Thread: java help with inversing matrices

Originally Posted by Norm

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.

Originally Posted by Norm

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.

Originally Posted by Norm

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.

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 ---

Originally Posted by helloworld922

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?

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);
}

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);
}

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);
	}

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

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

Originally Posted by helloworld922

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;

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