    Attached Files Attached Files

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    Default Re: I think I need an algorith

    If your too lazy to write a description of your problem and post it here, I way to lazy to read it out of an attached file.

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    Smile Re: I think I need an algorith

    Assignment 1: Rectangles
    Out: 24th August 2010 Due: 2nd September 2010 (Thu) at 11:59pm
    In this assignment you will write a class to represent an “axis-aligned” rectangle, i.e. a rectangle with only
    horizontal and vertical sides. You will write various methods to support different operations on Rectangle
    objects and finally, you will test your class by defining several rectangles and comparing them in various ways.
    A rectangle is represented by its lower left corner (x,y), width and height. Write a class that represents a
    rectangle as defined above. This class should have the following methods:
    1. A constructor that allows you to pass the position and dimensions of this rectangle to initialize the
    Rectangle object.
    2. A toString() method to print out the various variables of the rectangle in a reasonable way.
    3. A method area() that returns the area of the rectangle.
    4. A method overlaps(Rectangle a) that takes as parameter a Rectangle object ‘a’ and returns true if this
    rectangle overlaps with ‘a’.
    5. A method intersect(Rectangle a) that takes as parameter a Rectangle object ‘a’ and returns the rectangle
    that is formed by the intersection of this rectangle with ‘a’. Your method should return null if the two
    rectangles do not overlap.
    6. A method union(Rectangle a) that takes as parameter a Rectangle object ‘a’ and returns the rectangle
    that represents the union of this rectangle and ‘a’. The union of two rectangles is defined as the smallest
    rectangle that encloses both rectangles. Hint: The union and intersect methods are much more similar
    to each other than you might think!
    Now write another class with a main method. When you run the program, it should ask you for the following
    inputs from the keyboard in the following order:
    1. Number of rectangles
    2. Position (x,y) and the dimensions (width and height) of each rectangle in order.
    Finally, it should print the following in the following order:
    1. The areas of all the rectangles in the order that they were entered.
    (x,y)
    width
    height
    2. All pairs of rectangles that overlap each other. For each such pair, it should also print the position and
    dimensions of the intersection of the two rectangles.
    3. The total number of overlaps found in step 2.
    4. The position and dimensions of the rectangle obtained by the union of all the rectangles. This is the
    smallest rectangle in which all the input rectangles will fit. It is called the “bounding box”.
    See the accompanying file for an example of the expected input and output.
    Applications: Such rectangle classes are very widely used. You will find a similar class in almost any windowing
    toolkit/GUI library. They are used to manage overlapping text boxes, to detect whether a mouse is in a certain
    region of the window, etc. Another practical application of rectangles and their operations is in large-scale
    integrated circuit design where layouts on the chip are tested for consistency before fabricating the chip. If the
    layouts (usually rectangular) of two components overlap each other, the chip cannot be fabricated in a single
    layer. In practice more efficient algorithms are used to find all overlaps, but the concept is the same.
    Expectations from a perfect program:
    1. The class should have the same method names as above.
    2. The source code should be suitably commented.
    a. Inside every class should be a short explanation of what the class represents and what it is used
    for.
    b. Just before every method signature, there should be information about what the method does,
    what every parameter means and what the method returns, if anything.
    c. Just inside every method there should be information about what exactly the method does (in
    steps).
    3. The inputs should be taken in the specified order.
    4. The outputs are exactly as specified.
    What/How to submit: Turn in all your source code files (*.java) in a single zipped file on Blackboard. You do not
    have to turn in your .class files.

    package rectangle;
     
    import java.awt.Point;
    import java.awt.geom.Line2D;
     
    /**
     * @author Paul
     *
     */
    public class Rectangle {
    	private double width;
    	private double height;
    	private double area;
    	private double x;
    	private double y;
    	private Point p;
    	private Line2D.Double line1;
    	private Line2D.Double line2;
     
    	public Rectangle(double x, double y, double width, double height)
    	{
    		// makes the private variable height equal to the value of height in constructor.
    		this.height = height; 
     
    		// makes the private variable width equal to the value of width in constructor.
    		this.width = width;
     
    	this.x = x;
    	this.y = y;
     
    	line1 = new Line2D.Double(x, y, x, y - height);
    	line2 = new Line2D.Double(x, y - height, x + width, y - height);
    		//p = new Point(x,y);
     
     
     
    	}
     
     
     
     
     
    	public String toString()
    	{
    		System.out.println("width is: " + width + "; height is: " + height + "; area is: " + area + "; Point is: " + x + "," + y);
    		String str = "width is: " + width + "; height is: " + height + "; area is: " + area + "; Point is: " + x + "," + y;
    		return(str);
    	}
     
     
    	public double area()
    	{
    		area = width * height;
    		return(area);
    	}
     
    	public boolean overlaps(Rectangle a)
    	{
    		// I somehow need a switch structure here, but have never done one in my life before.  
     
                 /* 1. Draw the three cases on a piece of paper: two rectangles overlap
    partially, one rectangle is inside the other and two rectangles that do not
    overlap.
     
    2. You have to write a method that correctly identifies whether it is case 1
    or 2, in which case you return true. Or case 3, in which case you return
    false.
     
    3. Look at the simplest case, the first one. In terms of only
    (x,y,width,height) of both rectangles (i.e. 8 values in total) can you
    express the partial overlap? That is, try to come up with a condition
    involving these 8 variables that will evaluate to true when the two
    rectangles partially overlap. Once you have this, see how you must change
    this condition so that it works for the other two cases.
    		*/
     
     
    	}
     
    	public Rectangle intersect(Rectangle a)
    	{
     
    		if (a.overlaps(a) == true)
    		{
    			return(the new Rectangle);
    		}
     
    else
    return(null);
    	}
     
    	public Rectangle union(Rectangle a)
    	{
    		// I know that you use two coordinates, the lower left and the upper right, of the new Rectangle, to get the height and width of the new Rectangle but how do you get those two points?  
    	}
    }

    package rectangle;
     
    import java.util.Scanner;
    import java.io.*;
    import java.util.*;
     
    // this class is the test program.  It will ask the user for the number of rectangles and for the bottom-left corner
    // coordinate and the width and height and will output the area, all the pairs of rectangles that overlap each other, 
    // and also the lower-left corner postion and the dimensions of the rectangle formed by the intersection of each such pair
    // and also, it will output, by output I mean print out, the total number of overlaps, the position and dimensions of the rectangle that
    // will hold all of the other rectangles.  
    /**
     * @author Paul
     *
     */
    public class TestRectangleProgram {
     
    	/**
    	 * @param args
    	 */
    	static Scanner console = new Scanner(System.in);
    	public static void main(String[] args) {
    		// TODO Auto-generated method stub
     
    		// prints Enter the number of rectangles.
     
    		// creates int variable numOfOverlaps and initializes it to 0.
    		int numOfOverlaps = 0;
    System.out.println("Enter the number of rectangles.");
     
     
     
    // each array stores a value, xValue, yValue, widthArray, and heightArray.  
    // The array values, made by the user inputs in the for loop, 
    // will be called by another for loop that will print out
    // xCords[0], yCords[0],  widthArray[0], heightArray[0] + add a line
    // xCords[1], yCords[1], widthArray[1], heightArray[1] and so on
    // for the widthArray and heightArray, their values will be made to fill
    // areaArray like this in the for loop: areaArray[j] = rect[j].area() ,
    // where rect is an array of Rectangles the size of the number of rectangles.
     
     
    // user input for # of rectangles
    int numOfRectangles;
     
    	numOfRectangles = console.nextInt();
     
     
    // array of x values that will get filled in by the for loop.  The array size is the number of rectangles.  
    double[] xCords = new double[numOfRectangles];
     
    // array of y values that will get filled in by the for loop.  The array size is the number of rectangles.
    double[] yCords = new double[numOfRectangles];
     
    // array for the width that will get filled in by the for loop.  The array size is the number of rectangles
     
    double[] widthArray = new double[numOfRectangles];
     
    // array for the height that will get filled in by the for loop.  The array size is the number of rectangles
     
    double[] heightArray = new double[numOfRectangles];
     
    // array to store areas.  Array size is the number of rectangles.
    double[] areaArray = new double [numOfRectangles];
     
     
     
     
     
    // for loop will ask user for x-coordinate of rectangle, y-coordinate of rectangle, height of rectangle, and width of rectangle
    // until it has done so for the number of rectangles.  j is set to 0 and will increase until the for loop exits when the 
    // value of j reaches the number of rectangles.
    Rectangle[] rect = new Rectangle[numOfRectangles];
    for (int j = 0; j < numOfRectangles; j++)
    {
    	System.out.println("Enter the x-coordinate for Rectangle " + (j+1)+ " ");
    	xCords[j] = console.nextDouble();
    	System.out.println("Enter the y-coordinate for the Rectangle " + (j+1) + " ");
    	yCords[j] = console.nextDouble();
    	System.out.println("Enter the width for the Rectangle " + (j+1) + " ");
    	widthArray[j] = console.nextDouble();
    	System.out.println("Enter the height for the Rectangle " + (j+1) + " ");
    	heightArray[j] = console.nextDouble();
    	rect[j] = new Rectangle(xCords[j], yCords[j], widthArray[j], heightArray[j]);
    	areaArray[j] = rect[j].area();
    	System.out.println("Rectangle " + (j+1) + ": " + rect[j].toString());
     
     
     
     
    }
     
    for (int k = 0; k < numOfRectangles; k++)
    {
    	System.out.println("The area of Rectangle " + (k+1) + " is " + areaArray[k] + ".");
    }
     
    for(int v = 0; v < numOfRectangles; v++)
    {// beginning of for
    	for (int w = v + 1; w < numOfRectangles; w++)
    	{ // beginning of inner for
    		if (rect[v].overlaps(rect[w]) == true)
    		{ // beginning of if
    			numOfOverlaps = numOfOverlaps + 1;
    		} // end of if
    	} // end of inner for
    } // end of for
    System.out.println("The number of overlaps is: " + numOfOverlaps);
    for (int p = 0; p < numOfRectangles; p++)
    {
     
    }
    	}
     
    }

    How do you get it to perform the union(Rectangle a) method on each of the rectangles like so

    Rectangle r = Rectangle 1.union(Rectangle2.union(Rectangle3.union(Rectangl e4.union(Rectangle.union(etc))))

    or something like that



    How do you get it to make the intersect method work? I know you use two corners or something, but how do you get these points?
    Last edited by javapenguin; September 1st, 2010 at 08:23 PM.

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    Default Re: I think I need an algorith

    What are your questions? Again, I'm too lazy to read through all of your assignment and to try to figure out what you need to know and what you need to do. You'll have to read the assignment and come up with specific questions about the project where you want help.

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    Default Re: I think I need an algorith

    Quote Originally Posted by Norm View Post
    What are your questions? Again, I'm too lazy to read through all of your assignment and to try to figure out what you need to know and what you need to do. You'll have to read the assignment and come up with specific questions about the project where you want help.
    How to write a switch struture.

    Am sending a document with stuff in it:

    In this assignment you will write a class to represent an “axis-aligned” rectangle, i.e. a rectangle with only
    horizontal and vertical sides. You will write various methods to support different operations on Rectangle
    objects and finally, you will test your class by defining several rectangles and comparing them in various ways.
    A rectangle is represented by its lower left corner (x,y), width and height. Write a class that represents a
    rectangle as defined above.
    A method overlaps(Rectangle a) that takes as parameter a Rectangle object ‘a’ and returns true if this
    rectangle overlaps with ‘a’.
    A method intersect(Rectangle a) that takes as parameter a Rectangle object ‘a’ and returns the rectangle
    that is formed by the intersection of this rectangle with ‘a’. Your method should return null if the two
    rectangles do not overlap.
    A method union(Rectangle a) that takes as parameter a Rectangle object ‘a’ and returns the rectangle
    that represents the union of this rectangle and ‘a’. The union of two rectangles is defined as the smallest
    rectangle that encloses both rectangles. Hint: The union and intersect methods are much more similar
    to each other than you might think!

    1. Draw the three cases on a piece of paper: two rectangles overlap
    partially, one rectangle is inside the other and two rectangles that do
    not
    overlap.

    2. You have to write a method that correctly identifies whether it is case
    1


    or 2, in which case you return true. Or case 3, in which case you return
    false.

    3. Look at the simplest case, the first one. In terms of only
    (x,y,width,height) of both rectangles (i.e. 8 values in total) can you
    express the partial overlap? That is, try to come up with a condition
    involving these 8 variables that will evaluate to true when the two
    rectangles partially overlap. Once you have this, see how you must change
    this condition so that it works for the other two cases.

    I have received a number of questions regarding how to approach the intersect and the union methods. Here is a hint: it may be easier to think of a rectangle in terms of two points: lower-left and upper-right. To get the answer to both these methods, you must find these two points for the resulting rectangle in terms of what you know about the two given rectangles. After that subtracting the first point from the second will get you the width and height respectively.
    Attached Images Attached Images
    Last edited by javapenguin; September 1st, 2010 at 09:51 PM.

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    Default Re: I think I need an algorith

    I need to know how to set up a switch structure.

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    Default Re: I think I need an algorith

    Quote Originally Posted by javapenguin View Post
    I need to know how to set up a switch structure.
    By switch structure do you mean:
       int testInt = 10;
     
       switch (testInt) {
          case 1: system.out.println("int = 1");
          case 2: system.out.println("int = 2");
          ...ETC
       }

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    Default Re: I think I need an algorith

    You forgot the break statements.

    int testInt = 10;
    switch(testInt)
    {
    case 1:
        System.out.println("int = 1");
        break;
    case 2:
        System.out.println("int = 2");
        break;
    default:
        System.out.println("the int was not 1 or 2");
    }

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    Default Re: I think I need an algorith

    I thought I was missing something. I barely use switch statements.

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    Thumbs down Re: I think I need an algorith

    I have the problem where I'm not sure how to get a class that takes a Rectangle as a parameter to compare it to another Rectangle inside the class itself.

    	public boolean overlaps(Rectangle a)
    	{
     
    		if ((b.getUpperRightPosition().getX() < a.getBottomLeftPosition().getX()) 
    				|| (b.getUpperRightPosition().getY() < a.getBottomLeftPosition().getY()) 
    				|| (b.getBottomLeftPosition().getX() > a.getUpperRightPosition().getX())
    				|| (b.getBottomLeftPosition().getY() > a.getUpperRightPosition().getY()))
    				{
    				return(false);
    				} 
     
    				else
    				return(true);
     
     
     
    	}
     
    	public Rectangle intersect(Rectangle a)
    	{
    		if (((a.getBottomLeftPosition().getX() <= b.getBottomLeftPosition().getX()) && (b.getUpperRightPosition().getX() <= a.getUpperRightPosition().getX()))
    				&& ((a.getBottomLeftPosition().getY() <= b.getBottomLeftPosition().getY()) && (b.getUpperRightPosition().getY() <= a.getUpperRightPosition().getY())))
    				{
    				return(b);
    				}
     
    				else if (a.overlaps(b) == false)
    				{
    				return(null);
    				}
     
    				else
    				return(new Rectangle(innerRectangle.getBottomLeftPosition().getX(), innerRectangle.getBottomLeftPosition().getY(), innerRectangle.getUpperRightPosition().getX() - innerRectangle.getBottomLeftPosition().getX(), innerRectangle.getUpperRightPosition().getY() - innerRectangle.getBottomLeftPostion().getY()));
     
    	}
     
    	public Rectangle union(Rectangle a)
    	{
    		if (((a.getBottomLeftPosition().getX() <= b.getBottomLeftPosition().getX()) && (b.getUpperRightPosition().getX() <= a.getUpperRightPosition().getX()))
    				&& ((a.getBottomLeftPosition().getY() <= b.getBottomLeftPosition().getY()) && (b.getUpperRightPosition().getY() <= a.getUpperRightPosition().getY())))
    				{
    				return(a);
    				}
    	}
     
    	public Position getBottomLeftPosition()
    	{
    		return( new Position(x, y));
    	}
     
    	public Position getUpperRightPosition()
    	{
    		return(new Position( x + width, y + height));
    	}
    }

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    Default Re: I think I need an algorith

    how to get a class that takes a Rectangle as a parameter
    After over 200 posts and you don't know the difference between a class and a method. ?????

    When comparing two objects of the same class in a method of that class and that has one object passed to it as a parameter, use the "this" reference for the members of the current class and use the passed parameter to get at the other objects members:
    this.getX() >= parameter.getX()

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    javapenguin (September 2nd, 2010)

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    Smile Re: I think I need an algorith

    Quote Originally Posted by Norm View Post
    After over 200 posts and you don't know the difference between a class and a method. ?????

    When comparing two objects of the same class in a method of that class and that has one object passed to it as a parameter, use the "this" reference for the members of the current class and use the passed parameter to get at the other objects members:
    this.getX() >= parameter.getX()

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    Default Re: I think I need an algorith

    Ok how do I get something to perform the union method on Rectangles in an array of size n;

    Rectangle rectBig = rect[n-1].union(rect[n-2].union(rect[n-3].union(rect[n-4].union(rect[n-5].union(rect[n-6].union(etc)....union(rect[0]) ?

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    Smile One last question

    How do you get it to do something like this:

    where there is an array of Rectangles of size n;

    Rectangle superRect = rect[n-1].union(rect[n-2].union(rect[n-3].union(etc... .union(rect[0]))

    ^

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    Angry Help!!!

    I really need to know how to do that last thing with the union!!!

    Too late now! I had to turn it in!

    Last edited by javapenguin; September 2nd, 2010 at 11:37 PM.

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    Default Re: I think I need an algorith

    Much easier to use a loop.

    unionShape = empty shape
    for every rectangle in the list:
    unionShape = union(unionShape, rectangle)

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