Java Matrices Arrays

Question

This program adds two 3x3 matrices. It compiles and runs, but the output, instead of being:

1.0  2.0  3.0       0.0  2.0  4.0       1.0  4.0  7.0
4.0  5.0  6.0   +   1.0  4.5  2.2   =   5.0  9.5  8.2
7.0  8.0  9.0       1.1  4.3  5.2       8.1  12.3 14.2

It produces:

1.0  2.0  3.0       0.0  2.0  4.0       0.0  0.0  0.0
4.0  5.0  6.0   +   1.0  4.5  2.2   =   0.0  0.0  0.0
7.0  8.0  9.0       1.1  4.3  5.2       0.0  0.0  0.0

I'm not sure why the output displays as all zeros? The math in the program 'seems' right to me... Is there something I'm missing here?

import java.util.Scanner;

public class AddMatrices{

public static void main(String[] args){

    Scanner input = new Scanner(System.in);

    int N = 3;

    //get the users input and store it in the two arrays
    System.out.println("\nEnter matrix1: \n");

    //declare 2 arrays with the appropriate number of rows and columns in 
    //them to store the numbers in each matrix.
    //this is the first one.
    double[][] matrix1 = new double[N][N];

    for (int i = 0; i < matrix1.length; i++) {
        for (int j = 0; j < matrix1[i].length; j++) {
            matrix1[i][j] = input.nextDouble();
        }
    }

    //get the users input and store it in the two arrays
    System.out.println("\nEnter matrix2: \n");

    //declare 2 arrays with the appropriate number of rows and columns in 
    //them to store the numbers in each matrix.
    //this is the second one.
    double[][] matrix2 = new double[3][3];

    for (int i = 0; i < matrix1.length; i++) {
        for (int j = 0; j < matrix1[i].length; j++) {
            matrix2[i][j] = input.nextDouble();
        }
    }


    //call the addMatrix method and pass it the two arrays
    double[][] resultingMatrix = addMatrix(matrix1, matrix2);
    System.out.println("The addition of the matrices is ");



}//end of main method


//write the addMatrix method to add the two matrices and display the result

public static double[][] addMatrix(double[][] m1, double[][] m2){

    double[][] result = new double[m1.length][m1[0].length];

    for (int i = 0; i < result.length; i++) {
        for (int j = 0; j < result[0].length; j++){
            m1[i][j] += m2[i][j];
        }
    }

    for (int i = 0; i < m1.length; i++) {
        char plus = '+';
        for (int j = 0; j < m1[0].length; j++) {
            System.out.print("  " + m1[i][j]);
        }

        if (i == m1.length / 2)
            System.out.print("  " + plus + "  ");
        else {
            System.out.print("  ");
        }

        for (int j = 0; j < m2[0].length; j++) {
            System.out.print("  " + m2[i][j]);
        }

        if (i == m1.length / 2)
            System.out.print("  =  ");
        else {
            System.out.print("  ");
        }

        for (int j = 0; j < result[0].length; j++) {
            System.out.print("  " + result[i][j]);
        }
        System.out.println();
    }
return result;
}//end of add matrices

}//end of class

Differentiate logic from presentation. First add the matrices into a new matrix in memory, later print the results. Will make debugging your code much easier. Oh, and a debugger will allow you to follow how is your logic really working. — SJuan76
– SJuan76, Commented Dec 4, 2012 at 1:09

Bhavik Ambani · Accepted Answer · 2012-12-04 01:15:49Z

2

You're setting the added values to m1 instead of result. In your first double-for loop, just do:

result[i][j] = m1[i][j]+m2[i][j];

edited Dec 4, 2012 at 1:15

Bhavik Ambani

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answered Dec 4, 2012 at 1:09

awolfe91

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1 Comment

user1368970 Over a year ago

Yes, this makes much more sense now. Thank you!

Bhavik Ambani · Accepted Answer · 2012-12-04 01:15:40Z

1

I think you are never assigning the result to the variable results

you should change

m1[i][j] += m2[i][j];

to

result[i][j]=m1[i][j] + m2[i][j];

edited Dec 4, 2012 at 1:15

Bhavik Ambani

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answered Dec 4, 2012 at 1:10

Salchi13

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