How to sum rows and columns of a 2D array individually with Java?

Question

Here is the problem I'm working on: Write a program that reads an 3 by 4 matrix and displays the sum of each column and each row separately.

Here is the sample run: Enter a 3-by-4 matrix row by row:

1.5 2 3 4
5.5 6 7 8
9.5 1 3 1

Sum of the elements at column 0 is 16.5
Sum of the elements at column 1 is 9.0
Sum of the elements at column 2 is 13.0
Sum of the elements at column 3 is 13.0
Sum of the elements at Row 0 is: 10.5
Sum of the elements at Row 0 is: 26.5
Sum of the elements at Row 0 is: 14.5

and here is the code that I have come up with:

package multidimensionalarrays;

public class MultidimensionalArrays {

    public static void main(String[] args) {
        double sumOfRow = 0;
        double[][] matrix = new double[3][4];
        java.util.Scanner input = new java.util.Scanner(System.in); //Scanner
        System.out.println("Enter a 3 by 4 matrix row by row: ");
        //Prompt user to enter matrix numbers
        for (int row = 0; row < matrix.length; row++) {
            for (int col = 0; col < matrix[0].length; col++) {
                matrix[row][col] = input.nextDouble();
            }
        }
        double[] sumOfCol =new double[matrix[0].length];  
        for (int i = 0; i < matrix.length; i++) { //row
            for (int j = 0; j < matrix[i].length; j++) { //column
                sumOfRow += matrix[i][j];
                sumOfCol[j] += matrix[i][j];
            }
            System.out.println("Sum of the elements at row " + row + " is: " + sumOfRow);
        }
        System.out.println("Sum of the elements at column " + col + " is: " + sumOfCol);
    }             
}   

My problem is that when it gets to the end to print the sums for columns and rows, it is not recognizing the row or col variables. I've been playing with it and switching things around for probably hours now and I just can't seem to get this right, can someone help me with what I'm doing wrong? Also I don't know if I'm doing the column summing correctly?

Answer 1

In your matrix, it is a 3-by-4 matrix from the code segment double[][] matrix = new double[3][4];. The first index is the row index, and the second index is the column index.

Please note that your double[][] matrix is an array of arrays. Namely, matrix is an array of three arrays of length 4, and by convention, each sub-array can be seen as an array of objects arranged in one row. This is called row major order.

For example, in the array

0 1 2 4
0 1 3 9
0 1 5 15

it is really stored as

• matrix[0]: [matrix[0][0], matrix[0][1], matrix[0][2], matrix[0][3] and so [0 1 2 4]
• matrix[1]: [matrix[1][0], matrix[1][1], matrix[1][2], matrix[1][3] and so [0 1 3 9]
• matrix[2]: [matrix[2][0], matrix[2][1], matrix[2][2], matrix[2][3] and so [0 1 5 25]

Here is a simpler algorithm that uses loops to find the sum of values for each row and column value, but requires two passes:

/* After the prompt code segment and sumOfCol in the main method */

    // Row (major index)
    for (int row = 0; row < matrix.length; row++) {
        int rowSum = 0;
        for (int col = 0; col < matrix[row].length; col++) {
            rowSum += matrix[row][col];
        }
        System.out.println("Sum of the elements at row " + row + " is: " + rowSum);
    }

    // Column (minor index)
    // Assuming the length of each row is the same
    for (int col = 0; col < matrix[0].length; col++) {
        int colSum = 0;
        for (int row = 0; row < matrix.length; row++) {
            colSum += matrix[row][col];
        }
        System.out.println("Sum of the elements at col " + col + " is: " + colSum);
    }

The output is

Enter a 3 by 4 matrix row by row: 
0 1 2 4
0 1 3 9
0 1 5 15
Sum of the elements at row 0 is: 7
Sum of the elements at row 1 is: 13
Sum of the elements at row 2 is: 21
Sum of the elements at col 0 is: 0
Sum of the elements at col 1 is: 3
Sum of the elements at col 2 is: 10
Sum of the elements at col 3 is: 28

Answer 2

The variable counting the rows is called i at the point where you print the row sum.

For the column sums you need another loop to print them one by one.