2D array programs for ICSE Class 10, with the diagonals drawn out.

import java.util.Scanner;

class DiagonalSum {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Order of the square matrix: ");
        int n = sc.nextInt();
        int[][] a = new int[n][n];
        System.out.println("Enter " + (n * n) + " numbers:");
        for (int i = 0; i < n; i++)
            for (int j = 0; j < n; j++)
                a[i][j] = sc.nextInt();
        int left = 0, right = 0;
        for (int i = 0; i < n; i++) {
            left += a[i][i];            // row = column
            right += a[i][n - 1 - i];   // row + column = n - 1
        }
        System.out.println("Left diagonal sum = " + left);
        System.out.println("Right diagonal sum = " + right);
        int both = left + right;
        if (n % 2 == 1)
            both -= a[n / 2][n / 2];    // centre is on both diagonals
        System.out.println("Both diagonals together = " + both);
        if (left == right)
            System.out.println("The two diagonal sums are equal");
        else
            System.out.println("The two diagonal sums are not equal");
    }
}

// Sample input 1
4
5 1 2 8
3 6 9 0
4 7 2 1
6 2 3 4

// Output
Left diagonal sum = 17
Right diagonal sum = 30
Both diagonals together = 47
The two diagonal sums are not equal

// Sample input 2
3
4 9 2
3 5 7
8 1 6

// Output
Left diagonal sum = 15
Right diagonal sum = 15
Both diagonals together = 25
The two diagonal sums are equal

Run 1: 5 + 6 + 2 + 4 and 8 + 9 + 7 + 6. Run 2: the centre 5 is counted once, so 15 + 15 - 5 = 25.

import java.util.Scanner;

class HighestRow {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int[][] marks = new int[4][3];
        int[] total = new int[4];
        System.out.println("Marks of 4 students in 3 tests:");
        for (int i = 0; i < 4; i++) {
            for (int j = 0; j < 3; j++) {
                marks[i][j] = sc.nextInt();
                total[i] += marks[i][j];
            }
        }
        int best = 0;
        for (int i = 1; i < 4; i++)
            if (total[i] > total[best])
                best = i;
        for (int i = 0; i < 4; i++)
            System.out.println("Student " + (i + 1) + " total = " + total[i]);
        System.out.println("Highest total: student " + (best + 1));
    }
}

// Sample input
62 70 58
81 77 90
74 88 69
90 65 80

// Output
Student 1 total = 190
Student 2 total = 248
Student 3 total = 231
Student 4 total = 235
Highest total: student 2

Each row is one student and each column one test, so a row sum is a student total. The row totals are added while the marks are being read, which saves a second pair of loops. Printing best + 1 turns index 1 into the human "student 2".