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Thread: java help with inversing matrices

  1. #1
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    Default java help with inversing matrices

    hey, ive got to find the inverse of an 8x8 matrix. i have only coded a 2x2 matrix method. i have no clue or idea how to do anything larger than a 2x2. help is greatly needed.

    code:
    public static double[][] inverse(double[][] A) 
    { 
    double[][] result = new double[A.length][A.length]; 
     
    double ValueOfA = (A[0][0] * A[1][1]) - (A[0][1] * A[1][0]);
     
     
     
    result[0][0] = A[1][1];
    result[0][1] = -A[0][1];
    result[1][0] = -A[1][0]; 
    result[1][1] = A[0][0];
     
     
     
    result[0][0]/=ValueOfA;
    result[0][1]/=ValueOfA;
    result[1][0]/=ValueOfA;
    result[1][1]/=ValueOfA;
     
    for(int row= 0; row < result.length;row++)
    {
    for(int col = 0; col < result[row].length; col++)
    {
    System.out.print(result[row][col] + " ");
    }
    System.out.println();
    }
     
    return result; 
    }
    Last edited by helloworld922; February 6th, 2013 at 11:36 PM. Reason: please use [code] tags


  2. #2
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    Default Re: java help with inversing matrices

    What is the algorithm for inverting a matrix? Can you post the details of what is done and give an example of a matrix before and after?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: java help with inversing matrices

    Quote Originally Posted by Norm View Post
    What is the algorithm for inverting a matrix? Can you post the details of what is done and give an example of a matrix before and after?
    im sorry im a little confused on what your asking. the method i have above only works for a 2x2 matrix. the matrix im working on now is an 8x8. i need to find the inverse of that (8x8). ive done some research and ive looked thru my textbook but i dont understand the pseudo code and i cant find any examples throughout the internet coded for large matrices.

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    Default Re: java help with inversing matrices

    How will you know if the matrix has been correctly inverted if you can not describe what an inverted matrix is?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: java help with inversing matrices

    Quote Originally Posted by Norm View Post
    How will you know if the matrix has been correctly inverted if you can not describe what an inverted matrix is?
    i believe its finding the adjoint of the matrix divided by determinant. idk man. i know ive been checking with software on different websites. ive been working at this for days.

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    Default Re: java help with inversing matrices

    Without an algorithm for how to invert a matrix, you can not write a program to do it.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: java help with inversing matrices

    Quote Originally Posted by Norm View Post
    Without an algorithm for how to invert a matrix, you can not write a program to do it.
    sorry dude your stumping me. basically this is where im at. i got to find an inverse of a pre determined 8 x 8 matrix. this is all lol. i aint got the slightest clue on how to do this.

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    Default Re: java help with inversing matrices

    You need to find the algorithm to be able to write the code.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: java help with inversing matrices

    Wikipedia is your friend

    There are many different ways to do it, the simplest to implement is probably Gauss-Jordan Elimination.

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    Default Re: java help with inversing matrices

    whats been troubling me the most is being able to derive a method for this. ill take a look at what you sent me

    --- Update ---

    Quote Originally Posted by helloworld922 View Post
    Wikipedia is your friend

    There are many different ways to do it, the simplest to implement is probably Gauss-Jordan Elimination.
    so the identity matrix used is a whole new matrix? so starting out ive got 2 matrices correct. 1 identity and my starter matrix?

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    Default Re: java help with inversing matrices

    Yes, that's one way to do it. At the end your original identity matrix will be the inverted matrix and the original matrix will be the identity matrix. All you need to do at the end is copy the inverted matrix back.

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    Default Re: java help with inversing matrices

    ok having mad difficulty but heres some code: its not compiling can i get some help?

    public static double[][] GaussianEliminverse(double[][] A,int B)
    {
    double[][] b = new double[B][B];
    for (int row = 0; row < B; row++) {
    for (int col = 0; col < B; col++) {
    if (row == col) {
    b[row][col] = 1;
    } else {
    b[row][col] = 0;
    }
    }
    }

    double pivot, factor;
    int i, j, k;

    int n = b.length;

    for (i = 0; i < n; i++) {
    pivot = A[i][i];

    for (j = 0; j < n; j++)
    A[i][j] = A[i][j] / pivot;

    b[i][j] = b[i][j] / pivot;

    for (k = 0; k < n; k++) {
    if (k != i) {
    factor = A[k][i];

    for (j = 0; j < n; j++)
    A[k][j] = A[k][j] - factor * A[i][j];

    b[k][j] = b[k][j] - factor * b[i][j];
    }
    }
    }
    for (int row = 0; row < b.length; row++) {
    for (int col = 0; col < b[row].length; col++) {
    System.out.print(b[row][col] + " ");
    }
    System.out.println();
    }
    return (b);
    }

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    Default Re: java help with inversing matrices

    Please copy the full text of the error messages and paste it here.

    Please edit your post and wrap your code with
    [code=java]
    <YOUR CODE HERE>
    [/code]
    to get highlighting and preserve formatting.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: java help with inversing matrices

    Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
    at matrix.GaussianEliminverse(matrix.java:2387)
    at matrix.main(matrix.java:2048)

    on: b[i][j] = b[i][j] / pivot;


    this is my test:
    double [][] inversetest = { {1,2,3},{4,8,7},{3,2,4}};


    GaussianEliminverse(inversetest, 3);


    and code:
    public static double[][] GaussianEliminverse(double[][] A,int B) 
    {
    double[][] b = new double[B][B];
    for (int row = 0; row < B; row++) {
    for (int col = 0; col < B; col++) {
    if (row == col) {
    b[row][col] = 1;
    } else {
    b[row][col] = 0;
    }
    }
    }
     
    double pivot, factor;
    int i; 
    int j;
    int k;
     
    int n = b.length;
     
    for (i = 0; i < n; i++) {
    pivot = A[i][i];
     
    for (j = 0; j < n; j++)
    A[i][j] = A[i][j] / pivot;
     
    b[i][j] = b[i][j] / pivot;
     
    for (k = 0; k < n; k++) {
    if (k != i) {
    factor = A[k][i];
     
    for (j = 0; j < n; j++)
    A[k][j] = A[k][j] - factor * A[i][j];
     
    b[k][j] = b[k][j] - factor * b[i][j];
    }
    }
    }
    for (int row = 0; row < b.length; row++) {
    for (int col = 0; col < b[row].length; col++) {
    System.out.print(b[row][col] + " ");
    }
    System.out.println();
    }
    return (b);
    }

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    Default Re: java help with inversing matrices

    Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
    at matrix.GaussianEliminverse(matrix.java:2387)
    That's a runtime exception, not a compiler error.
    At line 2387 the index to an array went past the end of the array.
    Remember that the range of indexes for an array is from 0 to the array length-1.

    Some problems I see in the code:
    The loop control variables: i,j,k are defined outside of the for loops instead of inside the (). The value of those variables can be used outside the loop, but you must be careful.
    Another problem I see is that there are for() loops that are not using {} to surround the code that is looped through. Although allowed, using them will prevent errors and confusions.

    The posted code has lost its indentations which makes it hard to read and understand.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: java help with inversing matrices

    Please use tab-formatting on your code. Even with syntax highlighting it's extremely difficult to read code which is all left-aligned. Your problem should become apparent once you properly format your code.

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    Default Re: java help with inversing matrices

    sorry guys heres the code again:

    public static double[][] GaussianEliminverse(double[][] A,int B)           
    {
        double[][] b = new double[B][B];
        for (int row = 0; row < B; row++) {
    	for (int col = 0; col < B; col++) {
    		if (row == col) {
    			b[row][col] = 1;
    		} else {
    			b[row][col] = 0;
    		}
    	}
        }
     
        double pivot, factor;
        int i; 
        int j;
        int k;
     
        int n = b.length;
     
        for (i = 0; i < n; i++) 
        {
    	pivot = A[i][i];
     
    	for (j = 0; j < n; j++)
    	{
            	A[i][j] = A[i][j] / pivot;
    	}
     
    	b[i][j] = b[i][j] / pivot;
     
    	for (k = 0; k < n; k++) 
    	{
    	    if (k != i) 
                {
     
    	        factor = A[k][i];
     
    	         for (j = 0; j < n; j++)
    		 {
    		    A[k][j] = A[k][j] - factor * A[i][j];
    		 }
     
    		b[k][j] = b[k][j] - factor * b[i][j];
    				}
    	    }
    	}
    		for (int row = 0; row < b.length; row++) {
    			for (int col = 0; col < b[row].length; col++) {
    				System.out.print(b[row][col] + " ");
    			}
    			System.out.println();
    		}
    		return (b);
    	}

  18. #18
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    Default Re: java help with inversing matrices

    What happens now?
    I see that i,j,k are still defined outside the for loops. Why is that?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: java help with inversing matrices

    can you show me where i should put them. im trying man. im not sure what else to do in order to make this work

  20. #20
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    Default Re: java help with inversing matrices

    Remove the isolated, separate definitions for i,j,k

    Define them in the for loops:
      for(int i = 0; ....)
    same for j and k

    Look at how row and col were defined at the top of the method.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: java help with inversing matrices

    i took the variables out. im still getting the runtimeexception for b[i][j] = b[i][j] / pivot;

  22. #22
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    Default Re: java help with inversing matrices

    You shouldn't get a runtime exception if you've followed norm's suggestion, it should be a compile-time exception.

    Take a look at this part of your code:

    for (j = 0; j < n; j++)
    {
        A[k][j] = A[k][j] - factor * A[i][j];
    }
    b[k][j] = b[k][j] - factor * b[i][j];

    Can you see why this code will result in a runtime exception (or better yet, a compile-time exception if you did follow Norm's suggestion)?

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    Default Re: java help with inversing matrices

    Quote Originally Posted by helloworld922 View Post
    You shouldn't get a runtime exception if you've followed norm's suggestion, it should be a compile-time exception.

    Take a look at this part of your code:

    for (j = 0; j < n; j++)
    {
        A[k][j] = A[k][j] - factor * A[i][j];
    }
    b[k][j] = b[k][j] - factor * b[i][j];

    Can you see why this code will result in a runtime exception (or better yet, a compile-time exception if you did follow Norm's suggestion)?



    i have to have this project turned in tonight. i got to get this part, im stuck with what to do. can anyone please tell me what im doing wrong and fix the problem. its a outofboundsException at b[k][j] = b[k][j] - factor * b[i][j]; and b[i][j] = b[i][j] / pivot;

  24. #24
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    Default Re: java help with inversing matrices

    Look at the algorithm and see if the code is following it.

    If you removed the isolated definitions for i,j,k how does the code compile without an error?
    If you don't understand my answer, don't ignore it, ask a question.

  25. #25
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    Default Re: java help with inversing matrices

    i had to keep j isolated but i was able to remove i and j

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