Question

Here is my code, the point of which is to write a program that randomly inputs 20 numbers from 1 to 10 into a 2-D array of 5 rows and 4 columns. The program should output the 2-D array, and the sum of each row and the sum of each column. I can’t figure out where I’m getting this exception (here’s my total output):

Project 3...
2D Array elements:  6, 6, 7, 10, 4, 4, 0, 9, 0, 1, 3, 10, 1, 10, 10, 9, 10, 5, 8, 2, 
Sum of Rows: 115
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 4
    at arrayProject.projectthree(arrayProject.java:96)
    at arrayProject.main(arrayProject.java:19)

Here is my code:

public static void projectthree(){
    System.out.println("Project 3...");
    /*Write a program that randomly inputs 20 numbers from 1 to 10 into a 2-D array of 5 rows and 4 columns.
     The program should output the 2-D array, the sum of each row storing numbers in a parallel array and
     the sum of each column storing numbers in a parallel array*/
     int array[][] = new int[5][4];
     Random randomizer = new Random();
     for (int i = 0; i < array.length; i++){
        for (int j = 0; j < array[i].length; j++){
            array[i][j] = randomizer.nextInt(11);   //Assign random values to each element in the array
        }
     }
    System.out.print("2D Array elements: ");
     for (int i = 0; i < array.length; i++){
        for (int j = 0; j < array[i].length; j++){
            System.out.print(" " +array[i][j]+ ",");    //Output 2D Array
        }
     }

    int[] sumOfRows = new int[array.length];
    int[] sumOfColumns = new int[array[0].length];

    int sumR = 0;
    int sumC = 0;
    for(int row = 0; row < array.length; row++){
        for(int col = 0; col < array[0].length; col++){     //Sum of rows
        sumR += array[row][col];
        }
    sumOfRows[row] = sumR;
    }
    System.out.println(" ");
    System.out.println("Sum of Rows: " + sumR);


    for(int col = 0; col < array.length; col++){
        for(int row = 0; row < array[0].length; row++){
            sumC += array[row][col];
            }
    sumOfColumns[col] = sumC;
    }
    System.out.println("Sum of Columns:" + sumC);

}

Answer 1

Look at this loop:

for(int col = 0; col < array.length; col++){
    for(int row = 0; row < array[0].length; row++){
        sumC += array[row][col];
        }

You’re accessing array[row][col] but you should be accessing array[col][row]… or change the way you’re looping. Compare this loop with all the others which have worked:

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

Note how here i runs from 0 to array.length - 1, and is used as the first bit of indexing in the expression array[i][j].