1

I'm trying to write a code that multiplies two given matrices together and displays the results. Here is the code I have down so far:

public static void main(String[] args) {
    double firstMatrix[][] = {{1, 2}, {3, 4}, {5, 6}};
    double secondMatrix[][] = {{1, 2, 3}, {4, 5, 6}};

    double[][] thirdMatrix = multiplyMatrix((firstMatrix), (secondMatrix));

    System.out.println("The product of the matrices is ");
    for (int i = 0; i < thirdMatrix.length; i++) {
        for (int j = 0; j < thirdMatrix[0].length; j++) {
            System.out.print(firstMatrix[i][j] + " ");
            if (i == 1 && j == 2) {
                System.out.print("  *  ");
            } else {
                System.out.print("    ");
            }
        }
        for (int j = 0; j < thirdMatrix[0].length; j++) {
            System.out.print(secondMatrix[i][j] + " ");
            if (i == 1 && j == 2) {
                System.out.print(" = ");
            } else {
                System.out.print("  ");
            }
        }
        for (int j = 0; j < thirdMatrix[0].length; j++) {
            System.out.printf("%.1f", thirdMatrix[i][j]);
        }
        System.out.println();
    }
}

public static double[][] multiplyMatrix(double[][] a, double[][] b) {
    double c[][] = new double[3][3];
    for (int i = 0; i < c.length; i++) {
        for (int j = 0; j < b.length; j++) {
            for (int k = 0; k < a.length; k++) {
                c[i][j] += ((a[i][k]) * (b[k][j]));
            }
        }
    }
    return c;
}

But when I try to execute the program I receive an Array Index out of Bounds Error which shouldn't be the case since my for loops are making sure not to step out of bounds from the array. I don't understand where to change my code because it looks fine until I try to execute. Any help is really appreciated.

Community
  • 1
  • 1
Mel
  • 33
  • 7
  • The exception stack trace tells you what the index is, and where the exception is thrown. Don't ignore it. And use your debugger. – JB Nizet Nov 29 '18 at 20:22
  • hi @JBNizet I did what you said and was able to identify where it looks like the problem is. Namely at 'c[i][j] += ((a[i][k]) * (b[k][j]));' but am not sure where to go from here. Any advice? – Mel Nov 29 '18 at 20:40
  • You're getting the crash because you're trying to access a[2][2] which doesn't exist. And also b[2][1] which also doesn't exist. – onnoweb Nov 29 '18 at 20:58
  • hi @onnoweb thanks for the response. How can I fix this issue? – Mel Nov 29 '18 at 21:09

2 Answers2

1

If 1st matrix's dimensions is mxn and the 2nd's nxp then the product's dimensions mxp. Right?
So this 2nd level of the triple loop should iterate through the 2nd dimension of the product array:

for (int j = 0; j < b.length; j++)

should be:

for (int j = 0; j < b[0].length; j++)

b[0].length is the number of columns of the matrix b.
Also the 3d level of the triple loop should be:

for (int k = 0; k < b.length; k++)

b.length is the number of rows of the matrix b which is equal to the number of columns of the matrix a.
Edit
In a generalized version you could change this:

double c[][] = new double[3][3];

to

double c[][] = new double[a.length][b[0].length];
forpas
  • 160,666
  • 10
  • 38
  • 76
0
public static void main(String[] args) {
    double firstMatrix[][] = {{1, 2}, {3, 4}, {5, 6}};
    double secondMatrix[][] = {{1, 2, 3}, {4, 5, 6}};

    double[][] thirdMatrix = multiplyMatrix((firstMatrix), (secondMatrix));

    System.out.println("The product of the matrices is ");
    for (int i = 0; i < thirdMatrix.length; i++) {
        for (int j = 0; j < firstMatrix[0].length; j++) {// check this one
            System.out.print(firstMatrix[i][j] + " ");
            if (i == 1 && j == 2) {
                System.out.print("  *  ");
            } else {
                System.out.print("    ");
            }
        }
        for (int j = 0; j < thirdMatrix.length; j++) {
            if (i <= 1) {// you need are condition to stop
                System.out.print(secondMatrix[i][j] + " ");
                if (i == 1 && j == 2) {
                    System.out.print(" = ");
                } else {
                    System.out.print("  ");
                }
            }
        }
        for (int j = 0; j < thirdMatrix[0].length; j++) {
            System.out.print(thirdMatrix[i][j]);
        }
        System.out.println();
    }
}

public static double[][] multiplyMatrix(double[][] a, double[][] b) {
    double c[][] = new double[3][3];
    for (int i = 0; i < c.length; i++) {
        // Change b.length to b[0].length;
        for (int j = 0; j < b[0].length; j++) {
            // Change a.length to a[0].length;
            for (int k = 0; k < a[0].length; k++) {
                c[i][j] += ((a[i][k]) * (b[k][j]));
            }
        }
    }
    return c;
}
Gam Tech
  • 1
  • 2