2D Array Basics

All 15 2d array basics questions

Work through the interactive quiz above first. This is the full 2d array basics question set with worked solutions, so students can review any question after attempting it.

1. 1.Code Tracing 01 - 2D Array Basics: track the final printed value.

   int[][] grid = {{2, 5, 1}, {4, 3, 6}, {7, 0, 8}};
   int total = 0;
   for (int r = 0; r < grid.length; r++) {
     int c = (r + 0) % grid[r].length;
     total += grid[r][c] * (r + 1);
   }
   System.out.print(total);

   • 36
   • 32
   • 34
   • 29
   Show worked solution

   Correct answer: 32

   Each row chooses a column using modular arithmetic, then weights that cell by r + 1 before adding it to total.

2. 2.Code Tracing 12 - 2D Array Basics: follow the variable updates.

   int[][] grid = {{4, 7, 3}, {6, 5, 8}, {9, 2, 10}};
   int main = 0;
   int anti = 0;
   for (int r = 0; r < grid.length; r++) {
     main += grid[r][r];
     anti += grid[r][grid[r].length - 1 - r];
   }
   System.out.print(main + ":" + anti + ":" + (main - anti));

   • 36:17:19
   • 19:17:-2
   • 19:17:2
   • 17:19:-2
   Show worked solution

   Correct answer: 19:17:2

   The loop updates both diagonals at the same time. The final expression prints the two totals and their difference.

3. 3.Code Tracing 28 - 2D Array Basics: trace the branch and loop path.

   int[][] grid = {{7, 6, 1}, {6, 8, 11}, {7, 1, 13}};
   int count = 0;
   int total = 0;
   for (int r = 0; r < grid.length; r++) {
     for (int c = 0; c < grid[r].length; c++) {
       if (grid[r][c] >= 6) {
         count++;
         total += grid[r][c] - r;
       } else {
         total -= c;
       }
     }
   }
   System.out.print(count + ":" + total);

   • 6:48
   • 2:48
   • 7:48
   • 7:55
   Show worked solution

   Correct answer: 7:48

   The nested loops visit every cell. Matching cells increase both count and total, while nonmatching cells subtract their column index.

4. 4.Code Tracing 39 - 2D Array Basics: watch the index changes.

   int[][] grid = {{9, 8, 3}, {5, 10, 7}, {9, 3, 15}};
   for (int r = 0; r < grid.length; r++) {
     for (int c = 0; c < grid[r].length; c++) {
       if ((r + c) % 2 == 0) {
         grid[r][c] += r + c;
       }
     }
   }
   System.out.print(grid[0][0] + "-" + grid[1][1] + "-" + grid[2][2]);

   • 9-10-15
   • 9-8-5
   • 11-3-19
   • 9-12-19
   Show worked solution

   Correct answer: 9-12-19

   Only cells whose row-plus-column index is even are changed. The printed diagonal includes three of those updated cells.

5. 5.Code Tracing 50 - 2D Array Basics: evaluate the state change step by step.

   int[][] grid = {{11, 6, 5}, {4, 12, 9}, {11, 1, 17}};
   int total = 0;
   int count = 0;
   for (int c = 0; c < grid[1].length; c++) {
     if (grid[1][c] >= 10) {
       total += grid[1][c];
       count++;
     }
   }
   System.out.print(total + ":" + count);

   • 12:1
   • 25:3
   • 13:1
   • 22:1
   Show worked solution

   Correct answer: 12:1

   The loop traverses row 1, but only values meeting the threshold affect the total and count.

6. 6.What is printed by the following code segment?

   int[][] g = new int[3][4];
   System.out.print(g.length + " " + g[0].length);
   System.out.print(" " + g[2][3]);

   • 4 3 0
   • 3 4 0
   • 12 4 0
   • 3 4 null
   Show worked solution

   Correct answer: 3 4 0

   Rows come first in the declaration and every element starts at zero.

7. 7.What is printed by the following code segment?

   int[][] g = {{1, 2}, {3, 4, 5}};
   int t = 0;
   for (int r = 0; r < g.length; r++)
   {
       t += g[r].length;
   }
   System.out.print(t);

   • 4
   • 6
   • 5
   • 2
   Show worked solution

   Correct answer: 5

   Rows may differ in length, and summing the row lengths counts every element.

8. 8.What is printed by the following code segment?

   int[][] g = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
   int t = 0;
   for (int i = 0; i < g.length; i++)
   {
       t += g[i][i] + g[i][g.length - 1 - i];
   }
   System.out.print(t);

   • 15
   • 25
   • 45
   • 30
   Show worked solution

   Correct answer: 30

   The two diagonals total 15 each, and the centre element is counted twice.

9. 9.What is printed by the following code segment?

   int[][] g = new int[3][3];
   for (int r = 0; r < 3; r++)
   {
       for (int c = 0; c < 3; c++)
       {
           g[r][c] = r * 3 + c;
       }
   }
   System.out.print(g[1][2] + " " + g[2][0]);

   • 5 6
   • 6 5
   • 4 6
   • 5 7
   Show worked solution

   Correct answer: 5 6

   The formula numbers the cells in row-major order from zero.

10. 10.What is printed by the following code segment?

    int[][] g = {{1, 2}, {3, 4}};
    int[] row = g[0];
    row[1] = 20;
    g[1] = row;
    System.out.print(g[0][1] + " " + g[1][1]);

    • 2 4
    • 20 20
    • 20 4
    • 2 20
    Show worked solution

    Correct answer: 20 20

    Assigning the row reference makes both rows the same array object.

11. 11.What is printed by the following code segment?

    int[][] g = {{5, 1}, {2, 8}};
    int m = g[0][0];
    for (int[] row : g)
    {
        for (int v : row)
        {
            if (v > m) { m = v; }
        }
    }
    System.out.print(m);

    • 5
    • 2
    • 8
    • 16
    Show worked solution

    Correct answer: 8

    The enhanced form treats each row as an int array, and the search covers every cell.

12. 12.What is printed by the following code segment?

    int[][] g = new int[2][3];
    int c = 0;
    for (int[] row : g)
    {
        c += row.length;
    }
    System.out.print(c + " " + g.length);

    • 5 2
    • 6 3
    • 3 2
    • 6 2
    Show worked solution

    Correct answer: 6 2

    Two rows of three cells give six elements in total.

13. 13.What is the result of executing the following code segment?

    int[][] g = {{1, 2}, {3}};
    System.out.print(g[1][1]);

    • An ArrayIndexOutOfBoundsException is thrown.
    • 3 is printed.
    • 0 is printed.
    • The code does not compile.
    Show worked solution

    Correct answer: An ArrayIndexOutOfBoundsException is thrown.

    The second row holds only one element, so index 1 is out of range for that row.

14. 14.What is printed by the following code segment?

    int[][] g = {{1, 2, 3}};
    int t = 0;
    for (int c = 0; c < g[0].length; c++)
    {
        t += g[0][c] * (c + 1);
    }
    System.out.print(t);

    • 6
    • 14
    • 9
    • 12
    Show worked solution

    Correct answer: 14

    Each element is weighted by its one-based column position.

15. 15.What is printed by the following code segment?

    String[][] g = new String[2][2];
    g[0][0] = "a";
    int n = 0;
    for (String[] row : g)
    {
        for (String v : row)
        {
            if (v == null) { n++; }
        }
    }
    System.out.print(n);

    • 4
    • 1
    • 3
    • 0
    Show worked solution

    Correct answer: 3

    A grid of objects starts full of null and only one cell was assigned.