Multiplication of matrices/ arrays in java

Question

Hi I have this code for multiplication of matrices in Java. It is working (no errors) but giving me wrong answer of the multiplied matrix. The two matrices which are to be multiplied have been defined properly. Here's the code:

// position_new = multiply ( transformation (3x3) * transpose(3x1)  )
int rows=3, columns=1;
double multiply[][] = new double[rows][columns];
double sum;

for (int k = 0; k < columns; k++) {
    for (int e = 0; e < rows; e++) {
        sum = 0;
        for (int f = 0; f < columns; f++) {
            sum = sum + transformation[e][f] * transpose[f][k];
        }
        multiply[e][k] = sum;
    }
}
System.out.println("Multiplied Matrix:-");
for (int m = 0; m < rows; m++) {
    for (int n = 0; n < columns; n++)
        System.out.print(multiply[m][n] + "\t");
    System.out.print("\n");
}

Thanks a lot for the help in advance.

looks like some code is missing

Leo
– Leo

2014-04-07 11:46:04 +00:00
Commented Apr 7, 2014 at 11:46 — Leo
– Leo, Commented Apr 7, 2014 at 11:46

Andynedine · Accepted Answer · 2014-04-07 12:12:36Z

public void Miltiply(){
    // position_new = multiply ( transformation (3x3) * transpose(3x1)  )
    int rows=3, columns=1;
    int transformation[][]={{1,2,3},{1,1,1},{2,2,2}};
    int transpose[][]={{1},{1},{1}};
    double multiply[][] = new double[rows][columns];
    double sum;

    for (int k = 0; k < transpose[0].length; k++) {
        for (int e = 0; e < transformation.length; e++) {
            sum = 0;
            for (int f = 0; f < transpose.length; f++) {
                sum = sum + transformation[e][f] * transpose[f][k];
            }
            multiply[e][k] = sum;
        }
    }
    System.out.println("Multiplied Matrix:-");
    for (int m = 0; m < rows; m++) {
        for (int n = 0; n < columns; n++)
            System.out.print(multiply[m][n] + "\t");
        System.out.print("\n");
    } 
}

Output (M_3x3 X M_3x1 = M_3x1):

6.0 
3.0 
6.0

Jamey · Accepted Answer · 2014-04-07 12:24:13Z

0

your column/row variables are incorrect, it should be this:

// position_new = multiply ( transformation (mxn) * transpose(nxp)  )
int m = 3, n = 3, p = 1;
double multiply[][] = new double[m][p];
double sum;

for (int k = 0; k < p; k++) {
    for (int e = 0; e < m; e++) {
        sum = 0;
        for (int f = 0; f < n; f++) {
            sum = sum + transformation[e][f] * transpose[f][k];
        }
        multiply[e][k] = sum;
    }
}
System.out.println("Multiplied Matrix:-");
for (int i = 0; i < m; i++) {
    for (int j = 0; j < p; j++)
        System.out.print(multiply[i][j] + "\t");
        System.out.print("\n");
}

With the example you gave you were only looping over the first column of the transformation matrix (because columns == 1) This means you were doing a 1x3 * 3x1 matrix multiplication. :)

edited Apr 7, 2014 at 12:24

answered Apr 7, 2014 at 12:07

Jamey

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2 Comments

user3419155 Over a year ago

Hi what does transpose.[0].length do? I am sorry a bit new to java. I have transpose matrix defined but when i pasted the code, it says "Syntax error on token ".", class expected after this token"

Jamey Over a year ago

Sorry, my bad I have tidied it up to use variables again. @user3419155, that line gets the length of the second array dimension. You're probably missing transpose as a variable. Here is a mock up you can add:

int transformation[][] = new int[m][n]; transformation[0][0] = 7; transformation[0][1] = 5; transformation[0][2] = 2; transformation[1][0] = 9; transformation[1][1] = 4; transformation[1][2] = 8; transformation[2][0] = 2; transformation[2][1] = 4; transformation[2][2] = 3; int transpose[][] = new int[n][p]; transpose[0][0] = 4; transpose[1][0] = 5; transpose[2][0] = 1;

Giving: 55, 64, 31

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