GaussJordan.java

/**
 * A GaussJordan object holds a two dimensional array representing a matrix
 * containing a system of equations. Contains methods for performing a
 * Gauss-Jordan Elimination and putting a matrix into Reduced Row Echelon Form
 *
 * @author Thomas Coe
 * @version 1.0 October 8th, 2013
 */
public class GaussJordan{

    private double[][] augmentedMatrix;

    /**
     * Constructor for a GaussJordan object. Takes in a two dimensional double
     * array holding the matrix.
     *
     * @param matrix A double[][] containing the augmented matrix
     */
    public GaussJordan(double[][] matrix) {
        augmentedMatrix = matrix;
    }

    /**
     * Runs a Gauss-Jordan elimination on the augmented matrix in order to put
     * it into reduced row echelon form
     *
     */
    public void eliminate() {
        int startColumn = 0;
        for (int row=0; row<augmentedMatrix.length; row++) {
            //if the number in the start column is 0, try to switch with another
            while (augmentedMatrix[row][startColumn]==0.0){
                boolean switched = false;
                int i=row;
                while (!switched && i<augmentedMatrix.length) {
                    if(augmentedMatrix[i][startColumn]!=0.0){
                        double[] temp = augmentedMatrix[i];
                        augmentedMatrix[i]=augmentedMatrix[row];
                        augmentedMatrix[row]=temp;
                        switched = true;
                    }
                    i++;
                }
                //if after trying to switch, it is still 0, increase column
                if (augmentedMatrix[row][startColumn]==0.0) {
                    startColumn++;
                }
            }
            //if the number isn't one, reduce to one
            if(augmentedMatrix[row][startColumn]!=1.0) {
                double divisor = augmentedMatrix[row][startColumn];
                for (int i=startColumn; i<augmentedMatrix[row].length; i++) {
                    augmentedMatrix[row][i] = augmentedMatrix[row][i]/divisor;
                }
            }
            //make sure the number in the start column of all other rows is 0
            for (int i=0; i<augmentedMatrix.length; i++) {
                if (i!=row && augmentedMatrix[i][startColumn]!=0) {
                    double multiple = 0-augmentedMatrix[i][startColumn];
                    for (int j=startColumn; j<augmentedMatrix[row].length; j++){
                        augmentedMatrix[i][j] +=
                            multiple*augmentedMatrix[row][j];
                    }
                }
            }
            startColumn++;
        }
    }

    /**
     * Returns a String with the contents of the augmented matrix.
     *
     * @return A String representation of the augmented matrix
     */
    public String toString() {
        String text = "";
        for (int i=0; i<augmentedMatrix.length; i++) {
            for (int j=0; j<augmentedMatrix[i].length; j++) {
                text+=augmentedMatrix[i][j] + ", ";
            }
            text+="\n";
        }
        return text;
    }
}