Finding the QR decomposition of a matrix

Here’s an example of a Java program that finds the QR decomposition of a matrix using the Householder transformation method:

   import java.util.Arrays;

    public class QRDecomposition {

    public static void main(String[] args) {

        double[][] matrix = {

            {1, 2, 3},

            {4, 5, 6},

            {7, 8, 9}

        };

        double[][][] qr = decomposeQR(matrix);

        double[][] q = qr[0];

        double[][] r = qr[1];

        System.out.println("Matrix Q:");

        printMatrix(q);

        System.out.println("Matrix R:");

        printMatrix(r);

    }

    public static double[][][] decomposeQR(double[][] matrix) {

        int rows = matrix.length;

        int cols = matrix[0].length;

        double[][] q = new double[rows][cols];

        double[][] r = new double[cols][cols];

        // Initialize Q as an identity matrix

        for (int i = 0; i < rows; i++) {

            Arrays.fill(q[i], 0);

            q[i][i] = 1;

        }

        // Perform Householder transformations

        for (int k = 0; k < cols; k++) {

            double[] column = new double[rows - k];      

            // Construct the column vector

            for (int i = k; i < rows; i++) {

                column[i - k] = matrix[i][k];

            }

            double norm = euclideanNorm(column);

            double[] u = new double[rows - k];

            double[] v = new double[rows - k];

            // Calculate u

            u[0] = column[0] + Math.signum(column[0]) * norm;

            for (int i = 1; i < u.length; i++) {

                u[i] = column[i];

            }

            // Calculate v

            double uDotV = dotProduct(u, u);

            for (int i = 0; i < v.length; i++) {

                v[i] = u[i] / uDotV;

            }        

            // Calculate Q

            double[][] qk = new double[rows][cols];

            for (int i = 0; i < rows; i++) {

                for (int j = 0; j < cols; j++) {

                    qk[i][j] = q[i][j] - 2 * v[i] * dotProduct(v, q[i]);

                }

            }

            // Update Q

            for (int i = 0; i < rows; i++) {

                for (int j = 0; j < cols; j++) {

                    q[i][j] = qk[i][j];

                }

            }

            // Calculate R

            for (int i = 0; i < cols; i++) {

                for (int j = i; j < cols; j++) {

                    if (i == k) {

                        r[i][j] = dotProduct(v, matrix[k]);

                    } else {

                        r[i][j] = r[i][j] + v[i] * dotProduct(v, matrix[j]);

                    }

                }

            }

        }

        double[][][] qr = new double[2][][];

        qr[0] = q;

        qr[1] = r;

        return qr;

    }

    public static double euclideanNorm(double[] vector) {

        double sum = 0;

        for (double v : vector) {

            sum += v

 * v;

        }

        return Math.sqrt(sum);

    } 

    public static double dotProduct(double[] a, double[] b) {

        double sum = 0;

        for (int i = 0; i < a.length; i++) {

            sum += a[i] * b[i];

        }

        return sum;

    }

    public static void printMatrix(double[][] matrix) {

        for (double[] row : matrix) {

            for (double value : row) {

                System.out.print(value + " ");

            }

            System.out.println();

        }

        System.out.println();

    }

}

This program finds the QR decomposition of a given matrix using the Householder transformation method. The `decomposeQR()` method takes a matrix as input and returns a 3D array containing the matrices Q and R. The `printMatrix()` method is used to display the resulting matrices Q and R.

In this example, the input matrix is hard-coded as a 3×3 matrix. You can modify the program to accept a matrix of any size by adjusting the input matrix and the corresponding dimensions.