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Thread: 2 dimentional boolean array problems!

  1. #1
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    Unhappy two dimensional boolean array problems!

    Hi


    How can I do a matrix code with a boolean expression and then output it?

    I know that when you declare a 2 dimensional boolean array that all the elements are already false. So how can I display them elements that are false with a '.' and if true with a 'O'?

    Sorry for the dumb amateur question but I just cant do it!

    Thank you for any help given!

    I have tried this:

     
    	{
    		       for(int i=0;i<matrix.length;i++)
    		       {
    		          for(int j=0;j<matrix[i].length;j++)
    		          {
    		             if(matrix[i][j])
    		             {
    		               System.out.print("O");
    		             }
    		             else
    		             {
    		               System.out.print(".");
    		             }
    		          }
    		            System.out.println("");

    How ever the problem here is that I get a nullPointerException error appear.

    The full code is:


    /**
     * Class LEDDisplay - write a description of the class here.
     * 
     * @author (your name) 
     * @version (a version number or a date)
     */
    public class LEDDisplay
    {
    	/* private class constants */
    	private final int FONT_LETTER_HEIGHT = 5;
    	private final int FONT_LETTER_WIDTH = 6;
    	private final int LETTERS_PER_DISPLAY = 10;
     
     
    	/* instance variables */
    	private boolean [][] matrix;
     
    	/**
    	 * Constructor for objects of class LEDDisplay
    	 */
    	public LEDDisplay()
    	{
    		boolean[][] matrix = new boolean [FONT_LETTER_HEIGHT] [FONT_LETTER_WIDTH * LETTERS_PER_DISPLAY];
    	}
     
    	/*
    	 * This display() method will print a textual representation of matrix[][] onto the display pane.
    	 */
     
    	public void display()
     
    	{
    		       for(int i=0;i<matrix.length;i++)
    		       {
    		          for(int j=0;j<matrix[i].length;j++)
    		          {
    		             if(matrix[i][j])
    		             {
    		               System.out.print("O");
    		             }
    		             else
    		             {
    		               System.out.print(".");
    		             }
    		          }
    		            System.out.println("");
     
    }
    	}
    }
    I think that I need to add something to do with a 'char' >? But I am a complete amateur and have no idea where to sort this out!

    I just want to to output a display like this:
    .................................................. .................................................. ......................................
    .................................................. .................................................. ......................................
    .................................................. .................................................. ......................................
    .................................................. .................................................. ......................................
    .................................................. .................................................. ......................................

    at this stage as each '.' represents a false value in that 2 dim array.

    I also think that its something to do with the fact that I have declared matrix twice? Again I still have no idea how I can sort it!?

    The error I get at this stage is:
    Exception: line 2. java.lang.NullPointerException

    Which is crazy as line 2 is fine!

    I have been going crazy for weeks on this and what annoys me is that its such a simple thing! Please help me before I go crazy !

    Again thank you for any help you give on this!

    Regards
    Last edited by masterT; July 14th, 2011 at 11:55 PM. Reason: spelt dimensional wrong! sorry about that!


  2. #2
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    Default Re: 2 dimentional boolean array problems!

    Ok I have resolved this now! Thanks for looking.

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