Matrix.java

package com.company;

/**
 * Created by Benjamin R. on 30-07-2016.
 */
public class Matrix {    // Includes gaussian elimination methods
    private double[][] data;

    public Matrix(int m, int n) {
        data = new double[m][n];
    }

    public static Matrix gaussian(Matrix a, Matrix b) {
        int n = a.data.length;                // Number of unknowns
        Matrix q = new Matrix(n, n + 1);

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++)    // Form q matrix
                q.data[i][j] = a.data[i][j];
            q.data[i][n] = b.data[i][0];
        }

        forward_solve(q);                    // Do Gaussian elimination
        back_solve(q);                        // Perform back substitution

        Matrix x = new Matrix(n, 1);
        for (int i = 0; i < n; i++)
            x.data[i][0] = q.data[i][n];
        return x;
    }

    private static void forward_solve(Matrix q) {
        int n = q.data.length;

        for (int i = 0; i < n; i++) {    // Find row w/max element in this
            int maxRow = i;                // column, at or below diagonal
            for (int k = i + 1; k < n; k++)
                if (Math.abs(q.data[k][i]) > Math.abs(q.data[maxRow][i]))
                    maxRow = k;

            if (maxRow != i)                // If row not current row, swap
                for (int j = i; j <= n; j++) {
                    double t = q.data[i][j];
                    q.data[i][j] = q.data[maxRow][j];
                    q.data[maxRow][j] = t;
                }

            for (int j = i + 1; j < n; j++) {    // Calculate pivot ratio
                double pivot = q.data[j][i] / q.data[i][i];
                for (int k = i; k <= n; k++)    // Pivot operation itself
                    q.data[j][k] -= q.data[i][k] * pivot;
            }
        }
    }

    private static void back_solve(Matrix q) {
        int n = q.data.length;

        for (int j = n - 1; j >= 0; j--) {        // Start at last row
            double t = 0.0;                    // t- temporary
            for (int k = j + 1; k < n; k++)
                t += q.data[j][k] * q.data[k][n];
            q.data[j][n] = (q.data[j][n] - t) / q.data[j][j];
        }
    }

    public void setIdentity() {
        int i, j;
        int nrows = data.length;
        int ncols = data[0].length;
        for (i = 0; i < nrows; i++)
            for (j = 0; j < ncols; j++)
                if (i == j)
                    data[i][j] = 1.0;
                else
                    data[i][j] = 0.0;
    }

    public int getNumRows() {
        return data.length;
    }

    public int getNumCols() {
        return data[0].length;
    }

    public double getElement(int i, int j) {
        return data[i][j];
    }

    public void setElement(int i, int j, double val) {
        data[i][j] = val;
    }

    public void incrElement(int i, int j, double incr) {
        data[i][j] += incr;
    }

    public Matrix add(Matrix b) {
        Matrix result = null;
        int nrows = data.length;
        int ncols = data[0].length;
        if (nrows == b.data.length && ncols == b.data[0].length) {
            result = new Matrix(nrows, ncols);
            for (int i = 0; i < nrows; i++)
                for (int j = 0; j < ncols; j++)
                    result.data[i][j] = this.data[i][j] + b.data[i][j];
        }
        return result;
    }

    public Matrix mult(Matrix b) {
        Matrix result = null;
        int nrows = data.length;
        int p = data[0].length;
        if (p == b.data.length) {
            result = new Matrix(nrows, b.data[0].length);
            for (int i = 0; i < nrows; i++)
                for (int j = 0; j < result.data[0].length; j++) {
                    double t = 0.0;
                    for (int k = 0; k < p; k++) {
                        t += data[i][k] * b.data[k][j];
                    }
                    result.data[i][j] = t;
                }
        }
        return result;
    }

    public void print() {
        for (int i = 0; i < data.length; i++) {
            for (int j = 0; j < data[0].length; j++)
                System.out.print(data[i][j] + " ");
            System.out.println();
        }
        System.out.println();
    }
}