Codehs 8.1.5 Manipulating 2d Arrays Jun 2026

// Sum of a specific row int rowSum = 0; for (int col = 0; col < matrix[rowIndex].length; col++) rowSum += matrix[rowIndex][col];

arrayName[rowIndex][columnIndex]

swapColumns(test, 1, 2); System.out.println("\nAfter swapping col 1 and 2:"); print2D(test);

Input: [[1, 2, 3], [4, 5, 6]] Output: [[2, 4, 6], [8, 10, 12]]

Adding a new column to a 2D array requires modifying each row individually. You can use a loop to iterate over each row and add the new value. Codehs 8.1.5 Manipulating 2d Arrays

A common pitfall when manually creating 2D arrays is accidentally setting all rows to reference the same inner array, leading to unexpected behavior when modifying one row's elements. The best practice is to the array to ensure each row is a separate array instance. Here’s a step-by-step example to create a 3x3 grid filled with zeros:

To see the results of your manipulation, it is useful to print the array in a grid format:

This is the most common error. It happens if your loops go beyond the array size. Ensure your loops go < length , not <= length .

Before diving into manipulation, remember that Java handles 2D arrays as . // Sum of a specific row int rowSum

To change a value at a specific location, you use the syntax array[row][col] = newValue; . Remember, Java uses , meaning the first row is 0 and the first column is 0 . Step 2: Modifying Row by Row

for (int row = 0; row < matrix.length; row++) for (int col = 0; col < matrix[row].length; col++) // do something with matrix[row][col]

public static int[][] doubleArray(int[][] nums)

Let's look at a common scenario:

// 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;

The exercise presents you with a 2D array where the is set to 0 and needs to be updated with a specific value based on different rules for each row:

Use a nested loop and conditional assignment.

for(int row = 0; row < grid.length; row++) grid[row][2] = 5; // Sets third column in every row to 5 Use code with caution. Step 4: Printing the Array The best practice is to the array to

public class ArrayManipulator public static int[][] doubleArray(int[][] nums) nums.length == 0) return new int[0][0];