Question

I am having to write this program for school that multiplies a pair of two-dimensional arrays. I keep getting an out of bounds:3 error when I try to run the program.

I have gone over the code many times and can’t for the life of me figure out where the breakdown is happening. I ran a debug and this first breakpoint was at the if statement “if (a[0].length != b.length)” I couldn’t figure out why that was a breakpoint.

Could someone help me?

    public class Multiply {

    public static double[][] Multiply_2D_Arrays(double[][] a, double[][] b)

    { 
        if (a[0].length != b.length) // Check to see if the number of a's colums equals the number of b's rows
        {
            throw new IllegalArgumentException("Matrices don't match: " + a[0].length + " != " + b.length);

        }

        int a_rows = a.length;      // Defines the variable M as the row length of array a
        int b_columns = b[0].length;   // Defines the variable N as the column length of array b
        double[][] c = new double[a_rows][b_columns]; // This means that the dimensions of array c will be the rows of a by the columns of b

        for(int i = 0; i < a.length; i++)
            {
                for(int j = 0; j < b[0].length; j++)
                {
                    for(int k = 0; k < a[0].length; )
                    {

                        c[i][j] += a[i][k] * b[k][j];     //Iterates through each row and column of array a and b and then adding it to the dot point sum

                    }

                }
            }

            return c;      //returns the final new array c

        }

    public static void main(String[] args)

    {
        double[][] array1 = {{4.0, 5.0, 6.0}, {2.0, 1.0, 4.0}, {8.0, 7.0, 6.0}, {1.0, 1.0, 2.0}};
        double[][] array2 = {{5.0, 7.0, 7.0, 8.0}, {8.0, 8.0, 9.0, 2.0}, {10.0, 2.0, 3.0, 1.0}};
        double[][] array3 = Multiply.Multiply_2D_Arrays(array1, array2); //Calls the Multiply_2D_Arrays method

        for(int i = 0; i < array3.length; i++)
        {
            for (int j = 0; j < array3.length; j++)
            {
                System.out.print(array3[i][j] + " ");
            }
        }

        System.out.println();
    }

}

Answer 1

You have missed k++ in
for(int k = 0; k < a[0].length; )

Then you have an infinity loop

Also if you want to print the result as matrix, put System.out.println() inside for loop :

    for(int i = 0; i < array3.length; i++)
    {
        for (int j = 0; j < array3.length; j++)
        {
            System.out.print(array3[i][j] + " ");
        }
        System.out.println();
    }