Codehs 8.1.5 Manipulating 2d Arrays (2026)

for (int row = 0; row < array.length; row++) 
    for (int col = 0; col < array[row].length; col++) 
        System.out.print(array[row][col] + " ");
System.out.println();

Students often encounter "IndexOutOfBounds" errors or logic errors on this exercise. Here is how to avoid them:

Swap two rows:

function swapRows(matrix, rowA, rowB) 
  let temp = matrix[rowA];
  matrix[rowA] = matrix[rowB];
  matrix[rowB] = temp;
  return matrix;

Here is a complete Java class that you might submit for CodeHS 8.1.5, assuming the prompt asks you to implement swapping and a pattern fill:

public class Manipulating2DArrays 
// Prints the 2D array nicely
public static void print2D(int[][] arr) 
    for (int[] row : arr) 
        for (int val : row) 
            System.out.print(val + " ");
System.out.println();
// Swap two rows by reference
public static void swapRows(int[][] arr, int r1, int r2) 
    int[] temp = arr[r1];
    arr[r1] = arr[r2];
    arr[r2] = temp;
// Swap two columns by iteration
public static void swapColumns(int[][] arr, int c1, int c2) 
    for (int r = 0; r < arr.length; r++) 
        int temp = arr[r][c1];
        arr[r][c1] = arr[r][c2];
        arr[r][c2] = temp;
// Manipulation task 8.1.5 specific: Create diagonal pattern
public static void diagonalOne(int[][] arr) 
    for (int r = 0; r < arr.length; r++) 
        for (int c = 0; c < arr[0].length; c++) 
            if (r == c) 
                arr[r][c] = 1;
             else if (r + c == arr.length - 1) 
                arr[r][c] = 1; // Anti-diagonal also to 1
             else 
                arr[r][c] = 0;
public static void main(String[] args) 
    int[][] test = 
        9, 8, 7,
        6, 5, 4,
        3, 2, 1
    ;
System.out.println("Original:");
    print2D(test);
swapRows(test, 0, 2);
    System.out.println("\nAfter swapping row 0 and 2:");
    print2D(test);
swapColumns(test, 1, 2);
    System.out.println("\nAfter swapping col 1 and 2:");
    print2D(test);
diagonalOne(test);
    System.out.println("\nAfter diagonal manipulation:");
    print2D(test);

For example, if it says:

"Write a method that takes a 2D array and returns the sum of the diagonal elements"

I can give you this:

public int sumDiagonal(int[][] matrix) 
    int sum = 0;
    for (int i = 0; i < matrix.length && i < matrix[i].length; i++) 
        sum += matrix[i][i];
return sum;

Just reply with the exact text of the CodeHS 8.1.5 problem, and I'll write the complete code solution for you. Codehs 8.1.5 Manipulating 2d Arrays

The study of 2D arrays in computer science marks a transition from simple data storage to complex structural organization. In the CodeHS curriculum, specifically section 8.1.5, the focus shifts from merely creating these grids to the active manipulation of their contents. Mastering the manipulation of 2D arrays is a fundamental skill that allows programmers to manage spatial data, such as game boards, image pixels, and mathematical matrices, through the precise application of nested loops and index logic.

The core mechanism for manipulating a 2D array is the nested for loop. Because a 2D array is essentially an "array of arrays," a single loop is insufficient to reach every element. The outer loop typically iterates through the rows, while the inner loop traverses the columns of the current row. This structure provides a coordinate-like system, where every element is accessible via its row and column indices. In 8.1.5, learners practice how to use these indices not just to read data, but to modify it—whether by initializing a grid with specific values, updating a single entry, or transforming the entire data set based on a logical condition.

A significant challenge highlighted in this module is the "Row-Major" versus "Column-Major" traversal. In Java, 2D arrays are row-major by default, meaning the computer thinks of the data as a collection of rows. When manipulating these arrays, programmers must be careful with boundary conditions to avoid the common ArrayIndexOutOfBoundsException. For example, when swapping elements or shifting values, one must ensure that the index logic accounts for the array's length and the length of the individual subarrays. Successfully navigating these boundaries is what separates a novice from a proficient coder.

Practical application is the ultimate goal of 8.1.5. By manipulating 2D arrays, students can create algorithms that flip images, calculate the sum of specific regions in a grid, or manage the state of a Tic-Tac-Toe board. These exercises reinforce the importance of logical precision. A small error in a nested loop can lead to an entirely different outcome, teaching students the value of tracing their code and understanding the relationship between the index and the data it represents. for (int row = 0; row < array

In conclusion, CodeHS 8.1.5 is more than a lesson on syntax; it is a lesson in algorithmic thinking. By learning to manipulate 2D arrays, programmers gain the ability to handle multi-dimensional problems with efficiency. This mastery provides the necessary foundation for more advanced topics in software development, including graphics rendering, database management, and artificial intelligence, where data is rarely linear and structural manipulation is a constant necessity.

This guide covers the key concepts, common patterns, and step-by-step solutions you would need to understand and complete the exercises successfully.

Use array[row][col].

console.log(grid[0][2]); // 3 (row 0, col 2)