how to add elements in a 2d array

Question

I just want to know how this program works, and why the answer is 14. here is the code:

public class extra {
public static void main(String[] args){
    int[][] table = {{1,2,3},{4,5,6},{7,8,9}};
    int sum = 0;
    for( int i = 2; i > 0; i-- ) 
              sum += table[i][3-i];
              System.out.println(sum);

}

}

I understand the way the matrix is set up

123

456

789

but what is i in this problem, because I thought it was the number of rows, but since it is in a for loop, does it mean that i is the number in the matrix? Also how do the [i][3-i] come in to affect? The answer is 14, and I just want to know how it is 14.

Answer 1

It is only summing part of a diagonal, specifically table[2][1] which is 8, and table[1][2] which is 6.

The easiest way to see what is going on is to add an output statement in the loop:

for (int i = 2; i > 0; i--) {
  sum += table[i][3 - i];
  System.out.println(i + " " + (3 - i) + " " + table[i][3 - i]);
}

Answer 2

i, in itself, does not correspond directly to anything in the matrix. It is just the name of the variable that the for loop changes each time it loops.

The [i][3-i] is how the i interacts with table. On the first round of the for loop, the i will be equal to 2. Thus, sum will be increased by table[2][1], which is the 3rd row and the 2nd column of the matrix, which has a value of 8.

On the second round of the for loop, the for loop, the i will be equal to 1. Thus, sum will be increased by table[1][2], which is the 2nd row and the 3rd column of the matrix, which has a value of 6.

Therefore, sum will be equal to 8+6=14.

Answer 3

What the for loop does is as follows:

1. i = 2. Enter loop.
2. Add table[2][3-2] to sum. Sum is now 8, because table[2][1] = 8.
3. Decrement i by 1.
4. i = 1. Enter loop.
5. Add table[1][3-1] to sum. Sum is now 14, because table[1][2] = 6.
6. Decrement i by 1.
7. i = 0. 0 is not greater than 0, so we exit the loop. The sum became 14.

---

Two-dimensional arrays like int[][] table have two indexes. One for the "outer" array (or rows), and one for the "inner" ones (columns).

Let's use int[][] table = {{1,2,3},{4,5,6},{7,8,9}}; from your code as an example:

table[1][2]: 1 means we should look in the array at index 1, which is {4,5,6}. 2 means we should look at {4,5,6}'s index 2, which is 6. In other words table[1][2] == 6.

table[2][0]: 2 means we should look in the array at index 2, which is {7,8,9}. 0 means we should look at {7,8,9}'s index 0, which is 7.

Answer 4

for (int i = 2; i > 0; i--)

so starting at 2 it checks if i is greater than zero loops once then i-- subtracts 1 check again still greater than 0 loops again then subtracts 1 again checks if greater than 0 now 0 it is not greater than 0 so stops looping //thus loops 2 times

//[0] = 1st [1] = 2nd [2] = third ...

//counting in code starts at zero not 1 so and array of 3 counts a the spaces 0,1,2

int sum = 0;//sum starts at zero

//using the value of i translates as such

sum += table[2][3-2];//[2][1]this is 3rd group 2nd part so sum += 8
//then
sum += table[1][3-1];//[1][2]this is 2nd group 3rd part so sum += 6

0 + 8 + 6 = 14

Answer 5

Here's how to add elements in a 2D array in a easy way. First when you initialize a 2D array think of the first brackets [ ] as a column and the second bracket [ ] as column rows. For example: int[][] num = new int[10][5] which means 10 columns and 5 rows.

If you want to fill all the elements in a 2D array you must use two for loops:

int[][] num = new int[10][5];

for (int i =0; i < num.length;i++ ) {
     for (int x=0; x < num[0].length;i++) { //we used num[0] because we need the length of the rows not the columns
           num[i][x] = //any value you want to assign
          }
 }

Answer 6

your program takes table[2][1] (value of 8) and table[1][2] (value of 6) elements, sums them and prints as output (value of 14)

regarding your question in a title your main method should be more like this:

public static void main(String[] args) {
    int[][] table = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
    int sum = 0;

    System.out.println("Before\n");

    for (int i = 0; i < table.length; i++) {
        for (int j = 0; j < table[i].length; j++) {
            sum += table[i][j];
            System.out.printf("Sum after %d iteration: %d\n", i + j + 1, sum);
        }
    }

    System.out.println("\nIn total: " + sum);
}

i + j + 1 is a sum of current iteration which is sum of both axises, and since Java has 0-based indexed tables, it is increased by 1

Hope it helps!