nullpointer exception in 2D randomly generated array (gaussian elimination)

Question

I have found a null pointer exception when I was randomly generating a 2D array. I used the same methods before and it never gave me a null pointer exception.

Here is the main method.

    public static void main(String[] args) 
    {
    int row = 5;
    int column = 6;

    double[][] matrix = new double[row][column];

    for(int i = 0; i < row; i++)
    {
        for(int j = 0; j < column; j++)
            matrix[i][j] = Math.random()*20;
    }
    b = getB(matrix, row, column);
    printArray(matrix);
    System.out.println("");
    System.out.println("");


    elimination(row, column);
    backSubstitution(row,column);
    printSolution(column);

}

The debugger told me that the line that caused the exception lies in the findMaxPivotRow method inside the elimination method.

public static void elimination(int rows, int cols)
{
    int pivotRow;
    double multiplier;

    for (int r = 0; r< rows; r++)
    {
        pivotRow = findMaxPivotRow(r, rows);
        swapRow(r, pivotRow);

        for (int i = r+1; i < rows; i++)
        {
            multiplier = matrix[i][r] / matrix[r][r];
            b[i] -= (multiplier * b[r]);

            for (int j = r; j < rows; j++)
            {
                matrix[i][j] -= (multiplier * matrix[r][j]);
            }
        }
    }
}

public static int findMaxPivotRow(int k, int rows)
{
    double max_value = matrix[k][k];
    int max_row = k;

    for (int r = k; r < rows; r++)
    {
        if (Math.abs(matrix[r][k]) > max_value)
        {
            max_value = matrix[r][k];
            max_row = r;
        }
    }
    return max_row;
}

public static void swapRow(int k, int pivotRow)
{
    double[] swap;
    double multiplier;
    double swap_b;

    swap = matrix[k];
    matrix[k] = matrix[pivotRow];
    matrix[pivotRow] = swap;

    swap_b = b[k];
    b[k] = b[pivotRow];
    b[pivotRow] = swap_b;
}

The line that caused the exception is this line

double max_value =  matrix[k][k];

I searched the null pointer exception in 2d array online, and it tells me that the error is either the array is not initialized or the code is outside of the bounds. I used the debugger very thoroughly and it showed me that the indices are in the correct positions.

I know there are a lot of codes I have thrown at you, but you can read the main method, the first few lines of elimination and findMaxPivotRow method.

If you can find why that lined caused the null pointer exception, that would be very helpful. Thanks.

NullPointerException _cannot_ be caused by code going outside the bounds of the array in Java. The information you found was incorrect. — Robin Green, May 05 '18 at 06:06

score 0 · Answer 1 · answered May 05 '18 at 05:25

0

You have to pass the double[][] 'matrix' variable declared in main() to other methods so that they can work with it or keep that variable as a static member to be accessible by all the static methods.

answered May 05 '18 at 05:25

Siddhesh Salgaonkar

163
1
4

Thank you for the information, turns out I created two different arrays called matrix. One as a global and one in the main method. – Meme God May 05 '18 at 16:16

score 0 · Answer 2 · edited May 05 '18 at 06:17

The 2D Array matrix is not defined in your method findMaxPivotRow. Either you need to pass the matrix that you are declaring in the main method or you need to declare it as a global variable.

You can see this code where I am declaring as a global variable.

public class myClass{
    static double matrix[][];
    public static void main(String[] args) 
    {
        int row = 5;
    int column = 6;
        matrix = new double[row][column];

    for(int i = 0; i < row; i++)
        {
           for(int j = 0; j < column; j++)
               matrix[i][j] = Math.random()*20;
        }
        b = getB(matrix, row, column);
        printArray(matrix);
        System.out.println("");
        System.out.println("");


        elimination(row, column);
        backSubstitution(row,column);
        printSolution(column);
    }   
    public static void elimination(int rows, int cols)
    {
        int pivotRow;
        double multiplier;

        for (int r = 0; r< rows; r++)
        {
            pivotRow = findMaxPivotRow(r, rows);
            swapRow(r, pivotRow);

            for (int i = r+1; i < rows; i++)
            {
                multiplier = matrix[i][r] / matrix[r][r];
                b[i] -= (multiplier * b[r]);

                for (int j = r; j < rows; j++)
                {
                    matrix[i][j] -= (multiplier * matrix[r][j]);
                }
            }
            }   
   }    

   public static int findMaxPivotRow(int k, int rows)
   {
     double max_value = matrix[k][k];
     int max_row = k;

    for (int r = k; r < rows; r++)
        {
            if (Math.abs(matrix[r][k]) > max_value)
            {
                max_value = matrix[r][k];
                max_row = r;
             }
        }
        return max_row;
   }
}

public static void swapRow(int k, int pivotRow)
{
    double[] swap;
    double multiplier;
    double swap_b;

    swap = matrix[k];
    matrix[k] = matrix[pivotRow];
    matrix[pivotRow] = swap;

    swap_b = b[k];
    b[k] = b[pivotRow];
    b[pivotRow] = swap_b;
}

nullpointer exception in 2D randomly generated array (gaussian elimination)

2 Answers2